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Everything posted by DonPetru

  1. The Zener diode stabilizer circuit is well-known (fig. 1). The circuit provides practically a fixed stabilized voltage (which cannot be modified from the outside) having the voltage value of the Zener diode and has a low stabilization coefficient (<100). Being a stabilizer with parallel type adjusting element it is uneconomical for wide-range variable load (the stabilizer has high current consumption regardless of the load current value) and is used only at low load current (at most several hundred me). The circuit calculation consists in determining the limiting resistance RL. This resistance must allow the operating point of the diode to be maintained in the stabilizing region under the conditions of variation of the supply voltage of the circuit and of the current through the RS load. The voltage source U1 can always be represented and known by its voltage at empty E0 and its internal resistance Ri, which facilitates the calculation of this circuit: In practice, two cases usually appear related to the initial data of the calculation. The most common case is the one in which the circuit is powered from an existing source. The second case is the one in which the power supply can be adopted. An eventual capacitor C at the output of the stabilizer circuit could increase the splitting effect (on RL and rz) of the supply voltage pulses (if any) by shunting rz with the reduced capacitor reactance. Also the variations of the output voltage can be reduced due to variable components of the load current, by decreasing the output resistance of the stabilizer for high frequencies. At the same time, it is possible to use two or more Zener diodes with dynamic resistance and reduced temperature coefficients in series to achieve a Zener diode of higher voltage. Zener diodes with minimum temperature coefficient are those with UZ about 5.6V, and with minimum dynamic resistance are those with UZ ≈ 6.8 ... 8.2V. To increase the power dissipated by a Zener diode, if we do not have a proper Zener diode, we can use Zener plus bipolar transistor configurations, as I will show throughout this article. 1. Design of the stabilizer in case of an imposed source Initial data: the average value of the voltage on the Us load and the indication whether it can have the typical dispersion given in the catalog for the corresponding Zener diode; imitations of variation of the current of the load when it is fed to the voltage Us: Is_min, Is_max and the indication if they depend on the voltage on the load; the limits of variation of the empty supply voltages: E0min, E0max; internal resistance of the power supply: Ri; the total permissible variation of the voltage on the load due to the variation of the supply voltage E0 and the load current IS: sUsmax. The average value of the voltage on the load should normally be adopted close to the nominal values of the voltages on the fabricated Zener diodes. Unfortunately, these voltages can only be used as starting values in calculations, because they are defined in the catalog at certain currents, or the actual currents by diodes in the stabilizing circuits are known only after the design is completed. This situation occurs especially when it is not possible to sort the stabilizing diodes (on a standard product or in the absence of a measuring device). If during use of the stabilizer it is possible to remain without load (empty) must be imposed Ismin = 0. Otherwise, the stabilizer may accidentally remain empty and the Zener diode may be destroyed. In order to prevent the use of the empty stabilizer, if this is done on a printed circuit separately from the load circuit, a properly sized resistor parallel to the Zener diode (to ensure Ismin) ). If the circuit is supplied from a rectifier, then above the voltage E0min (where Smin is the stabilization coefficient). the pulses overlap. These do not practically affect the operation of the diode because they are reduced by approx. S min (where S min is the stabilization coefficient. The total variation of the voltage on the load ∆Usmax is the variation that appears after the circuit is made (for a given diode and a resistance, the dispersion and the tolerance do not intervene anymore). 2. Stabilizer design In order to calculate the RL limitation resistance and to check the stabilizer quality, the following steps must be completed: a). A Zener diode with the nominal voltage Us close to the given Us voltage and with the limit currents meeting the approximate condition is adopted: Sometimes the limits of the current through the Zener diode are missing. The maximum current can be determined with approximation to the relation: where PdMAX [W] represents the maximum dissipated power of the diode, given in the catalog only for an average ambient temperature of -20 ... 300C and Uzmax is the maximum possible value of the voltage on the diode due to the manufacturing dispersion. The minimum current will be established based on the table in the catalog, depending on the stabilization demands (at low currents the dynamic resistance of the Zener diodes is higher and the lower stabilization). For better stabilization, it is advisable to adopt an IzMIN current equal to the current from which the dynamic resistance is approximately constant and low. This current is generally: By sorting, we can often find Zener diodes with IzMIN noticeably smaller than the above value. The limits of the Uz voltage due to the manufacturing dispersion are read from the catalog: Uzm (minimum) and UzM (maximum). If the dispersion of the Uz voltage given in the catalog is not allowed in a specific application, it is necessary to select, for the practical realization or even before the calculations are made, the diodes that present in the middle of the current range IzMAX - IzMIN Uz voltage as close to the Uz voltage. imposed. b). Since, in order to simplify the calculations, in the continuum the voltage-current characteristic of the Zener diode will be considered linear (with constant dynamic resistance), it is necessary to specify the coordinates of a point of it ("unknown point"): Iz and Uz as well as the value of the resistance dynamic rz. The known point can be retrieved from the catalog, in which case Uz has the limits of Uzmin and Uzmax, due to the manufacturing dispersion. For a series production of the circuit is practically the only solution, instead, for the achievement of unique ones, the Zener diodes can be selected and measured. c). If it is specified in the design data that Ismin and Ismax also depend on the supply voltage (the load behaves like a linear resistance), the limits of the equivalent load resistance are determined: Because the current limits have been given to load the voltage at the US voltage. d). The limit values of the resistance RL with the relations are determined: If the Zener diodes are selected and the Uz voltage from the known point does not show dispersion, then in the above relations it is considered: If it is specified in the design data that Ismin and Ismax do not depend on the Us supply voltage, then the computational relations (6) and (7) become: In order for the problem to have a solution, in both cases it must result: If this condition is not met, the adopted Zener diode does not have the maximum current sufficiently high for the application to be resolved and a diode with the same voltage U z must be adopted , but with the value of current I zMAX higher (with higher P dMAX dissipated power immediately). e). If the condition (11) is met and the two resistances are approximately equal, then R L will be adopted with the tolerance "t" very low (1%), of the nearest normalized value, in case of series production of the circuit or even select the resistance of the value resulting in calculations, in case of unique ones. If the two limits are noticeably spaced, it is recommended to adopt the normalized RL resistance as close to the RLMAX (without exceeding this by the positive tolerance), when the main performance of the stabilizer - the stabilization coefficient is optimal and the current through the Zener diode is within an area close to IzMIN (less power dissipated per diode). f). The performances of the stabilizer are calculated: - the stabilization coefficient: with: - output resistance: g). The maximum total voltage variation produced by the variation of supply voltage and load current is determined: which must be smaller than the one imposed on the initial data of the project. Otherwise it will be necessary to either select the Zener diodes with lower rz dynamic resistance, or to adopt a Zener diode with higher dissipated power - which has a lower dynamic resistance - but which consumes a higher current from the power supply (it is not an economic solution). h). In order to stabilize the medium voltage on the load, one of the current limits is determined by the Zener diode, for example Izmin, in the absence of the Uz voltage dispersion: where: with "t" = tolerance of the limitation resistance adopted, in percentages and: i). Determine the voltage on the diode without dispersion at the Izmin current: Calculate the average voltage on the load without taking into account the dispersion: This being also the average tension on the load, it will be compared with the value Us initially given by making a decision on its acceptance. Some increase in Uzmed voltage can only be achieved if the RLMAX is significantly higher than the RLMIN and the RL close to RLMAX has been adopted. In this case, the RL is closer and the RLMIN is adopted. k). The extremes of the US voltage are determined taking into account the dispersion and the variation. The possible dispersion of the Uzmed voltage is the same as the dispersion of the Uz voltage given in the catalog (if the diode is not selected). So the voltage at the output of the stabilizer can be between the values: which includes both the dispersion effect and the variation due to the modification of the voltage E0 and the load current Is. It is also possible to include simple and the effect of varying the voltage Us extremes of ambient temperature to the normal temperature on the voltage U zmed . it). Check if can operate in the hollow stabilizer in the case of the initial data I smin different from 0. For this condition to be fulfilled: with: m). The nominal power of the resistance R L is established : 3. Design of the stabilizer where the source can be adopted It is possible to establish a program of calculations by which to adopt the stabilizing diode and obtain the voltage E 0 and the resistance R L , starting from the maximum allowed total variation of voltage on load (excluding dispersion). As long as this calculation occupies a significant space, it will no longer be presented. One solution that uses the calculations from the previous case is to adopt a diode with the right voltage, with a current domain: and determining a voltage E 0 which has the limits that meet the condition given by formula (1). The internal resistance Ri of the source can be adopted provided that a relative voltage drop λ = 0.1 ... 0.2 (as in rectifiers) occurs on this one: In continuation the calculations can be carried out as in the previous case, resuming with an increase of the voltage E0 if the total variation of imposed voltage is not realized or with a possible reduction of it if a total variation is obtained much greater than the imposed one. In case the stabilizer will be supplied from a rectifier, the voltage Ur0 and the current Ir0 are required for its calculation. For this purpose relations are used: where λ is determined from the relation (26) using the current denominator Izmax + Ismin when the stabilizer will not operate normally in the empty or Izmax0 otherwise. Izmax current can be calculated using the correspondingly modified (16), (17), (18) relations. Increasing the load current From the above, it turns out that the Zener diode also consumes electricity from the source, which depends on the current through the Iz diode and on the stabilized voltage. This is why a quite varied range of Zener diodes are manufactured, which differ not only by the stabilizing voltage (Uz), but also by the maximum current it supports, respectively the maximum power output that is the product between the maximum current supported by the IzMAX diode and the voltage. Maximum zener UzM. The most commonly encountered Zener diodes support 1W power, which for an ordinary 10V diode (example: PL10Z, 1N4740 etc) means a maximum diode current of only 100mA, a value that in some situations is not satisfactory. Zener diodes are also made of tens of watts but unfortunately they are quite expensive. If, however, we want to achieve a stabilizer like the one in Fig. 1 for larger currents, what to do? One possibility would be to connect several Zener diodes of the same type in parallel. This is not a recommended solution, because, besides the economic aspect, the inherent dispersion of the diode parameters would adversely affect the stabilization and the currents through those diodes would be unevenly distributed. However, there is a simple, better and very cheap solution. Thus, realizing an assembly like the one in figure 2a, a Zener diode results, whose maximum dissipation multiplies approximately with the amplification factor in current β. For example, a transistor of type 2N3055 having β ≈ 30, using a Zener diode with Pd = 1W, can obtain an equivalent diode that will be able to dissipate a power of about 20W. The stabilization will be the better the β the greater. Therefore, there is the temptation to choose transistors with β as high as possible, but the choice of the transistor is not made first by the value of the amplification factor in the current, but by the current and the maximum power dissipated by it. If in the above example a BC109 transistor, which can have β ≥ 500, a diode of more than 500W, theoretically, this transistor cannot withstand a 100mA base current and no 500W dissipated power. . Figure 2 presents three variants for the proposed purpose. Regarding Figure 2a, it should be mentioned that if the load current decreases below the value of the minimum diode opening current (IzMIN), in order to maintain the stabilization performance it is indicated to mount a resistor between the base and the transistor emitter as in figure 2b, of which value is determined by the relation: The need to install the transistor on a suitable radiator, if any, should not be overlooked. The equivalent Zener diode, obtained according to the diagram in Figure 2a, is frequently used in overload protection mountings, by limitation, or as a safety, if the ballast resistor (RL) is replaced by a fuse. The use of the diode-transistor combination, presented above, has another advantage. Zener diodes have a positive temperature coefficient and the base-emitter junction of the npn silicon transistors has a negative temperature coefficient. A judicious choice of these components, and especially the optimal choice of the current through the transistor (because the value of the temperature coefficient depends on this current), allows to achieve an equivalent diode almost perfectly thermally compensated. Increasing the stabilization factor in Zener diode stabilizers The simplicity of Zener diode stabilizers "is paid" by more modest performances than in the case of integrated stabilizers or error amplifier schemes. However, a considerable improvement in performance can be achieved very simply, using two (or more) circuits (see fig. 1) in cascade. Thus, if the variation of the output voltage, relative to the variation of the input voltage for the first floor can be written: then for the two floors we will have: where Rd1 and Rd2 are the dynamic resistors of the diodes, which are usually much lower than RL1 and RL2. So the value of the report will be considerably lower. Of course, the Zener voltage of the first diode will have to be higher than the second and the larger this difference, the more RL2 will have to be higher, so the stabilization will be better. The disadvantage of the solution lies in the need to have a primary source of higher voltage and to decrease the efficiency, because on the RL2 resistance an additional dissipation will occur. Design example To dimension a Zener diode voltage stabilizer that provides a Us = 6V voltage, with the dispersion according to the catalog, for a load that can be considered resistive having: Ismin = 10mA, Ismax = 80mA. The supply voltage is obtained from a rectifier having a nominal (external) output characteristic as in Fig. 3 (Ri = Rir = ∆ur / ∆ir = 3V / 0.15A = 20 Ω). The voltage of the network, therefore also the voltage E0, shows variations of +/- 7%. The total variation of the voltage on the load (excluding the dispersion) is maUmax = 0.3V. The circuit is intended for a large series product that will work at the normal ambient temperature. A stabilizing diode of type 1N4735 with IzMAX = 146mA, Uzm = 5.58V and UzM = 6.82V is provisionally adopted. It is calculated: The limits of the empty voltage of the source are determined: The fulfillment of the condition given by the formula (2) is verified: So the diode corresponds in principle to the given application. The following data are extracted from the datasheet for diode 1N4735: Uz = 6.2V at current Iz = 41mA, rz = 2 Ω and PdMAX = 1W. The load resistance limits are: The permissible limits for the resistance R L are determined : Because RLMAX> RLMIN, the problem has a solution. The maximum permissible tolerance of the RL resistance is then determined: Normalized resistors can be adopted: RL = 133 Ω, tolerance ± 1% or RL = 137 Ω, tolerance ± 1%. In order to have a better stabilization I will adopt the higher value, namely 137 Ω. The minimum value of the limiting resistor RL shall be: The performances of the stabilizer are: which is a very good value for a Zener and Rieş≈ diode stabilizer rz = 2 Ω. The maximum total variation of load voltage (excluding dispersion) is determined: The resistance is calculated: The minimum diode current in the absence of dispersion is: The voltage on the diode at this current, without dispersion, will be: Medium tension on load, without dispersion: which is close to the value Us = 6V imposed in the statement and can be accepted. The extremes of the load stabilizer voltage are: Check that the stabilizing diode supports the current that appears when the stabilizer is running empty: The nominal power of the RL resistance is established: A rated power resistance of at least 3W shall be adopted. Capacitor value of capacitor C is calculated by the formula: We will choose a normalized capacitor of 680uF with the characteristic nominal voltage greater than Usmax = 6.958V, for example: 10V. When we do not have 137 Ohm resistors we can call in parallel several resistors, equivalent to the adopted value (137 Ohms), higher ohmic resistance and less dissipated power per piece but with the sum of the powers dissipated by each resistance normalized equal to min. 3W. Bibliography: Dumitrescu M. - Voltage and current stabilizers ", Technical Publishing House, Bucharest, 1965; Cătonanu, VM - Electronic Technology", Didactic and Pedagogical Publishing House, Bucharest, 1981; V. Vulpe et al - "Dioda Zener", Technical Publishing House, Bucharest, 1975.
  2. DonPetru


    Somehow, the capacitor is a kind of battery. Although they work in totally different ways, capacitors and batteries also store electricity. Inside the batteries, a series of chemical reactions produce electrons on one terminal and "absorb" them from the other. A capacitor is simpler than a battery, in the sense that it does not produce electrons, but only stores them. So the capacitor is a passive electrical device that stores energy in the form of an electric field between two fittings charged with an equal electrical charge, but of opposite sign. It is also known as the capacitor. The unit of measurement, in the international system, for the electric capacity is the farad (noted F). The capacitors can be of several types (electrolytic, tantalum, etc.), being made both in surface mounted device (SMD) and THD (trough hole device) technology. Inside the capacitor, the terminals are connected to two metal plates separated by a dielectric (non-conductive) material. You can easily make a condenser with two pieces of aluminum foil separated by a piece of paper. It will not be a very good capacitor in terms of its capacity to accumulate electricity, but it will work. In theory, any non-conductive material can be dielectric. However, for practical applications, specific materials are used to obtain the best results. Mica, ceramics, cellulose, porcelain, Mylar, Teflon and even air are some of the dielectric materials used in capacitors. The dielectric used dictates the type of capacitor and what it is used for. Depending on the size and type of the dielectric, some capacitors are better for high frequency electric currents, while others for high voltages. Capacitors can be used for almost any purpose, from small capacitors on your computer to huge capacitors that power a bus. NASA uses glass-dielectric capacitors to turn on the huge electrical systems of shuttles and space stations. Here are some of the dielectrics used in capacitors and what capacities they give them: Air - often used in radio circuits; Mylar - often used for clock circuits, stopwatches and other time measuring devices; Glass - good for high voltage applications; Ceramic capacitors - used for high frequencies, in devices such as antennas, X-ray or MRI scanners; Super capacitors - used on electric and hybrid machines. 1. The capacitor circuit In a schematic representation of an electrical circuit, a capacitor is thus presented, depending on its type. When you connect a capacitor to a battery, here's what happens (see fig. Below): The plate of the capacitor that is attached to the negative terminal of the battery accepts the electrons produced by it; The plate of the capacitor that is attached to the positive terminal of the battery loses electrons, which go to the battery. Once charged, the capacitor will have the same electrical voltage as the battery (if the battery has 1.5 volts, then the capacitor will also have 1.5 volts). For a small capacitor, the capacity is small. Large capacitors can withstand a little more electrical charge. You can find capacitors as much as a can that can support enough electrical charge to make it possible to light a bulb for a minute or more. Even nature has a "capacitor" that works in the form of lightning. One "plate" is the clouds, and the other is represented by the earth, and the lightning appears between these two "plates". Obviously, with a capacitor of this size, enough load can be stored to produce lightning. Pictured above is a circuit consisting of a bulb, a battery and a capacitor. If the capacitor is large enough, you will notice that when you connect the battery, the bulb will illuminate as the battery current flows to the capacitor and this accumulates electrical charge. The light of the bulb will increase in intensity until the capacitor reaches its maximum capacity. If you disconnect the battery from the circuit and replace it with a wire, the current will flow from one capacitor board to the other. The bulb will initially emit an intense light which then decreases in intensity as the capacitor discharges. 2. Farad's The capacity of electrical charge storage by the capacitor, or capacitance, is measured in the International System in units called pharyngeal units. A capacitor of a farad can store a charge coulomb at one volt. A coulomb has 625 X 1016 electrons. An ampere represents a rate of electrons of 1 coulomb per second, so a capacitor of a farad can sustain one ampere per second at a voltage of one volt. A capacitor of a farad is quite large. For this reason, capacitors are usually measured in farult submultiples, in units such as microrodar (mF) and picofarad (pF). To get a sense of how big a farad is, think about it: A standard alkaline AA battery has about 2.8 amps / hour; This means that a standard alkaline AA battery can produce an electric current of 2.8 amps and a voltage of 1.5 volts for one hour (about 4.2 watts per hour such a battery can keep a light on 4 watts approximately one hour); Let's say the battery produces current with a voltage of one volt to simplify calculations. To store the energy of an AA battery with a current intensity of 2.8 amps and a voltage of one volt in a capacitor, you should have a capacitor of 3600 X 2.8 = 10080 headlights, because one ampere per hour equals 3600 amps. per second. 3. Energy stored As opposite charges that accumulate on the plates of a capacitor due to a separation of the loads, a voltage develops in the capacitor due to the electric field of these loads. The increase of the energy is done against the electric field, being separated more electrical charge. The energy (measured in SI in joules) stored in the capacitor is equal to the action required to determine the voltage in the capacitor and, implicitly, the electric field. The stored energy is calculated by the formula: where "V" is the voltage (electrical voltage) at the capacitor terminals. The maximum energy that can be stored (safely) in a capacitor is limited by the maximum electric field that the dielectric material can withstand before breaking down. Therefore, all capacitors using the same dielectric material have the same energy density (joules per cubic meter). 4. Applications The difference between a capacitor and a battery is that a capacitor can discharge all the electrical energy in a fraction of a second, while a battery needs minutes to fully discharge. This is why the electronic flash of a camera uses a capacitor - the battery charges the capacitor for a few seconds, and it releases all the charge it has accumulated in the flash lamp almost instantly. This makes large capacitors extremely dangerous - TVs and other devices containing large capacitors have warnings about opening them. Capacitors are used in electrical circuits in several different ways: Sometimes they are used to store energy and use it at high speeds. This makes lightning. Large lasers use this technique to generate bright, instantaneous spark. Condensers can also remove pulses. If a DC power path "has pulses", a capacitor can absorb "peaks" and fill "valves", so smooth the (voltage) waveform. A capacitor can block DC voltage. If you connect a capacitor to a battery, when fully charged, the current will no longer flow between the poles of the battery. However, any alternate signal passes through an unsteady capacitor. This is because the capacitor is charged and discharged with the fluctuations of the current, making it appear that it passes through it. 5. History of the capacitor The invention of the capacitor varies somewhat depending on who you ask. There are documents that indicate that German researcher Ewald Gorg von Kleist invented the condenser in November 1754. Several months later, Pieter van Musschenbroek, a Dutch professor at Leyden University came up with a similar device, called the Leyda bottle, which is considered the first capacitor. Because Kleist did not have very detailed journals and notes, he was often regarded as a contributor to condenser development, not his inventor. However, over the years, it was established that their research was independent and that it was just a coincidence between them. Leyda bottle is a very simple device. It consists of a glass jar, half filled with water, and lined inside and outside with metal foil. The glass acts as a dielectric, although water has long been believed to be the key ingredient. There is usually a metal wire or chain that is passed through a cork stopper (or other insulating material). The chain is bent on something that will distribute the load. Once the current is transmitted into the cylinder, it should be able to withstand two opposing but equal charges until it is connected to a wire, producing a spark or shock. Benjamin Franklin used the Leyda bottle in his experiments with electricity and found that a piece of flat glass works just like the Leyda bottle, thus developing the flat capacitor or Franklin square. Several years later, the English chemist Michael Faraday used capacitors in the first practical applications in trying to store unused electrons from his experiments. This was the first usable capacitor, made of large oil barrels. Faraday's progress on capacitors is what allowed us to "transport" electricity over long distances. As a result of Faraday's accomplishments in the field of electric fields, the unit of measure of capacitor capacity is the farad. 6. Electrical circuits 6.1 DC sources The dielectric material between the plates is an insulator that stops the flow of electrons. A constant electrical current flowing through a capacitor "stores" electrons on one plate and takes the same amount of electrons from the other plate. This process is called "loading". The current passing through the capacitor results in the charge separation from within it, which develops an electric field between the plates of a capacitor, developing a voltage difference between plates equivalent to the electric current passing through it. This voltage (V) is directly proportional to the amount of separate load (Q). Since current (I) in the capacitor is the rate at which the charge (Q) is "forced" through the capacitor (dQ / dt), it can be mathematically expressed: where "I" is the current flow in the conventional direction, measured in amperes, "dV / dt" is the voltage derived time, measured in volts per second, and "C" is the capacitance measured in farars. For circuits with a constant (continuous) voltage source and consisting only of resistors and capacitors, the voltage passing through the capacitor may not exceed the voltage at the source. Thus, a balance is reached where the voltage passing through the capacitor is constant and the current from the capacitor is zero. For this reason, capacitors are said to block DC. 6.2 Alternative power sources The right current for a capacitor is the alternating current, which changes its direction periodically. This type of electrical current is best suited for a capacitor because it charges the plates alternately: in one direction it charges one plate and when it switches it charges it on the other. Except when the electric current changes direction, the capacitor current is different from zero throughout a cycle. For this reason we can say that the capacitors let the alternating current "pass". However, electrons never pass directly from one plate to another unless the dielectric material is broken. Such a situation leads to physical damage to the capacitor and sometimes to the electrical circuit in which it is located. Because the voltage passing through the capacitor is proportional to the integral of the electric current, with the sine of the waves in the alternating current or the signal of the circuits, in this phase an angle of 90 degrees results, the current driving the voltage of the transit angle. It can be shown that the voltage of the alternating current passing through a capacitor is in the quadrant with the alternation of the current through the capacitor. This means that the voltage and the electric current are offset by a quarter of a cycle. The amplitude of the voltage depends on the amplitude of the current divided by the product of the frequency of the current with the capacitance. 6.3 Impedance The phase voltage rate ("phase" is a complex number, which represents the amplitude of a sinusoidal function of time) that traverses an element of the circuit towards the phase current. That element is called the impedance (Z). Impedance describes a measure of the opposition of an AC current. For a capacitor, this is calculated by the formula: where: is the capacitive reactance, ω = 2πf is the angular frequency ("f" is the frequency, "C" is the capacitance measured in farars, and "j" is the "imaginary unit"). From here, the capacitive reactance is the imagined negative component of the impedance. The negative sign indicates that the electric current brings the voltage to 90 for the sine signal, as opposed to the inductor, The impedance is similar to the resistance resisted by a resistor. The impedance of a capacitor is inversely proportional to the frequency, so for alternating currents with very high frequencies, the reactance is close to zero - so a capacitor is close to a short-circuit to alternating currents with high frequency. In contrast, for low-frequency alternating current, the reactance grows outside the capacitor, so this is an open circuit for a low-frequency alternating current. This behaviorally dependent frequency describes most capacitor functions. The reactor is so called because the capacitor does not waste energy, but stores it. In electrical circuits, as in mechanics, there are two types of loads: resistive and reactive. The resistive loads (they resemble by analogy with an object that moves on a rough surface) dissipate the energy from the circuit in the form of heat, and the reactive loads (they resemble by analogy with an object that moves on a surface where the friction is very small ) stores the energy, introducing it back into the circuit. Also significant is the impedance which is inversely proportional to the capacitance, as opposed to inductors and resistors, where the impedance is proportional to the resistance and the inductance, respectively. So the impedance formula for serial and parallel bonding is the inverse of the resistive case; in series, the sum of the impedances, and in parallel the sum of the conductance (conductivity). 6.4 Serial or parallel connection Capacitors in a parallel configuration have the same potential difference (voltage). Their total capacitance (Ceq) is given by the formula: The reason for connecting capacitors in parallel is to increase the total stored energy. In other words, increasing the capacity increases and the energy that can be stored. The formula by which this can be calculated is The electrical current passing through the capacitors connected in series remains the same, but the voltage passing through each capacitor may be different. The sum of the potential differences (voltages) is equal to the total voltage. Their total capacitance is given by the formula: In parallel, the effective surface of the combined capacitor has increased, increasing the total capacitance, while, in series, the distance between the plates is effectively reduced, thus reducing the total capacitance. Basically, connecting the capacitors in series means economically obtaining high-voltage capacitors, which can be used, for example, to stabilize the electrical current coming from a high-voltage source. Three 600-volt capacitors connected in series will "create" an 1800-volt capacitor. This is of course the capacitance obtained from the connection of capacitors in series, each having a third of the total capacitance. Opposite these results, it is possible to obtain, by connecting the same capacitors in parallel, we obtain a 3X3 capacitor array, with the same total capacitance with a single capacitor, but operable at three times lower voltages. In this application, a resistor will be connected "in front" Another application of this arrangement is the use of polarized capacitors in alternating circuits; the capacitors are connected in series, with reverse polarity, so at any time, a capacitor will be non-encoder. 7. Types of capacitors [Ordered by dielectric material:] a). Vacuum: two electrodes, usually of copper, are separated by a vacuum. The insulating coating is usually made of glass or ceramic material. They typically have small capacities between 10 and 1000 peaks and voltages over 10 kilowatts (kV) are often used on radio transmitters and other devices that use high-voltage electrical currents. This type of capacitors can be fixed or variable. Variable vacuum capacitors can have a minimum to maximum capacitance ratio of over 100, allowing any starting circuit to cover a full decade of frequencies. The vacuum closest to perfection between the dielectrics with the tangential loss equal to zero. This allows the transmission of electricity without significant loss and heat release. b). The air: a capacitor which has air dielectric material, consists of two metal plates, usually made of aluminum or silver plated brass. Almost all air capacitors are variable and are used in radio wave circuits. c). Metallic plastic foil: these capacitors are made from a high quality polymer foil (usually polycarbonate, polystyrene, polypropylene, polyester, and for high quality capacitors, polysulfone), and a metal foil or layer that covers this plastic foil. They have good quality and stability and are suitable for timer circuits. Suitable for high frequencies. d). Small: Similar to those of metal foil. Often for high voltage. Suitable for high frequencies. Expensive. e). Paper: Use for relatively high voltages. Now outdated. f). Glass: Use for high voltage. Expensive. Stable temperature coefficient over a wide range. g). Ceramics: Thin, alternative, metal and ceramic layers. The temperature and capacitance depend on the ceramic material used as a dielectric, and whether they are in class I or II. They have (especially those in the second class) a high dissipation factor, a high frequency dissipation coefficient, their capacity depends on the voltage crossing them, their capacity changes with age. However, they are used in many low-precision coupling and filtering applications. Suitable for high frequencies. h). Electrolytic aluminum: Polarized. Similar in structure to those with metal foil, but the electrodes (plates) are made of pickled aluminum plates to gain a larger surface. The dielectric is made of a material soaked in a substance called electrolyte. They may have large capacities, but suffer from low tolerances, high instability, gradual loss of capacity especially when subjected to heat and electrolyte leakage. They tend to lose their capacity at low temperatures. Not suitable for high frequency currents. i). Electrolytic tantalum: similar to aluminum-electrolytic capacitors but with better temperature and frequency characteristics. High dielectric absorption. High leakage. They have higher performances at low temperatures. j). OS-CON: Capacitors made from a solid-electrolyte polymerized organic semiconductor that offer a longer lifetime and a higher cost. k). Supercapacitors: Manufacture from carbon aerogels, carbon nanotubes or high porosity electrodes. Extremely high capacity. it). Epic capacitors ": They are made of two insulated conductors that have been twisted. Each conductor behaves like an electrode of the capacitor. Epic capacitors" are a form of variable capacitors. Small changes in capacitance (20% or less) are obtained by twisting and loosening the two conductors. m). Varactor or varicap: There are special capacitors with reverse diode thresholds whose capacitance decayed with the voltage. Use inter alia in loops locked in phase. Bibliography: http://en.wikipedia.org/wiki/capcitor (www.wikipedia.org) http: //electronics.h...s.com/capacitor (www.howstuffworks.com)
  3. 1. Overview A bipolar transistor consists of three alternating zones - PNP or NPN - made on the same single crystal. The middle area is very thin compared to the others and bears the basic name (B). Extreme areas are named based on external polarization, emitter (E) and collector (C). The three regions have ohmic contacts that are pulled out of the transistor capsule and are called terminals. Depending on the type of zones (N or P) that are alternated, there are two categories of transistors: NPN and PNP (fig. 1). Due to the embodiment, two PN junctions appear: the emitter-base (EB) junction and the base-collector (BC) junction which can be assimilated with two semiconductor diodes. In practice, if we do not have a catalog, we may find a transistor whose structure is not known (NPN or PNP). In this situation, a method of identifying the structure of the bipolar transistor is used according to the indications in fig.2. This method assumes the existence of a measuring device, more precisely a digital multimeter, with the switch set to the "diode test" position. The donor or acceptor atoms of the emitter and the collector are much larger than the base (about 100 times). For the transistor to operate, the EB junction is polarized directly and the BC junction in the opposite direction with a voltage much higher than the EB junction. The following will explain the operation of a commonly used NPN transistor (Fig. 3). The concentration of carriers in the emitter (electrons) is much higher than in the base and because the EB junction is directly polarized from an external source U EB (fig. 3), there is a massive injection of electrons from the emitter (represented by an arrow) in the region of the base where it finds a much smaller number of holes. These gaps recombine with a small part of the injected electrons. Due to the fact that the base is very thin, most electrons pass through this region and enter the collector area. The BC junction being polarized in the opposite direction (the positive Ucb voltage is applied to the base), an electric field appears which accelerates, the electrons coming from the base to the collector. In the region of the collector the electrons coming from the base are recombined with the gaps coming from the power supply. It is thus noted that although the BC junction is polarized in reverse, it passes through a large current, almost equal to the direct current of the EB junction. This is the main property of the transistor effect which can be stated as follows: a high current can be passed through a polarized junction if the polarity junction is directly in the immediate vicinity (the base is very small). If the thickness of the base is large (greater than the diffusion length of the carriers from the base emitter) then the transistor effect is non-existent and the two series junctions are independent. Figure 3 shows the flows of load carriers through the transistor. The emitter current is composed of two components: The current I EN is due to the majority electrons and the current I EP is the inverse current (due to the gaps) of the BE junction which is very small and can be neglected. The collector current is formed by a fraction "α" of the electron current of the emitter and the inverse current of holes of the junction BC noted I CB0 : The factor α has usual values of the order of 0.900 ... 0.999. The current I CB0 is desirable to be as small as possible. It is thus called the "quality factor" of a transistor. In most applications this current can be neglected for current transistors. The base current is determined by the inverse current of the BE (I EP ) junction , the recombination current of the electrons with the base gaps. (I RB ) and current I CB0: Based on the above considerations we can write the fundamental relation of the transistor operation: For PNP transistors, the operation is identical, with the observation that the external polarizations are of opposite direction and the majority carrier flow is formed by gaps. The current directions as well as the polarizations for the two types of transistors are shown in fig. 1. If neglected I CB0 can define a coefficient that shows how many times the collector current is higher than the base current. This factor expresses the transistor's DC amplification and shows how a small base current leads to a much larger collector current. For current transistors, having a value very close to 1, it results for β large values generally between 10 and 1000. The DC amplification factor depends on the temperature and the size of the collector current. It increases with increasing temperature and decreases at high collector currents. Thus, the transistor effect consisted in modifying the current of voids (starting from the emitter and reaching the collector) by modifying the polarization voltage of a directly polarized junction, namely the polarization voltage of the emitter - base junction. 2. Static characteristics of bipolar transistors The transistor manufacturer indicates in the catalog sheets the graphical connection between the transistor currents and the continuous voltages applied between the terminals. This represents the static characteristics. Any of the transistor contacts can be selected as a voltage reference point. In practice, most often the voltages refer to the emitter or in other words the most common mode of connection in transistor schemes, is the common-emitter (EC) connection, which will be described later. Fig. 4 presents for a silicon NPN transistor these characteristics and the mounting with which they can be determined. The sizes l B and U BE are input sizes, and I C and U CE are output sizes. Input feature - represents the variation of the base current depending on the voltage U BE . It is determined by maintaining from P 2 a constant UCE voltage and varies with P1 the polarization voltage of the base U BE . I B and U BE are measured. This feature is similar to a directly polarized PN junction (Fig. 4c). Occurrence of current I B and implicitly of current I C, occurs only when a voltage threshold U D called open voltage isexceeded. This voltage depends on the semiconductor material. Thus for transistors with AND the opening occurs for U BE voltages between about 0.5 V and 0.65 V and for those with Ge between 0.1 V and 0.2 V. The output characteristic - expresses the variation of the collector current I C according to the U CE for different values of base current I B . For its determination, a certain value of the basic current I B is set with P 1, which is kept constant. Then with P 2 the voltage U CE C is changed accordingly. The study of this characteristic shows that at a constant base current, the collector current increases very little with U CE and in practice it is often considered independent of this voltage. I C depends essentially on I B and therefore on UBE (fig.4b). The transfer feature - shows the dependence of the collector current on the base current. It is determined by simultaneously adjusting the P1 and P2 to keep the U CE At the same time measure the variation of I C according to I B . This feature is a straight line whose inclination depends on the DC amplification factor (fig. 4d). 3. The operating modes of the transistors From the point of view of the polarization mode of the three junctions there are three operating modes: Normal active regime (zone II of fig. 4b). The transistor has the directly polarized BE junction and the BC junction in the opposite direction. The limits of this regime are determined by the condition of canceling one of the polarization voltages. The collector current of the transistor is controlled by the base circuit. Locking or cutting mode (zone III of fig.4b). The junctions BE and BC are polarized in the opposite direction. The current passing through the transistor is very small (in the order of nanoamperes) and is due to the thermally generated minority carriers. The maximum inverse voltage that can be applied to the BE junction in locked mode depends on the type of transistor and is specified in the catalog. Thus for high frequency transistors with Ge it is about 0.3 V, for transistors with Si of 3-7 V and for transistors with Ge allied of 10-20 V. In case of exceeding this voltage, the BE junction behaves like a diode Zener with a very steep characteristic, an important reverse current appears and if there is no limiting resistance, the transistor is destroyed by thermal packing. The saturation regime (zone I of fig. 4b). The junctions BE and BC are directly polarized. The currents flowing through the transistor are mainly limited by the external circuit. This regime may also occur on a transistor to which the polarization sources are applied for operation in the normal active region. Thus, if in Fig. 4a, the potentiometer P 2 is replaced by the resistance R C connected between the collector and + E C , it can be seen that by increasing the voltage U BE it can be reached that at a certain moment the current Ic increases to a value that all supply voltage to fall on Rc. There will thus be a limitation of the collector current to the value: where the voltage U CE is very close to zero and marks the boundary between the normal active regime and the saturation regime. Until the Ics value is reached through an IBS current, a normal active regime can be considered practical for a transistor: Increasing the base current above the I BS value will no longer produce a proportional increase of the collector current, remaining at the I CS value which it cannot be exceeded by being limited by the external circuit. However, the emitter current will continue to increase with the difference between the existing base current and the I BS value . In other words, a saturation current I CS through a transistor whose value depends on the external sizes RC and E C can be obtained if a minimum I BS current is injected into the base . The collector-emitter voltage obtained is called saturation voltage - U CEsat . 4. Dynamic regime of bipolar transistors The transistor as a circuit element can be considered as an active quadripole (fig. 5). Since it has only three electrodes, one will be common to the input and output. This electrode or terminal will serve as a voltage reference point and is considered at zero potential (ground). When operating in dynamic mode the currents and voltages on the transistor contacts are variable in time. Depending on the common terminal chosen, there are three fundamental modes of connection: with the common base (BC), with common emitter (EC) and with common collector (CC) (fig.6). a). The common base circuit (BC) is characterized in that the signal is applied between the base and the emitter and the load resistance R C is mounted between the collector and the base (in terms of which the sources E B and E 0 are presented in sc). Due to the high value of the input current which is the emitter current, the amplification in the current is close to the unit and the input impedance is reduced to tens or hundreds of ohms. This is a disadvantage in the case of multi-stage amplifiers where the low input impedance dampens the output impedance of the previous floor, which requires the use of complicated adaptive circuits. However, the BC mount is widely used in high frequency amplifiers, being preferred to the EC mount where the transistor-collector-specific reaction capacity can produce the floor auto-oscillation. In BC connection, this capacity only appears in the output circuit. The output impedance is high, in the order of hundreds of kOhmi or MOhmi. The gain of the voltage is also large, and in the particular case when there is the collector and emitter resistors R C and R is equal (for low frequencies) about their ratio R C / R E . The phase of the output signal is identical to that of the input signal. This can be explained in the simplest way: if the input voltage tends to increase, then the potential of the emitter will increase, which will decrease the collector current and thus increase the collector voltage (ie the output voltage). b). Circuit with common collector (CC) it is characterized by the fact that the input signal is applied between the base and the collector and the load resistance R E is connected between the emitter and the collector (from the point of view of E. E B and E C are in sc). If the diagram in Fig. 6c is redrawn as in Fig. 6d, we notice that only a fraction of the input voltage U1 is applied between the base and the emitter (U BE ). This will produce a variation in the current I B , I E and I C . The emitter current produces on the load resistance R E an output voltage U 2 lower than the input voltage (U 1 = U BE + U 2). Therefore the voltage amplification is subunit (0.09 - 0.95). Due to the low value of the input current (the base current), the amplification of the current and the input impedance are high. The output impedance is very low. As an actual value, the input impedance is in the order of tens of kilos Ohms, and the output impedance is in the order of tens of ohms. Both impedances are β, I C and R E dependent . Due to this particular feature of the two impedances, the DC connection is used in practice especially for adaptation. Since the voltage amplification is almost unitary, the floor is also called a repeater on the transmitter, which practically reproduces the input signal as amplitude and phase. c). Circuit with common emitter (EC). The input signal is applied between the base and the emitter and the load resistance is connected between the collector and the emitter. Since the input current, which is the base current, has a low value, compared to I E , the input impedance is higher than at the BC connection, which allows the production of multi-stage amplifiers without special adaptation measures. Also the output impedance is relatively high being in the order of tens or hundreds of kilohms. The voltage gain, if a resistor R E is considered in the emitter circuit, is given approximately, for low frequencies, by the ratio R C / R E, and the amplification in current is the factor ß. It is the assembly most often used in practice as a result of the above. As an important observation it should be noted that the amplified signal in voltage at the output is in phase with that of the input. Thus, if a variation of the input voltage (U BE ) is assumed in an upward direction, this causes an increase of the current I B and therefore I C , which leads to an increase of the voltage drop on R C and ultimately to a decrease of the output voltage (U CE ). In the table below the characteristics of the basic schemes of fig. 6 are concentrated. Figure 7 shows an amplifier stage in EC connection with an NPN transistor. Considering that the voltage of the US signal source is zero, we observe that the EC power supply is divided on the Rc and the transistor between the collector and emitter according to the relation: This equation can be represented in the plane of the static output characteristics (fig.4b and fig.7b) by a straight line AB and also called a straight line whose ends are characterized by: By choosing a polarization of the base (EB) a current I BO can be established whose characteristic intersects the right of charge at point P. This point is called a point of It also corresponds to a collector current I CO and a voltage U CE corresponding to the output characteristics . However, if an alternating voltage component U S is superimposed on the base polarization, the base current varies: I B = I B2 - I B1 . This will determine in the collector circuit variations of the collector current (A / c) and the collector-emitter voltage (U CE ) around the static value I C0 , respectively U CE0 . In other words, if an AC signal is applied to the base circuit, the same signal is amplified in the collector circuit, but also amplified in antiphase. The amplification transistor depends on the external sizes and R B , R C . The amplification factor defined above is a parameter that expresses the ratio I C / I B in cc or at low frequencies (about 1 kHz). As the working frequency increases, the ratio of the value of the collector AC to the value of the base AC becomes smaller than β and in this situation a new parameter "h 21e " is defined and also called the direct current transfer ratio. It decreases as the frequency increases. The rate at which it becomes equal to 1 and is called the cutoff frequency is denoted by f t . As an observation to this parameter, it should be noted that the cutoff frequency f T in the EC connection it is lower than the BC connection, where it is β times higher. This justifies the use of BC mounting in high frequency amplifiers. The voltage amplification of this floor can be expressed as in the case of a pentode by: where R C is the load impedance and S is the slope of the transistor. The slope is a parameter that shows how much the output current (collector) varies in mA for 1 V input voltage variation (U BE ). It is expressed in mA / V. A common feature of bipolar transistors is that the slope increases almost linearly with the current, ie approx. 35 mA / V for each mA of the collector current. For example, if a transistor has I C = 5mA then S = 35 * 5 = 175mA / V and if R C = 1K we have a voltage amplification A u = 175 * 1 = 175. At high currents, close to the maximum permissible collector current, the slope is smaller and no longer increases linearly with I C . Also the slope depends on the working frequency. The linear law is generally valid at low frequencies. At medium and high frequencies it decreases with the frequency reaching approx. 25-30% T at frequencies close to f * T . This is mainly due to the time required to travel through the base thickness, at frequencies of the order of (0.1-0.2) * f T it becomes comparable with the frequency period and the collector current ceases to immediately track the instantaneous variations of the base current. As a result there is a reduction of the amplification and a phase difference between the output current and the input current. Positioning the static operating point "P" (fig.7b) on the load line is particularly important. They depend on the operation of the transistor in linear or non-linear regime as well as the time it drives from the total of a period. Fig. 8 presents some particular situations of static point positioning used in practice. If a linear operation is desired, the static point will be in the middle of the load line in M 2. In this case the transistor will conduct the entire signal period (360 ° or 2π). Alternans collector voltage are symmetrical and may be almost equal to half the maximum amplitude of E C . The transistor is said to be in Class A operation . If the static point is close to the saturation zone "M 1 " then one of the alternations will be limited and nonlinear operation will occur. By choosing the operating point "M 4 " at the intersection of the load line with the EC axis , a half-period (180 ° or π) transistor conduction is obtained. The signal in the collector will have only one alternation and the transistor will operate in class B. The direct current consumed from the source is null in the absence of the signal and it increases as it increases. This regime allows high energy efficiency (up to 80%). However, if the operating point is M 3, which is close to the cut-off point of the transistor, then it is possible that from a certain level of the input signal, the transistor will lead for less than one period. The transistor will operate in class AB and the power consumption from the source when no input signal is applied is small. This mode of operation is used in high-performance audio amplifiers to reduce the connection distortion. From a practical point of view, it sometimes appears that the transistor needs to run for less than half a period. In this case the operating point will be M 5 and the transistor will operate in class C. For this, the input circuit will have a polarization of the base which will allow only from a certain level of the positive alternation of the input signal the transistor to be open. This regime has the highest energy efficiency and is used in RF amplification or frequency multiplication (the percentage of harmonics is high due to the shape of the collector current pulse) of the transmitters. One last class of operation is the D class. In this case, the transistor works in switching mode, blocking saturation. The power dissipated by the transistor in the two states is minimal and very large amplifications can be obtained with very high efficiency. The disadvantage is that the input part is complicated. 5. biasing the transistors in the above was found that EC and BC junctions were two separate sources is polarized with B and E C . This practically creates many difficulties. Therefore, the most widespread polarization mode is that which uses a common power source - as shown in Fig. 9 for an NPN transistor in EC connection and class A amplification regime. Source E C it provides both the collector current and the basic current necessary to position the static operating point on the load line in the desired area (fig. 9a). The major disadvantage of this scheme is that due to the large dispersion of the transistor parameters (IB, β) on the one hand, and on the other hand due to their variation with temperature, the static operating point cannot be controlled in practice. The diagram in figure 9b removes the dispersion of the base current and thus of ß, by mounting a polarization divider RB1, RB2 by which a current of about (10-20) is established * I B. Thus, the voltage of the base is practically stabilized. Without having the claim that there are no exceptions, for the circuit in Fig. 9b (which is the most common), the following domains for the circuit resistors can be considered as usual, if the transistor is low power: The emitter resistor R E which introduces a reaction negative, it has a great role in improving the parameters of the scheme in terms of temperature. Thus, due to the increase in temperature, currents I C and I E tend to increase. It will also increase the potential of the emitter to the mass and how the base is maintained at a constant voltage due to the divisor R B1 and R B2, a decrease of UBE voltage and therefore of IC, IE currents will result. The disadvantage of this scheme is that due to the relatively low values of resistors R B1 and R B2 a process of reducing the input impedance of the floor takes place, which is especially annoying in multi-stage amplifiers. To eliminate this disadvantage, a boostrap connection can be used to polarize the base, which also preserves the advantages of the previous scheme due to the existence of the base divider (fig. 9c). Since in the EC connection the signal on the emitter is in phase with the base one, a positive reaction at the input (ie an increase of the signal in the base) is applied through capacitor C, which translates into increasing the impedance in the input. 6. The switching transistor The The operating modes in the blocking and saturation state have been described previously (point 3). The switching mode of a transistor means a dynamic mode in which the transistor operates alternately, saturated-blocked (see fig10). Figure 10 shows the diagram of a switch in CE connection with an NPN transistor, as well as the current pulses with the corresponding times. Right lock is characterized transistor load operating point A where I C is zero and U EC = E C . Also, the saturation of the transistor corresponds to an operating point B, which is obtained by injecting a minimum basic current I BSmin . A current I CS = β * I BSmin is obtained in the collector . In practice, however, a current I BS > I BSmin is applied to guarantee transistor saturation . The collector current can no longer increase and then I CS <β * I BS . The collector voltage in this case will be very low: U CEsat = (0,1..0,5) V. We consider that until the moment t 0 the transistor is blocked by the negative value U 1 of the input signal (U i ) applied on the base. At this time, there is a jump in the input voltage from the value U 1 to the positive value U 2 , soon followed by the jump of the base current from zero to I B1 > I BSmin . [adv_2] Due to the fact that the charge carriers (electrons) injected quickly by the emitter in the base need a time to reach the collector, the current I C will hold at zero a delay time t t after which it begins to increase to the stationary value. The CS . The time when the current increases from zero to 0.9 from its final value is called the rise or rise time - t r. It follows that from the time t 0 when the saturation switch command was applied, until the moment the current of the collector reached 0.9 of the maximum value passed a time called "direct switching time": If at time t 3, the input signal decreases sharply from the value U 2 positive to U 1 negative, the base current will also tend to decrease sharply from I B1 to I B2 changing its meaning due to the fact that in the base region there are load carriers in big number. The resistance R B has the role of limiting the base current to the value I B2 and thus protecting the BE junction. The surplus of electrical charge in the base area will cause the I CS to maintain a "t S " time after which it begins to decrease. During this time the load stored on the basis of the wave and the name of the storage time take place. At time t 4 saturation occurs in the transistor output and the operating point will shift from B to A in a time T C . During this time the basic task continues to be evacuated until it is almost completely canceled. The time t C is called the fall time and is defined as the time when the collector current drops from the value I CS to 0.1 * I CS . "Reverse switching time" means the time interval from the moment the locking command is applied to the moment when the collector current drops to 0.1 from its maximum value and is: Note: In the catalogs of the various manufacturers, they are still meeting for some time. above and the following notations with meaning: t d = t i ; t f = t c . From a practical point of view, it is desirable that U CEsat be as small as the high value of the current I CS passing through the transistor will produce a high power dissipation. For this the current I B will have to be considerably increased compared to I BSmin . However, the increase will not have to be exaggerated, as the storage time will increase and thus the switching properties of the transistor worsen. The typical case of operation in this regime is the final floor of the BO on the TVs. 7. Limit parameters of transistors In the catalog sheets made available to the beneficiaries of the transistor manufacturers, a series of parameters must be specified which should not be exceeded, as follows: Maximum junction temperature. It depends on the nature of the semiconductor material. For Si transistors we have T jmax = 125-175 ° C, and for those with Ge, T jmax = 80-100 ° C. This is usually ensured by the choice of current regimes and judicious voltages and where appropriate by special cooling measures. It should be noted that the lower operating limit for all types of transistors is T jmin = 65 ° C. The maximum collector current of a transistor type depends on a number of factors from which we mention the value of the power dissipated under saturation regime, the threshold to which the decrease of the amplification factor in the current is allowed. This current corresponds to the permanent and noted in the catalog I Q . There is also defined a peak current "I CM " which can only be reached in pulses with a maximum duration well established. This is limited by the existence of irregularities of the flat forms of the junctions where in certain areas high densities of current can occur which produce a dangerous heating and therefore the destruction of the transistor. The maximum base current allowed in permanent mode is noted with I B , and in pulse regime I BM . The emitter-base inverse voltage represents the maximum allowable voltage that can be applied to the EB junction in the blocking direction. The importance of this parameter has been explained in detail, at point 3. in the catalogs it is denoted with U EB0 . The index 0 shows that it is determined for the situation with the collector empty (I C = 0). The maximum collector voltage depends on the transistor connection mode. Thus in the catalogs the following voltages are specified: U CB0 is the reverse voltage applied to the collector-base junction when the emitter is open or empty (I E = 0). Since the emitter-base junction is inert, the transistor behaves like a reverse polarized diode. It is the highest voltage the transistor can withstand. U CEX represents the collector-emitter voltage with the EB junction blocked by a reverse voltage to the normal situation when it is open. It is smaller than U CB0 . U CES is the collector-emitter voltage when the EB junction is shorted from the outside. As this junction will be slightly activated, the U CES voltage is slightly lower than the U CEX ; U CER represents the collector-emitter voltage when a resistor is connected between the emitter and the base. It is even smaller than U CES and U CEX . The CEO represents the collector-emitter voltage with the base empty (I B = 0). It is usually the lowest voltage. It should be noted that these voltages cause a reverse polarization of the collector-base junction, and the base-emitter junction may be in the mentioned situations. The inverse voltages presented correspond to the inverse (residual) currents: I CB0 , I CE0 , I CER , I CES and I CEX . The largest residual current is I CE0 = 3 * I CB0 . This doubles as the temperature rises by approx. 70 0 C. These currents are the limit inverse currents which must not be exceeded, in Fig. 11 the behavior of the transistors at high voltages is shown. The maximum reverse voltages that a transistor can withstand are in the area where an avalanche process called the first piercing begins. This regime does not become dangerous as long as these voltages together with the corresponding residual currents remain inside the parabola which represents the maximum dissipated power (fig. 11a). The maximum reverse currents are the lower the voltages increase. In case of exceeding these voltages due to reaching the Avalanche threshold Up, the multiplication process can no longer be controlled and the transistor enters the secondary throughput manifested by the decrease of the voltage drop between the collector and emitter and the current increase (fig. 11b). The maximum dissipated power theoretically represents the power dissipated on the two junctions: since in the normal active regime UBE << UCB, we can practically consider: in the plane of the output characteristics, equation (14) represents a parabola which together with the maximum collector current and the collector voltage- maximum emitter delimits the allowed area of operation (fig.12). Maximum power dissipation is also noted in catalogs and P M . In the case of power transistors, the dissipation is ensured by the installation of radiators whose calculation takes into account the temperature of the junction of the capsule and the environment as well as the thermal resistance that intervenes between the junction and the environment. 8. Application The circuit of Fig. 13, supplied with E C, is given= 18Vcc and using a BC109B type transistor having β = 300 and I CB0 negligible. Knowing that the transistor must work at the static point I C = 3mA, U CE = 8V, U BE = 0.6V, the values of resistances R 1 , R 2 , R 3 and R 4 are required . Solution We write the following characteristic equations resulting from the application of Kirchhoff's theorem in fig.13: Since β = 300 we can write: Because α≈1 we can write: At the same time, System (15) being two equations with four unknowns, we choose: R 4 = 1 kOhmi and R 2 = 10 kOhmi, according to the domains indicated in table 2. From the first equation of the system (15) it results: Which is rounded to the nearest standard, from the 10% tolerance class, that is R 3 = 2.2kΩ . From the second equation of the system (15) it is written: A standardized value of R 3 = 39kΩ will be adopted . OBS. Having known the resistances R1 and R 2 , the voltage E B can be calculated using the voltage divider relation: E B = E C (R 2 / (R 1 + R 2 )). 9. Setting the static point to the desired value If the circuit calculated in the previous application is performed experimentally and the static operating point is measured, deviations from the values initially imposed will be found. These are due to the following causes: the circuit cannot be made with the calculated values of the resistors, but with their standardized values, which differ slightly from the first ones. Moreover, the resistances belonging to the tolerance class 10%, for example, have actual values different from the standard value written on them by ± 10%; parameter ß can have a dispersion around the nominal value of -50% to + 100%, for the same type of transistor, a very common case in practice; the static operating point varies with the temperature and it is very likely that the temperature of the transistor will differ from that given in the catalog as a reference. Therefore, in order to bring the static operating point to the desired value, the following adjustments can be made (see circuit in fig. 13): a). For the modification of the U EC voltage only , it is taken into account the explanation of the physical operation of the transistor, which, as shown, can be considered a constant current generator between the emitter and the collector. Therefore, by changing the resistance R 3 , the voltage drop on it is changed, the difference up to the value (E C -R 4 · I E ) being taken by U CE (see the first equation in the system (15)). So increasing R 3 decreases U CE and vice versa, with the precaution of not increasing too much on R 3 , which would result in the transistor leaving the normal active region of the characteristics. It should be noted that at this setting the collector current remains constant. b). To change the collector current, it can be operated either by changing R 4 or the base divider R 1 , R 2 . Usually the last option is preferred: if the resistance R1 is increased, the positive potential brought by the divisor R 1 , R 2 on the basis of the transistor decreases, the emitter-base junction is weaker polarized and therefore the collector current decreases (to the limit, if R1 → ∞ the base has the potential of mass and I C= 0). Obviously, if R1 decreases, the collector current will increase. Similarly, if R 2 is reduced, the fraction of + E C that is brought to the base decreases and the collector current will decrease as well (at the limit, if R 2 = 0, the base gets the potential of mass and I C = 0). Conversely, if R 2 increases, the collector current increases. It is observed that with the change of the collector current, the voltage drop on the resistors R 3 , R 4 also changes , which in turn results in the change of the collector-emitter voltage, ie the increase of I C corresponds to the decrease of U CE and vice versa. 10. Variation of the static operating point with the temperature In any of the polarized circuits (fig.6), at a temperature increase, the collector current tends to increase, which leads to the decrease of the U EC voltage . The increase of the collector current is due to the following causes: When the temperature rises, the residual current of the base collector junction increases strongly, thus contributing to the increase of the collector current For a directly polarized junction, crossed by constant current, if the temperature rises, the voltage at its terminals decreases. Therefore, in the case of the transistor, raising the temperature will lead to the decrease of the base-emitter voltage, for constant emitter current. It turns out that if the polarization circuit keeps the UBE constant, there is an increase in the transistor current, so also the collector current. Experimentally, an increase of the amplification factor in current with the temperature is constant. Therefore, if the polarization circuit provides a constant injection of current into the base, an increase in the collector current will increase with increasing temperature. In conclusion, the collector current is a function of temperature through I CB0 , U BE and ß. 11. Methods for stabilizing static operating point variations The problem of stabilizing the static operating point in relation to temperature is one of the critical problems that occur in semiconductor devices. The variation of the temperature should not only mean the variation of the ambient temperature (although this is important), but also the heating of the transistor when it is crossed by electric current (by simply turning on the circuit). Therefore, the variation of the temperature means the variation of the temperature of the junctions, which can have multiple causes. The methods used for stabilization are of two types : Linear stabilization methods. These methods consist of introducing suitable resistors (in size and position) in the polarization circuit (Fig. 14a). Thus, it can be found experimentally (and verified by calculation) that in general, the introduction of a resistor in the transistor emitter has a favorable effect, the more pronounced the higher the resistance value. Nonlinear stabilization methods. These methods consist of introducing nonlinear elements (eg diode, thermistor, Zener diode) into the polarization circuit (fig. 14b and c). It is intended that, by temperature variation of a parameter characteristic of the nonlinear element, to compensate the tendency of variation of the collector current. For example, let's look at the circuit in figure 14b, where the diode D is made of the same material as the transistor and has residual current I0. The mechanism of compensation is as follows: at an increase in ambient temperature, the initial tendency of the collector current is to increase. At the same time, the residual current of the diode is increased, so the voltage drop on the resistor R 1 increases , which causes the potential of the base to decrease, and the collector current to have a decreasing tendency. This compensates for the initial growth trend of I C . To choose the correct diode we will have to write the equation: from which the value of I0 is derived. Another common circuit is the one drawn in Fig. 14c, in which the resistance R 2 is a thermistor. The circuit acts as follows: a rise in ambient temperature is assumed. It entails the increase of the collector current; at the same time the value of the resistance R 2 decreases, so the continuous potential brought on the base of the transistor by the divider R 1 and R 2 is reduced . As a result, there is a tendency to decrease the collector current which compensates for the initial tendency. However, in order to make them sensitive to the variation of the junction temperature, the compensation circuits discussed are provided with a thermal coupling as tight as possible between the transistor and the diode, respectively the thermistor. Thus, in the case of the power transistors placed on the radiator, the non-linear device can be mounted in the body of a screw which is then screwed into the radiator as close to the transistor. 12. Transistor types and families Various codes are used to identify different types of transistors. For example, in the European code the first letter signifies the nature of the semiconductor material: A = Germanium, B = Silicon. C = gallium-arsenic, D = indium-anti-monium. The second letter shows the scope: C = transistor of small and low frequency signals; D = low frequency power transistor; F = high frequency transistor; L - high power and high frequency transistor; S = switching transistor; U = switching power transistor. In the American code, the transistors are designated by the 2N code. The last figures indicate the respective type of transistor. At the current stage of electronics development the base material is silicon which is less affected by temperature. It allows transistors to be obtained in a wide range of powers and frequencies. It is also the main element of integrated circuits. In certain electronic circuits it is necessary to sort the bipolar transistors. In general, the sorting criteria refer to the deviation of the parameter ß (current amplification factor) from a value ß taken as a reference in the respective application. The deviation can be positive or negative and is expressed as a percentage. The more rigorous the selection criteria, the more likely there is to obtain a more expensive circuit, because it is necessary to purchase and sort more transistors. In more demanding applications, transistors are sorted not only by the factor ß, but also by other factors, such as: the noise factor. The noise factor is defined as a ratio between the signal / noise ratio at the output of the transistor and the same ratio but referred to the input. Following are the main types and families of Si transistors used in consumer electronics. a). Transistors with Si and AF switching, low power. The family of NPN transistors in TQ-18 metal capsule, includes as representative types: BC 107, 108, 109. The complementary PNP types in the same capsule are: BC 177, 178, 179. By using waterproof plastic capsules, the technology has become more productive and cheaper by 20-40%. From the class of low power transistors in plastic capsule are: BC173, BC174, BC546, BC556, BC550, BC560, BC639, BC640 etc. b). Medium-power Si transistors The most common family is BD 135, 137, 139 (NPN) together with its complementation BD 136, 138-140 (PNP). They are generally used in final floors with powers of up to 3-4 W. The maximum dissipated power is about 12W at a capsule temperature of 25 ° C and the maximum collector current of IA. In practice, however, it is not used at currents greater than 0.5 A since (3 greatly decreases above this value. For amplifications at currents of order 0.5-1A other families are used: BD 233-235-237 (NPN) and BD 234-236-238 (PNP). The maximum dissipated power is 25W, and the maximum collector current of 2A. The capsule is plastic type SOT 32 or TO-126 with the collector removed to the surface to allow direct contact with the radiator. for a good cooling . Transistors with Si NPN for the final video floors These transistors have the task in the TVs to amplify the complex video signal from a level of 3-4 Vvv to an amplitude of 90-100 Vv as it is needed for a modern kinescope. As the spectrum of a luminance video signal ranges from 0 to 5 MHz, the gain in the EC connection must be uniform in the band (20-35 times). It follows that the cutoff frequency must be high (fT> 50 MHz). To achieve the desired output level, the supply voltage of the floor is high and therefore the UCE0 is between 100 and 300 V. The usual load resistances of 3-5 kOhms will determine an average current of 10-30 mA which implies, from reliability reasons, maximum collector currents between 50 and 100 mA. Also for stable operation at high frequencies in the connection ??, d). RF and FI-MA-MF Si transistors These transistors equip the high frequency blocks of the radio receivers, the FI-MA-MF amplifiers, as well as the AFI-sound from the TVs. Also some types (BF214) can be used as oscillator in FIF channel selectors. The limit parameters are of the order: Pmax = 120-300 mW; I C = 15-30 mA; U CE0 = 20-25 V. However, to work in a frequency range from 0.15 to 20 MHz, these transistors have a high cut-off frequency (200-300 MHz) and a low collector-base capacity (0 5-LPF). e). Transistors with Si for AFI video-sound TV. The high frequency range (30-40 MHz) in which this transistor has to work involves, on the one hand, high cutoff frequencies of the order of 400-600 MHz, and on the other hand CCB reaction capacities reduced by half compared to the types. earlier. Are they usually used in connection? without neutrodyne and in adjustable amplification (RAA) or non-adjustable mode. The reduction of the collector-base reaction capacity is achieved through an integrated screen during the manufacture between the metal foil island which is the base contact and the collector area. The screen, being connected to the transmitter the effect of parasitic capacity is greatly diminished. Because the screen is a layer P made in the base material N of the collector, a PN junction appears that acts as a diode connected between the emitter and the collector. Therefore, in the current measurements with the ohmmeter, these transistors between the collector and the emitter have the character of a diode and not a blocking state in both directions. Representative families are BF 167-173 in metal capsule TO-72 and BF 198-199 in plastic capsule similar to BC transistors. Types BF 199 and BF 173 are used in fixed amplification mode, and types BF 167, BF 198 in adjustable amplification (RAA) mode. The adjustment can be done in voltage or current. f). Transistors with Si for FIF - FIU domains These transistors equip the TV channel selectors for receiving FIF (50-230 MHz) and UIF (470-860 MHz) bands. Due to the high working frequencies, the cutting frequency is high, the low reaction capacity and the low noise factor. NPN transistors were used in the first step of introducing Si transistors into the channel selectors. The representative family consists of types BF 180-181-200, which result from the same technology, the sorting being made by the noise factor F. g). Power Transistors with Si for AF These transistors are used in AF amplifiers with output powers of tens of W, regulators or voltage sources. The most common family is 2N3055, where the sorting is done taking as criteria the voltage U CE0 and ß at a specified collector current. The typical transistor of this family is characterized by: U CE0 = 60 V. I C = 15A and P M = 117 W. The figure of 117 W for the maximum dissipated power is valid for ideal cooling conditions, ie the transistor is considered mounted on a radiator. infinitely so that the temperature of the capsule does not exceed 25 ° C. h). Transistors with Si for horizontal sweep The operation of the final stage of BO from the pulse TVs requires for the used transistors, high working voltages and currents, high switching frequency and speed as well as low saturation voltages. Because the operation of a final stage of the BO implies the existence of a bipolar switch, in some types of transistors, a fast diode is installed internally in contrast to the collector current. This diode is called a parallel recovery diode. Bibliography [1] - Schett Z. et al. - "Semiconductors and applications" - Facla Publishing House, Timişoara, 1981 [2] - Găzdaru C., Constantinescu C., - "Guidance for Electronists Vol.I" - Teh. Publishing House, Bucharest, 1986 [3] - Vasilescu G., Lungu Ş. - "Electronica" - Didactic and Pedagogical Publishing House, Bucharest, 1981
  4. DonPetru


    1. Introduction What do resistors do? The resistors limit the electrical current. To illustrate a simple application: connect a resistor with a LED in series. The electric current is high enough to make the LED light up, but not high enough to cause damage. In the example above the electrical resistance has the role of limiting the value of the current through LED to a nominal value specific to its use (given by the catalog). At the same time, by limiting the current a potential difference will appear on the electrical resistance inserted with the LED and LED. Both potential additions should give the voltage supply to the circuit, in our case 9V. Electrical resistance characterizes any electrical conductor. For example, for a homogeneous conductor, the value of the resistance is: where: ρ is the resistivity of the material from which the conductor is made, measured in ohm · meter; l is the length of the conductor, measured in meters; S is the cross-section of the conductor, measured in square meters. In an electrical circuit, the value of the resistance is calculated using Ohm's law , being equal to the ratio between the voltage U applied to the source terminals and the intensity I of the current flowing through the conductor. 2. Variation of electrical resistance according to temperature The ohmic resistance of metals increases with temperature and of coal and liquids decreases as their temperature increases. The electrical resistance of copper increases by 4% at a heating of 10 ° C. How the ohmic resistance of an electric conductor varies with temperature can be determined using the following relation: where: t2 is the final temperature; t1 is the initial temperature; R2 is the electrical resistance of the material at t2 (final resistance); R1 is the electrical resistance of the material at t1 (initial resistance); α is the temperature coefficient (specific to each material and represents the variation of the resistance of one ohm of the respective conductor to an increase of its temperature by 1 ° C). 3. The symbol of the electrical resistance In the electronic diagrams the symbol of the fixed resistor in the form of rectangle (symbolization according to the European standard IEC) or the symbol "zigzag" (according to the American and Japanese standards) is used. [adv_1] Resistors are used with converters to form a subsystem of a sensor. Converters are electrical components that convert energy from one form to another, where one form of energy between the two (which is converted, or is to be converted) is electrical. A light dependent resistor, or LDR, is an example of an input converter. Changing the brightness of the light on the surface of the LDR results in changes in its resistance. As I will explain later, an input converter is often connected with a resistor to form a circuit, called a potentiometer. In this case, the voltage of the electrical current coming out of the potentiometer will have a voltage that will reflect the changes of the light that falls on the surface of the LDR from the potentiometer composition. Microphones and switches are input converters. Speakers, incandescent lamps and LEDs are output converters. In other circuits, resistors are used to direct the electrical current to certain parts of the circuit or can be used to determine the voltage gain of an amplifier. Resistors are used in conjunction with capacitors to produce delays. Most electronic circuits need resistors to work well and it is important to find among the dozens of types of resistors available on the one with the correct value, in, or M, for the particular application we want to use. 4. Fixed value of the resistors Figure below shows the construction of a carbon film resistor: During manufacture, a thin carbon film is placed on a small ceramic bar. The resistive coating is spiraled in an automatic machine until the electrical resistance between the two ends of the bar is as close as possible to the correct value. Metallic bars and ends are added and the resistor is covered with insulation and finally the lines on the insulation are painted to indicate the value of the resistor. Carbon film resistors are cheap and easy to find, with values of ± 5-10% of their nominal value. "Metal foil and metal oxide resistors are manufactured in a similar method, but have ± 1-2 % of their nominal value There are some differences in the performance of these two types of resistors, but the performance does not affect their use in simple circuits. Butter coil resistors made by wrapping a wire on a ceramic support. They can be made extremely precise and can be used in multimeters, oscilloscopes and other measuring instruments. Through some types of coil type resistors, strong electrical currents can pass, without overheating the resistor and are used in power sources and other high current circuits. 5. Color code How can you find the value of a resistor on the colored strips on it? Each color represents a number, as shown in the table below. The first band on a resistor is interpreted as the FIRST FIGURE of the value of a resistor. For the resistor shown below, the first band is yellow, so the first digit is 4. The second band gives us the SECOND FIGURE. For the resistor in the image, it is purple, so the second digit is 7. The third band is called MULTIPLICATOR and is not interpreted in the same way as the other 2. It tells us how many zeros we have to add according to the numbers we have. Being red, its value is 2. So the value of the resistor in the image is 4700 ohms, that is 4700 ohms or 4.7kohms. The remaining band is called TOLERANCE. This was the percentage of the accuracy of the resistor value. Most carbon film resistors have a yellow colored tolerance indicating that it is ± 5% of face value. When you want to read the value of a resistor, look for the tolerance band, usually yellow, and position it to the right, reading the value of a resistor is not a complicated operation, but requires little practice. To write this operation as a mathematical equation, we will note the first digit with A, the second with B, the multiplier with N, and the tolerance with X. The formula is: AB x 10 N ± X%. Color tolerance values are: More about color code The color code as shown above allows us to find the value of a resistor greater than 100 Ohms. How do we find out the value of a resistor less than 100 Ohms? For 12 Ohms, on the resistor, the first band will be brown, the second red, and the third black. So the first digit will be 1, the second 2, and the multiplier 0 shows that no zero is added to the first two digits. Now we can indicate any value over 10 Ohms. But how do we proceed to indicate values less than 10 Ohms? For values less than 10, the multiplier will be golden. For example, the colors brown, black, gold (they are in order, from the first band to the multiplier) indicate the value of 1 Ohm, and the colors red, red, gold, indicate the value of 2.2 Ohm. So, if the multiplier is golden, the number of the first and second digits is divided to 10. Metal sheet resistors, which have a tolerance of ± 1% or ± 2%, often have a code consisting of 4 bands. It works the same way, except that the first three bands are interpreted as numbers, and the fourth as a multiplier. For example, a resistor with a metal sheet of 1 kOhmi has the bands: brown, black, black, brown and brown or red for tolerance. E12 and E24 values If you have experience in circuit construction, you have noticed that the resistors usually have values of 2.2 kOhmi, 3.3 kOhmi or 4.7 kOhmi and do not have integer values, such as 2 kOhmi, 3 kOhmi, 4 kOhmi etc. Manufacturers do not produce resistors with these values - why? The answer has to do with the fact that resistors are not precisely manufactured, with some tolerance. Look at the table below, which shows the E12 and E24 series values. Resistors are made in multiples of these values. For example: 1.2 ohms, 12 ohms, 120 ohms, 1.2 kOhmi, 12 kOhmi, 120 kOhmi and so on. We consider the values 100 Ohms and 120 Ohms, close in the E12 series. 10% of 100 Ohms is 10 Ohms, while 10% of 120 Ohms is 12 Ohms. A resistor marked with 100 Ohms and with a tolerance of 10% can have any value between 90 Ohms and 110 Ohms, while a resistor marked with 120 Ohms and the same tolerance can have any value between 108 Ohms and 132 Ohms. From a practical point of view, all that matters to you is to know that carbon film resistors are available in multiples of E12 and E24. often, for a calculated resistance that you want to use in a particular application, you will need to choose the closest value to the E12 or E24 series. 6. Limitation of electric current You are now ready to calculate the value of a resistor connected in series with an LED. Look at the following figure: An ordinary LED needs a current of 10 mA and a voltage of 2 V during operation. The power supply of the circuit is 9 V. What is the voltage crossing R1? The answer is 9-2 = 7 V (the voltage passing through the elements of a connected circuit in series must be added to the voltage of the power source). Now we have two information about the electric current that crosses R1: - it has the intensity of 10 mA; - has the voltage of 7 V. To calculate the resistance, the formula is used: We substitute the values of V and I: This formula uses the fundamental units of measurement, ie volts for voltage, amps for intensity and ohms for resistance. In this case, 10 mA had to be transformed into amps, resulting in 0.01 A. If the value of the current intensity is in mA, the value of the resistance will be in kOhmi: The calculated value for R1 is 700. What is the closest value in the E12 / E24 series? Resistors with values of 680, 750, and 820 are available. 680 is the ideal choice. This should allow an electrical current with an intensity slightly greater than 10 mA. Most LEDs support electrical currents up to 20 mA, so it's perfect. 7. Connecting the series and parallel resistors In a circuit that contains resistors connected in series, the electric current is the same in all its points. The circuit in the diagram shows 2 resistors connected in series to a 6 V power source. It does not matter where in the circuit we measure the electric current, it will be the same. The total resistance is given by the formula: Total R = R 1 + R 2 In our circuit, total R = 1 + 1 = 2 kOhmi. But what intensity will the current passing through it have? The formula is: I = V / R = 6/2 = 3mA. Note that the current intensity is given in mA, if the replaced resistance is in kOhmi. If the resistor is replaced in Ohmi, then the current intensity will be given in amps. So through both resistors a current of equal intensity passes. What is the electrical voltage at the terminals of R1? The formula is: V = I * R or U = I * R. Substituting, we obtain: V = 3mA x 1kOhm = 3V What will be the electrical voltage at the terminals of R2? It will also be 3 V. It is important to note that the sum of the electrical voltages that cross the two resistors represents the voltage of the power supply of the assembly. The circuit below shows two resistors connected in parallel to a 6V battery. The circuits with resistors connected in parallel provide alternative paths for the electric current. The total resistance of such a circuit is calculated by the formula: This is called the product formula over the sum and works for any two resistors connected in parallel. An alternative formula is: This formula can be expanded to allow multi-resistor calculation, but both formulas are correct. What is the total resistance of this circuit? The intensity of the electric current can be calculated by the formula: [adv_1] How is the current in this circuit compared to the current in the circuit with the resistors connected in series? Is bigger. It is more sensitive. By connecting the resistors in parallel and making alternate paths through which the electrical current can pass, it is easier for him to pass there. What is the current intensity that passes through each resistor? Because it is divided, and the resistors have equal values, the electric current passing through R1 will have 6 mA and will be equal to that passing through R2. The electrical voltage at the resistance terminals R1 is: V = I * R = 6mA x 1kOhm = 6V. This is equal to that of the energy source. One end of R1 is connected to the anode of the current source and the other is connected to the cathode of the current source. With no other electrical component in the path, the voltage passing through R1 must be equal to that of the current source, ie 6 V. But what is the voltage of the electric current that crosses R2? For the same reason, it is also 6 V. IMPORTANT: When the components (resistors, capacitors, coils) are connected in parallel, the electrical voltage at their terminals is the same. Below is a slightly more complex circuit, with resistors also connected in series, and in parallel: To find the total resistance, the first step is to calculate the resistance of the resistors connected in parallel. We know from the calculations made in the above circuit that the total resistance of oxide resistors of 1 kOhmi is 0.5 kOhmi, so the total resistance of the circuit is 1 + 0.5 = 1.5 kOhmi. The intensity of the electrical current of the source is: I = V / R = 6kOhmi / 1,5mA = 4mA. This is the intensity of the electric current that crosses R1. What is the intensity of the electric current that crosses R2? Since there are two identical paths, on which R2 and R3 are located, through them an electric current with an equal intensity of 2 mA will cool. The electrical voltage at the resistance terminals R1 is: V = I * R = 4mA x 1kOhm = 4V. Because R 2 and R 3 are equal, then the electric currents that cross each one are equal, respectively I 2 = I 3 = 2mA. Again, the sum of the electrical voltages on R1 and R 2 , R 3 is equal to that of the power source, in our case of the battery. 8. Power dissipated by resistors When an electrical current crosses a resistor, the electrical energy is converted into heat. This can be seen in an incandescent bulb, in which the electric current flows through the filament, which emits heat and light. The power of a bulb, a resistor, or any other component, is defined as the power of converting the electric current into light, heat, or any other form of energy. The power is measured in watts (W) or milliwatt (mW) and is calculated by the formula: P = U * I = I 2 * R = U 2 / R where P is the electrical power, U - the electrical voltage at the resistance terminals, I - the electrical current that crosses the resistance. What is the electrical power of a resistor crossed by an electric current of 100 mA current and voltage at 6 V terminals? P = U * I = 6V * 100mA = 600mW = 0.6W. 0.6 W represents the heat generated by the resistor. To prevent overheating, it must have the ability to dissipate heat at the same rate at which it produces it. The ability of a resistor to dissipate heat depends on its surface. A small resistor with a small surface area cannot dissipate heat quickly enough and it is very likely that it will overheat if strong electric currents pass through it. Larger resistors, with a larger surface area, can dissipate heat more efficiently. The figure below shows resistors of different sizes. The power of a carbon foil resistor used in most circuits is 0.5 W. This means that a resistor of this size can lose heat at a maximum rate of 0.5 W. In the previous example, the calculated heat rate loss was 0.6 W, so a larger power resistor of 1 W or 2 W. will be needed. Some resistors are built to allow the passage of very strong electrical currents into the aluminum housing with "fins". "aluminum to increase the surface and dissipate heat more efficiently. Bibliography: https: //homepages.we...cs/resistor.htm
  5. If you think you know everything about LEDs then I invite you to read the following article where we have gathered the most important useful information about LEDs. Also in this article we have gathered some electronic schemes that can be used to power LEDs. 1. What are LEDs? An LED ( English : light-emitting diode, means a light-emitting diode) is a semiconductor diode that emits light at the direct polarization of the pn junction. The effect is a form of electroluminescence. An LED is a small light source, most often accompanied by an electrical circuit that allows modulation of the shape of the light radiation. Most of the time they are used as indicators in electronic devices, but more and more have started to be used in power applications as light sources. The color of the light emitted depends on the composition and state of the semiconductor material used, and may be in the infrared, visible or ultraviolet spectrum. LEDs are used to provide white and color light in compact lanterns, bulbs and luminaires as well as in a wide range of electronic devices. 2. Brief history. Electroluminescence was discovered in 1907 by HJ Round, using a silicon carbide crystal and a primitive semiconductor metal detector. The Russian Oleg Vladimirovich Losev was the first to create the first LED in the 1920s. His research has been around the world, but it has not been used for decades. In 1961, Bob Biar and Gary Pittman, discovered that by applying current to an alloy of gallium and arsenic, it emits infrared radiation. The first LED in visible spectrum (red) was made in 1962 by Nick Holonyak, when he was working at General Electric Company. A former student of his, M. George Craford, invented the first yellow LED and improved the illumination factor of the red and red LEDs - orange about ten times in 1972. By 1968 the visible LEDs and the LEDs infrared cost a lot, almost $ 200 and could not be used only for minor applications. The first large-scale corporation to manufacture LEDs was Monsato Corporation, producing LEDs for display in 1968. These were taken over by Hewlett Packard and integrated into the first alphanumeric computers. The first widely marketed LEDs were used to replace incandescent indicators, first on expensive equipment such as labs and tests, then later on TVs, radios, telephones, computers, even watches. These red LEDs could only be used for indication because the light emission was not sufficient to illuminate a surface. Over the years other LED colors have been discovered, with higher lighting capabilities. The first LED with high lighting capacity was made by researcher Shuji Nakamura in 1993 from an InGaN alloy. It was awarded in 2006 with the Milennium Technology Prize for its invention. In 2008, the most powerful LED commercialized belonged to the South Korean company Seoul Semiconductor. A single LED in the Z-Power P7 series achieves 900 Lumen performance at 10 watts, so an efficiency of 90 lm / W, equivalent to a regular 75W bulb. On May 12, 2010, Nexxus Lighting presented the most powerful LED lamp available on the market, with an efficiency of 50 Lumen / Watt. The brightness of the PAR38 Array LED lamp is comparable to that of an ordinary / standard 75 Watt bulb reaching 985 Lumen at a consumption of only 18-20 Watt, while being variable. On April 12, 2010, Toshiba presented the prototype of the most powerful LED lamp for domestic and industrial use, with an efficiency of 120 Lumen / Watt. [4] The brightness of the led lamp is comparable to that of a standard / standard 100 Watt bulb, reaching 1690 Lumen. On December 18, 2012, Cree presented the XLamp MK-R LED Lamp with an efficiency of 200 Lumen / Watt and a dimension of 7 x 7 mm. [5] The brightness of the led lamp is comparable to that of a 120 Watt incandescent bulb, reaching 1769 Lumen at 15 W and 85 ° C. 3.Simbol. Classification. Construction. 3.1 Symbol 3.2 Classification. Construction. The LEDs fall into two broad categories: a) Low power LEDs (<1W); b) High power LEDs. Depending on the type of construction the LEDs are divided into: a) THT LEDs (case a below); b) SMD LEDs (case b below); c) Power LEDs (one or more high power SMD LEDs). Depending on the number of colors rendered we distinguish: single, two-color and three-color LEDs (the latter is also called RGB LEDs). 4. Efficiency and operating parameters of LEDs Typical indicator LEDs are designed to operate at most with 30 ... 60 mW of electricity. Around 1999, Philips Lumileds introduced power LEDs capable of running continuously with one watt. These LEDs are based on much larger semiconductor to be able to absorb high power. Also, the semiconductor molds were mounted on metal supports to allow heat transfer from the LED mold. One of the key benefits of LED lighting sources is the high light efficiency. The white LEDs match quickly and have successfully replaced standard incandescent lighting systems. In 2002, Lumileds made five-watt LEDs with a luminous efficiency of 18-22 lumens per watt (lm / W). In comparison, a conventional incandescent bulb of 60 or 100 W has an efficiency of about 15 lm / W, and standard fluorescent lighting up to 100 lm / W. Since 2012, the Lumiled catalog presented a table with the most efficient light sources according to color. In September 2003 the company Cree introduced a new type of blue LED that consumes 24 mW with a consumption of only 20 milliamperes. The commercial variant that produced white light with an efficiency of 65 lm / W at 20 mA, became at that time the brightest white LED on the market being four times more efficient than the standard incandescent bulbs. In 2006, they presented a white LED prototype with a light efficiency of 131 lm / W at 20 mA. Nichia Corporation has developed an LED with a luminous efficiency of 150 lm / W at a current of 20 mA. By comparison, XLAMP Cree LEDs, which have been commercially available since 2011, have an efficiency of 100 lm / W consuming 10W and can go up to 160 lm / W with only 2W consumed. In 2012, Cree launched a white LED capable of 254 lm / W. General lighting requires high power LEDs, one watt or more. Typical operating currents for such devices start at 350 mA. Note that these levels of efficiency are achieved only by the monobloc LEDs and could be obtained at a low temperature in a laboratory. The lighting operates at higher temperatures and with losses in the supply circuit, so the resulting efficiency is much lower. The US Department of Energy, after testing the commercial LED lamps intended to replace incandescent or CFL lamps, showed that the average efficiency in 2009 was approximately 46 lm / W (the performance of the tested LEDs varying between 17 lm / W and 79 lm / W). On February 3, 2010 the company Cree issued a press release regarding a prototype LED laboratory that has an efficiency of 208 lm / W, at room temperature. The color temperature was reported at 4579 K. In December 2012, Cree issued another press release announcing the commercial availability of LEDs with an efficiency of 200 lm / W at room temperature. 5. Lifespan and failure rate Solid-state devices, such as LEDs, are subject to quite low wear if they operate at low currents and low temperatures. Many LEDs designed in the 1970s and 1980s are still in service and in the early 21st century. The typical lifetime of an LED is between 25000 and 100,000 hours, but the heat transfer with the environment and the way we choose the operating current (see section 8 the notion of ILED_OPTIM) can significantly prolong or shorten this time. 6. Colors and materials Conventional LEDs are made from a variety of inorganic semiconductor materials. The table below shows the colors available with a range of wavelengths, voltage drops and materials: Blue and ultraviolet LEDs Blue LEDs are based on a semiconductor band gap made of gallium nitride (GaN) and indium gallium nitride (InGaN). They can be combined with red and green LEDs to produce the impression of white light. The modules that combine the three colors are used in large video screens and in adjustable color programs (using RGB LEDs). The first blue LEDs were produced in 1971 using gallium nitride by Jacques Pankove of RCA Laboratories. These devices had too little light to be useful in practice and research into gallium nitride (GaN) devices slowed down. In August 1989, the Cree Company introduced an indirect semiconductor tape gap made of silicon carbide, resulting in the first commercially available blue LED. The silicon carbide (SiC) LEDs had very low efficiency, no more than approx. 0.03%, but they emitted in the blue portion of the visible light spectrum. At the end of the 1980s, significant progress was made in epitaxial growth and doping with gallium nitride-type carriers, which led to the launch in the modern era of gallium nitride (GaN) optoelectronic devices. Starting from this base, high-brightness blue LEDs were demonstrated in 1993. High brightness blue LEDs were invented by Shuji Nakamura of Nichia Corporation, using gallium nitride, which revolutionized LED lighting, making high power LED light sources feasible. At the end of the 1990s blue LEDs became widely available. They have an active region consisting of one or more quantum gaps in indium gallium nitride (InGaN) which is between several thick layers superimposed on gallium nitride (GaN), called plating layers. By the relative variation of the In / Ga ratio in quantum gaps of InGaN, the light emission can theoretically be varied from purple to amber. Various Al / Ga ratios of aluminum gallium nitride (AlGaN) can be used for the manufacture of tiles and quantum layers for ultraviolet LEDs, but these devices have not yet reached the technological efficiency and maturity level of InGaN blue / green devices / GaN. If pure GaN is used in this case to form the active quantum layers, the device will emit near-ultraviolet light with a maximum wavelength centered around 365 nm. Green LEDs manufactured using the InGaN / GaN system are much more efficient and brighter than green LEDs made from materials that do not contain nitride, but practical devices still have too low yields for high brightness applications. White light There are two main ways to produce white light emitting diodes (WLEDs) or LEDs that generate high intensity white light. One is to use individual LEDs that emit the three primary colors: red, green and blue, and then blend all the colors to form white light. The other way is to use a phosphor material to convert monochromatic light from blue or UV to wide-spectrum white LEDs in the same way that fluorescent tubes work. There are three main methods of blending colors to produce white light using LEDs: - Blue LED + Green LED + Red LED (mix color, can be used as backlight for screens); - in the vicinity of UV or UV LED + RGB phosphor (LEDs that produce white light with a shorter wavelength than when using blue to excite an RGB phosphor); - Blue LEDs + yellow phosphorus (two complementary colors combine to form white light - more efficient than the first and most frequently used methods). Due to metamerism (or changing the color of an object viewed in different lights or with different power spectral distributions), it is possible to produce quite different spectra that appear white. 7. LED performance The table below highlights the performance parameters for three types of lamps analyzed and a forecast of the performance of LED lamps in 2017. Also in the table was calculated the "total lamp life". This parameter represents the measured luminous flux accumulated over the entire life of the lamp and is measured in megalumen-hours. The light efficiency for an LED lamp in 2012 is 65 lm / W. The last row in the table is calculated by making the ratio between the luminous efficiency of the other lamps and that of the LED lamps from 2012. The scalar impact of future LEDs is expected to be smaller as future LED performance will improve and designers will continue to seek and increase the quality of materials and components used in their construction. 8. How do we control the LEDs? Light emitting diodes (LEDs) are constructed from PN semiconductor junctions. When the LED is directly polarized the electrons are able to recombine with the holes inside the device and release energy in the form of photons. The current passing through the PN junction of the LED will have to be limited and influence the brightness of the LED. The setting and limitation of the current can be done in several ways: using an external resistor to limit the direct current through the LED; using a constant current source to establish a definite and stable current through LED (Constant Current - CV); a DC-DC converter in constant voltage mode (Constant Voltage - CV). The above methods will be detailed in point 8 of this article. 8.1 Powering the LEDs using a resistor One of the simplest ways we can control the lighting of an LED is to attach a resistor to it to limit the LED current to an appropriate value (a value that indirectly respects and results from the catalog data. LED, which ensures the proper functioning of the LED). If we attach to this circuit a simple switch to stop and start the LED operation, it is the simplest circuit to power an LED (fig.3a). Careful! a) without a resistor or a similar ballast circuit that limits the LED current to a certain value, the LED will burn; b) the battery plus will have to coincide with the electrical path that makes contact with the LED anode, respectively minus the battery with the LED cathode; c) the identification of the anode and cathode terminals for an LED is done as in figure 4. Figure 3b shows the case where we have several identical LEDs that we want to power from a battery using a single resistor. The mathematical formulas for calculating the value of the resistor are the following: - for figure 3a: R 1 = (U battery - U LED1 ) / I LED1 or taking into account that we have a safe LED the simplified formula can be applied: R 1 = U battery / I LED1 ; - for figure 3b: R 2 = (U battery - U LED2 - U LED3 - U LED4 ) / I LED , where I LED will be selected so that it is supported by all LEDs and will have to ensure the proper functioning of all LEDs. This scheme applies when all LEDs are of the same type, so they have the same characteristics. For example, in the case of LEDs with a diameter of 3mm but which have different colors - as in the figure above - a current of 5mA can ensure the correct functioning of all LEDs. Attention, it is not recommended to power the LEDs by applying the solution in fig.3b when we have LEDs that do not have the same LED_OPTIM . In the case of ordinary low power LEDs, such as those in Figure 2, the optimal LED current recommended for use in calculations is shown in Table 5. Note: ILED_MAXIM may differ from one LED model to another, respectively from one color to another. TheLED_MAXIMvalues higher than those presentedcan be found in the catalog. Note, in the calculations, the values in column ILED_OPTIMwill always be used, which will ensure an appropriate voltage drop on the LED for its or their proper functioning. Figure 5 shows a method of identifying the operating voltages of the LEDs (noted in the technical documentation in English with VF). 8.2 Powering the LEDs via a constant current source (DC - Constant Current) or a constant voltage source (CV - Constant Voltage) We have previously seen the simplest method by which we can power an LED using a common resistor. The method is simple but has a major disadvantage: due to temperature variations and taking into account any variations in supply voltages (that is, if we do not use a battery but a power supply with transformer and rectifier bridge with capacitive filter), the LED current can record considerable variations leading to the decrease of the LED life. To combat this phenomenon, several types of circuits have been designed to provide a constant current or a constant voltage at the LED terminals. Figure 6 shows graphically the functions corresponding to the three modes of LED power supply: using a constant voltage circuit (fig. 6a), using a constant current generator (fig. 6b) and the CC-CV version that combines the properties of the circuits of constant current with those of constant voltage. After analyzing the three graphs in fig.6 a question will appear quite common among the developers of LED lighting applications, namely: which is the best method of controlling the LEDs: CC or CV or CC -CV? Throughout this article I will try to bring enough arguments to complement your current experience to make the appropriate decision in various situations. I said earlier that LEDs are semiconductor devices that need a certain current in order to operate, then you will surely wonder why companies supply both LEDs with constant current (DC) power supply and power solutions. constant voltage supply (CV)? The main reason is that companies want to give designers enough options to enable them to optimize the lighting system. If several LEDs are connected in series, the most efficient way to power them is to connect them to a constant power supply. If the LEDs are connected in parallel, there may be a problem with the current distribution through each LED. A possible alternative to this power mode is the placement of an external component or an active electronic component that controls the LED current. Although this strategy provides the same current through each LED, the method results in a less efficient lighting solution, What is the difference between a constant current source (DC) and a constant voltage source (CV)? Figure 6 shows the three characteristics of the three distinct modes of operation of the LED power supply. The X axis shows the load increase, and the Y axis shows the output voltage of the LED power module. The blue line represents the voltage and the green line is the output current. To begin with, we will analyze the performance of the constant voltage power supply (fig. 6a). As the name suggests, the circuit returns to the output a constant voltage as the load current increases (symbolized in the figure by the English term "load"). The load current will be able to increase until a moment when the circuit enters the current limiting mode, in order to prevent circuit damage. Figure 6b shows the characteristic of a constant current source. In this case, if the load changes (increases or decreases), the current will remain constant. Figure 6c shows the characteristics of a circuit that combines two modes of operation. Initially, the circuit acts as a constant voltage source. Once the maximum permissible load current is reached, the circuit control loop will adjust the load current to a constant value while simultaneously reducing the output voltage. This type of approach has many benefits and allows the designer to achieve greater efficiency using modern solutions based on CV-CC sources. In the last period, a lot of types of integrated circuits have been developed that use the characteristic of fig. 6c, some of them being presented in this article. 8.3 Series and / or parallel LEDs? --- Serial connection --- Powering multiple LEDs in series avoids uneven brightness due to current variation. So, all LEDs will see the same current for the same brightness level. The output voltage of the driver will be equal to: V OUT = V F X n where V F is the nominal operating voltage of the LED and n is the number of LEDs connected in series. For example, if V F = 2V and we have 5 LEDs connected in series then the output voltage of the LED driver will be 10V. Most LED drivers are DC / DC type low voltage converters. Care must be taken to keep the input voltage within nominal limits so as not to exceed the output level above the appropriate limit. When LEDs are connected in series, the output current of the driver will be equal to: I OUT = I F ; where I F is the nominal current of the LED, a very important catalog date. So all LEDs connected in series will see the same current, in our example I will consider: I F = 30mA. Advantages of LED series connection: low circuit complexity; each LED sees the same current; high efficiency (no ballast resistor required). Disadvantages of LED series connection: the output voltage of the driver can become quite high for LEDs connected in series; over the lifetime, LEDs can unevenly change their operating parameters, leading to overcharging of some and undercharging of others, which will cause much faster LED string failure or brightness. uneven; if a LED fails, the brightness of the entire serial connection is interrupted. A short-circuited LED has a reduced effect on the overall brightness of the circuit but can cause overvoltage of the other LEDs in series, if the LED driver is not provided with a current reaction by which to automatically adjust the output voltage to the corresponding value - I mention this because most LED drivers made with linear voltage stabilizer integrated circuits, output a fixed voltage, which does not automatically adjust according to the nominal current consumed by LEDs. In this situation, it may happen that at one point the fixed output voltage is too high for the n-1 LEDs remaining in operation. --- Parallel connection --- Suppose we have three rows of LEDs connected in parallel. Through each row or string of LEDs a current circulates that I will note with: IF1, IF2and IF3. The LED driver will need to provide a constant output voltage equal to nsx VF, where nsis the number of LEDs in a string. Then the output current of the LED driver will have to be the sum of the currents that flow through the three LED strings connected in parallel, namely:I OUT = I F1 + I F2 + I F3 If: IF1= I F2 = I F3 = I F , then: I OUT = 3 * I F . The output voltage of the driver will remain the same as initially calculated, ie those 10V, if we have five LEDs connected in series on each row with VF = 2V. If we consider IF = 30mA then the output current of the driver will be 90mA. So, by connecting the LEDs in parallel, the output voltage may be the same as in the case of serial connections if we have the same types of LEDs, but the current required for their power supply will increase depending on how many LEDs we connect in parallel. The major advantage of using parallel LED connections is that we can use a larger number of LEDs which, if connected in series, would require a higher supply voltage than the nominal output of the driver. of LEDs. For example, if the upper voltage limit of an LED driver is 28V and V F = 2V, we can connect at most 14 LEDs on a single string. If the driver has a current capacity greater than IF, suppose 0.3, and I F = 0.03A, that means we can connect at most 0.3 / 0.03 = 10 LED strings in parallel. Thus the total number of LEDs connected to the output of a driver, both serial and parallel, will reach 14 x 10 = 140 LEDs. The biggest problem with parallel connections is that the small differences in tolerances or manufacturing dispersion of the components of a circuit can lead to significant differences in the current absorbed by each string of LEDs. This will have repercussions on the perception of the intensity of the color or the brightness of an LED, reaching even in extreme cases when the failure of one or more LEDs will cause the entire circuit to be switched off. In order to eliminate or better said to minimize the consequence mentioned above, a series of balancing (balancing) resistors must be connected in series with each LED string: R B1 , R B2 & R B3, which will help to compensate for the current variations caused by the typical V F LED differences. Small imbalances of V F within a series of LEDs connected in series could cause a significant variation of the current I F through the string. The typical value of the ballast resistance is less than 20 Ohms. In other situations, a better current generator with transistor or a current mirror with transistors is used to better balance the currents connected in parallel instead of a common resistor. The ballast or balancing resistors in the structure of the constant current generators or the current mirrors will be used continuously to compensate for the small Vbe variations. To keep the current constant regardless of temperature variations, the transistors used in such a manner will have to be "compensated / thermally connected". Mounting them on the same radiator is a common method to do this. Advantages of parallel LED connections: the possibility of supplying a large number of LEDs. Disadvantages of parallel LED connections: Low efficiency; Increasing circuit complexities; Low reliability. The low reliability is caused by a considerable risk of current variations occurring. A shorted LED will increase the current I F through the remaining series LEDs. An increased current will cause the other LEDs in the string to fail. If the junction of an LED breaks, this will cause all LEDs in a row to stop working. So, both possible defects are the main cause of low reliability for such an LED connection. 8.4 Matrix connection To help improve the reliability of parallel connections, the matrix connection can be used where the LEDs are connected on horizontal and vertical supply sides. This mode of connection, called in specialty literature and cross connection, is nothing more than the connection of LEDs in series and in parallel. In this connection, the required output voltage and current is the same as when connecting LEDs in parallel, where the number of LEDs that can be powered without exceeding the maximum voltage allowed by the driver is much higher than in the case of series of LEDs. However, the matrix connection is somewhat more error tolerant, no balancing resistors are used for parallel operation and the efficiency of the connections is much improved. However, current distribution on the sides of the matrix remains a problem. An unequal distribution of current (caused by component tolerances) can lead to visible differences in brightness. For example, differences in thermal characteristics may cause a variation of current that will inevitably lead to the problem previously reported over time. A shorted LED will cause a vertical power supply to be turned off and the remaining LEDs will continue to operate normally. If an LED is not open, only the remaining LEDs on that row will be able to operate. Therefore, the matrix connection allows individual control of a large number of LEDs using a driver with a lower output voltage than when using serial or parallel connections. 8.5 Multichannel connection Usually, it is recommended to use the LED serial connection whenever possible because this connection mode avoids the thermal distribution problems encountered in the case of matrix and parallel connections. The most robust connection is to use a separate driver for each LED string or a single multi-channel driver. This connection mode combines the reliability advantages of serial connections with a high current capacity characteristic of matrix and parallel connections. The obvious disadvantage of such an approach is the increased costs and complexity. 9. Protective circuits LEDs are extremely reliable devices, with average lifespans approaching 50,000 hours. By far the most common failure is the gradual degradation of the light intensity to 50% of the nominal value. However, failures also occur due to mechanical stress or temperatures, misuse, packaging defects etc. The most common and "catastrophic" failure for LEDs is to stop them. When this happens, as we have seen, in the case of the serial connection, all the LEDs in the string are interrupted. A frequent cause that leads to this type of defect ("interrupted led") is the application of excessive excessive voltage. The use of a constant current source will protect the LEDs from the defect described above. However, the components may be subjected to under / over voltage which may be induced by external circuits or events. Thus, in order to prevent and eliminate the above phenomenon, a protective device (PDX) is connected in parallel with each LED. These devices are nothing more than switches, which if one of the LEDs fails to open, the PDX shuts off to ensure continuity of operation for the other LEDs in the string. Once the LED is replaced, the PDX will reset automatically. Usually, to keep costs to a minimum, a PDX connects in parallel on two serial LEDs. 10. LED mountings Regardless of type, color, size or power, all LEDs work best when powered by a constant power source. The manufacturers mention the characteristics of the LEDs, such as: light efficiency, color, etc., at a certain current (denoted by IF) and at a certain operating voltage (denoted by VF). Most integrated circuits designed to pilot LEDs are designed to deliver good output voltage stability across a range of currents. Therefore, it can be quite difficult to determine which LED method is best for a particular application. From the beginning we try to avoid that solution by which each LED is supplied by a constant current source, because the method, In applications that require a large number of LEDs, it becomes economically inefficient. As a result, during the development of LED applications, it was tried to adopt that power solution that would lead to: a minimum number of constant current sources and a maximum number of LEDs, in conjunction with an arrangement through which to the best luminous efficiency is obtained. Thus have appeared various solutions of serial / parallel connection of LEDs, with the disadvantages and the advantages of rigor, solutions that I will list below. in conjunction with an arrangement to obtain the best luminous efficiency. Thus have appeared various solutions of serial / parallel connection of LEDs, with the disadvantages and advantages of rigor, solutions that I will list below. in conjunction with an arrangement to obtain the best luminous efficiency. Thus have appeared various solutions of serial / parallel connection of LEDs, with the disadvantages and advantages of rigor, solutions that I will list below. 10.1 Power supply of LEDs through constant current generator with transistors One of the oldest methods of supplying one or more LEDs is shown in Fig. 8 below. 10.2 Power supply of LEDs through constant current generator with LM317 LM317HV adjusts ~ 1.23V between ADJ and V OUT terminals . The current through LEDs is given by the relation: I LED = 1.23 / R. The advantage of this circuit is that by maintaining a constant current with the help of voltage control on the resistor R, the voltage at the LED terminals is also maintained constant. In the case of the circuit in Fig. 15, the current through each of the three LED branches connected in series is given by the relation: ILED = 0.6 / Rsense. 10.3 LED driver with LM2941 Another driver similar to LM317 is LM2941. LM2941 is a voltage regulator that allows up to 26V input voltage. LM2941 regulates 1.275 V between ADJ and GND terminals. Figure 16 is an LED driver with LM2941 capable of delivering 354 mA. 10.4 Led driver with LT3021 The LT3021 is another linear voltage regulator with a maximum admitted voltage of 10V input at a current of 0.5A (fig.17). The LT3021 maintains constant 0.2V between the ADJ and GND terminals. The LED current is given by the 0.2 / R ratio. If the nominal voltage of the LED is 3.6 V, the number of LEDs connected in series is equal to two. 10.5 LED driver with TLE4242G An integrated circuit that can be used to power LEDs is TLE4242 (fig. 18), where VREF is 177 mV between ADJ and GND pins. The maximum input voltage at the input is 42 V. Using a 5.1 Ohm resistor the LED current will be 34.7mA. Switched LED drivers A switched LED driver is linked to how the switching sources work and are built. The switching voltage regulator maintains a constant voltage at different current loads. Therefore, switching sources intended for powering LEDs can maintain a constant current through LEDs at any voltage drop on the LED, provided the overvoltage protection and thermal packing of the source are operable or, in other words, exist. in the construction of the source. Some examples of switching power source topologies are: BUCK - where the output voltage is generally lower than the input voltage; BOOST - where the output voltage is generally higher than the input voltage; BUCK - BOOST, is an up / down voltage structure, with inverted output; SEPIC (single-ended primary inductor converter). This converter borrows the functions of the buck and boost converters, increasing or decreasing the output voltage, even though at one point the input voltage may be lower than the output voltage. The control strategy allows to obtain a much lower noise source, in conjunction with a minimum number of external components. FLYBACK. A topology based on the functions of buck and boost converters, but instead of inductors contains a transformer that galvanically isolates the output input. Before choosing a particular topology to build or power an LED driver, I recommend that you study the technical specifications of each integrated circuit. In applications that use switching sources as a voltage source, in order to power one or more LEDs in series, it is recommended that the LEDs be connected to the output of the source before mounting power because the source will have to operate in empty an output voltage greater than the nominal operating voltage of the LEDs. 10.6 LED driver with L6902 (fig.14) L4902 is a "buck" switching LED driver. Table 6 below shows the values of the various components required to obtain a certain LED current. Resistors R1 and R3 have the role of providing overvoltage protection (typically 23.3V) in case it is interrupted, for example, due to an LED failure, supplying all LEDs. If the operating voltage of an LED is 3.6V then 6 LEDs can be connected in series. 10.7 LED driver with L4973 Another buck topology driver topology is shown in Fig. 15, where the input voltage is 48Vdc. In this case, up to 12 LEDs with Vf = 3.6V can be connected in series. Resistors R1, R2 and the internal voltage reference of 5.1 V, reduce the voltage response Vfb to 0.5 V. 10.8 LED driver with LTC3783 (fig.16) This LED flyback driver can provide 150 mA to several LEDs -s connected in series. Surge protection triggers at 130 V and deactivates at 120 V. The number of series LEDs that can be connected is: 120 / 3.6 = 33 LEDs. The PWM terminal can be used to control the brightness of LEDs connected in series. 10.9 LED driver with MAX5035 (fig.17) This buck topology supports input voltages between 7.5 and 30V. LED current is typically 350mA and the output voltage for LED power supply is 12V. 3 3.6V LEDs can be connected in series. V CONTROL is a linear voltage for controlling the brightness of the LED, by which the LED current is modified according to the following equation: I LED = [V REF x (R 1 + R 5 ) - V CONTROL x R 1 ] / (R 5 x R P ) Comments: R P is the parallel equivalent resistance of resistors R2, R3 and R4. V REF is typical 1.22 V. V CONTROL is a voltage for controlling the brightness, by default of the LED current, which starts from 0 V for the maximum LED current and can reach the maximum output voltage. BIBLIOGRAPHY: - http://www.energystar.gov - http://en.wikipedia.org/wiki/LED - http://www.cree.com/ - http: //www.talkingel...plifier- P2.html - Datasheets: LM317HV, LM2941, LT3021, TLE4242G, L6902D, L4973, LTC4930, LTC3783, MAX5035. - Maxim AN3668 High-Efficiency Current Drive for High-Brightness LEDs. - Maxim AN3639 Design of a Nonisolated, Flyback LED Driver Circuit.
  6. DonPetru

    Operational amplifier

    1. The generalities Necessity of the miniaturization of electronic circuits has led to the inclusion in a single capsule of several discrete components, such as: transistors, diodes, resistors, etc. In this way, the integrated circuits have been achieved, where most of the components of a circuit are included in a single capsule. If in this capsule we transpose the scheme with discrete components of an operational amplifier then we can say that the integrated circuit is an operational amplifier (fig. 1). Operational amplifiers (AOs) are high-amplitude direct current amplifiers initially designed to perform certain mathematical operations, having differential inputs and, usually, a single output. Although an operational amplifier is an ideal amplifier, with infinite amplification, infinite band and perfectly flat frequency response, infinite input impedance and output impedance 0, without temperature drift, in practice, the operational amplifier has the following characteristics: - impedance large entrance; - low output impedance; - very high profit (over 50 000); - very large band and very flat frequency response; - drift with very low temperature. Constructively, an operational amplifier consists of: two inputs (inverters and non-inverters), one output and the power terminals, as shown in fig.2. There are many applications of the operational amplifier, including the inverter, the non-inverting amplifier, the voltage repeater, the summing amplifier, the integrating amplifier, the differential amplifier and the compiler. To determine the specific application, different external components are connected to the operational amplifier. 2. Brief history 1941 - The first electronic tube operational amplifier. The first operational amplifier was found in the US patent no. 2401779 "Additive Amplifier", recorded in 1941 by Karl D. Swartzel Jr. from Bell laboratories. This design used three electronic vacuum tubes to achieve a gain of 90dB and operated at a symmetrical voltage of ± 350V. The circuit had a single non-inverting input quite similar to the differential inverting and non-inverting inputs of the present operational amplifiers. During the Second World War Swartzel's invention proved very valuable, being used in the control of the M9 artillery, in a system designed by Bell laboratories. This artillery control system worked with the SCR584 radar system to improve the target range, reaching almost 90%, 1947 - The first operational amplifier with inverting and non-inverting inputs In 1947, the operational amplifier was for the first time formally defined and named in a paper by Professor John R. Ragazzini of Columbia University. In this paper a footnote mentions that the operational amplifier was designed by a student whose work proved to be important. This operational amplifier, designed by Loebe Julie, has been superior in several respects. He had two major innovations. In the input circuit he used a triode to reduce the drift of the output circuit and, more importantly, it was the first operational amplifier that had two inputs (inverters and non-inverters). Differential inputs have made a whole host of new features possible. 1948 - The first stabilizing chopper with operational amplifier In 1949, Edwin A. Goldberg designed the stabilizer chopper with AO. It is composed of a common operational amplifier and an ac amplifier that operates in parallel with the AO. The chopper picks up the AC signal by switching the DC voltage between rated and ground, at a fast rate (60Hz or 400Hz). This signal is then amplified, rectified, filtered and feeds the non-inverting input of the operational amplifiers. This has greatly improved the gain of the operational amplifiers by significantly reducing the thermal drift and the dc offset. Unfortunately, any AO that was used with the chopper could not use the non-inverting input for any other purpose. However, the much improved features of the AO stabilizer chopper increased the use rate of operational amplifiers. Techniques that will usually use non-inverting input will not be very popular until the 1960s when operational amplifier integrated circuits appear. In 1953, operational amplifier electronic tubes became commercially available with the launch of George A. K2-W, Philbrick Researches, Incorporated. 1961 - The first operational amplifier integrated circuit Once the transistor was born in 1947 and the silicon transistor in 1954, the concept of integrated circuit became a reality. The introduction of the planar process in 1959 made transistors and integrated circuits stable and cheap enough to be marketed. By 1961, the first integrated circuits of operational amplifiers were produced. These AOs were actually small circuit boards with edge connectors. Usually, they allowed manual selection of resistors, in order to improve certain things, such as offset voltage and temperature drift. In 1961, the P45 could be powered at ± 15 V and had a gain of 94dB and could accept input signals within ± 10V. 1962 - The first modularized AOs By 1962, several companies produced modular plates that could be introduced into printed circuits. These packages were extremely important leading to the inclusion in a single capsule of the operational amplifier. Once this is done, the operational amplifiers could very easily be included in different schemes, resulting in smaller circuits. 1963 - The first integrated circuit operational amplifier in monolithic technology In 1963 it was designed by Bob Widlar, from Fairchild Semiconductor, the first operational amplifier in monolithic technology. Monolithic integrated circuits consist of only one chip. Unlike the chip there are also discrete circuits, only with pieces (discrete IC) or several free chips connected on a circuit board (hybrid IC). Almost all modern operational amplifiers are monolithic integrated circuits; however, this first integrated circuit was not very successful. Issues such as uneven supply voltage, low gain, and a small dynamic range, could not secure a dominant position among operational amplifiers until 1965, when μA709, designed by Bob Widlar, was launched. 1968 - Launching μA741 - the most used operational amplifier The popularity of monolithic operations increased even more with the launch of the LM101 operational amplifier integrated circuit in 1967, which solved a variety of issues, but also with the subsequent launch of μA741 in 1968. The uA741 integrated circuit was extremely similar to LM101 except for the fact that it included a 30 pF capacitor for compensation inside the capsule, while LM101 required external compensation. This minor difference made the uA741 one of the most used operational amplifiers, the location of the pins of which later became a reference. This operational one is still in production and has become ubiquitous in electronics, many manufacturers of electronic components making this classic chip, recognized under the simple name of 741. 1966 - The first operational amplifier "varactor bridge" From 741, there were several different directions taken in the design of operational amplifiers. Operators "varactor bridge" began to be produced in the 60's, are characterized by low input currents and are the best operational amplifiers available, having a high power supply noise rejection capacity and can handle hundreds of volts, at their entrances. In the 1970s, high speed, low input currents, could be achieved using FET transistors. These will be largely replaced by MOS transistors in the 1980s. During the 1970s, several operational amplifiers with a single power supply were available. With a single power source the input and output voltages can be as low as the negative supply voltage instead of at least two volts above it. The result is that it can work in many applications with the negative power supply pin connected to the ground of the signal source, thus eliminating the need for a separate negative power supply. 1972 - The first integrated circuit with four AOs included in the capsule LM324 was the first integrated quad amplifier operational circuit, later becoming an industrial standard. In addition, the encapsulation of multiple operational amplifiers in the same capsule, led in the 1970s to the birth of operational amplifiers in hybrid capsules. These AOs have generally improved existing versions of monolithic operations. As the supply voltages of the analog circuits decreased (like the digital ones), it was necessary to make the low voltage operational amplifiers, precisely to follow this trend. Thus, symmetrical supply voltages of +/- 5V or only 5V or even lower have been reached. 3. Characteristic Sizes a. Differential amplification factor (gain) in the open loop A0 represents the ratio between the output voltage variation (V0) and the differential input voltage (see Figure 4): b. The common amplification factor in the open loop AMC represents the ratio between the variation of the output voltage and the arithmetic mean of the input voltages: This parameter results from the fact that, even if the two input voltages, are equal to zero, the voltage is output at the output of the operational amplifier. In the ideal case, of the perfect operational amplifier A MC = 0. c. The offset voltage at the input U EI is the value of the DC voltage applied to one of the inputs of the circuit for which the output is zero: V = 0. d. Input polarization current - iB, where iB is the average value of the input currents: e. Common rejection factor CMR It is the ratio between the differential amplification factor A0 and the common amplification factor: A MC . According to the above, since in a perfect amplifier A MC = 0, it results in this case: C MR = ∞ f. The open loop treadmill It is the frequency range (range) in which the amplification decreases to the value of A U / √2 (-3 dB) from the maximum value: a U . 4. Operation of the operational amplifier 4.1 Inverter operational amplifier (fig.5) In the case of this type of amplifier, the signal is amplified on the terminal marked with (-), and the terminal (+) is connected to the table (fig. 5). Applying the first theorem of Kirchhoff around the node at the input the relation is obtained: where: - the current given by the voltage V 1 ; - the reaction current, which appears through the loop formed by the resistance; - the current through the input of the operational amplifier. Because: But: Because: And so the gain: The (-) sign is observed indicating that the output voltage is in the phase position as compared to the input voltage. Some properties of the inverter operational amplifier can be deduced from this relation. - Multiplication with a constant, putting the condition: so the output voltage reproduces the input voltage, multiplied by k times. - Sharing with a constant . If: so the output voltage is a fraction of the input voltage. - Repeater circuit : - Addition circuit : If more voltages are applied to the inverter input, by means of resistors, at the output, a signal is obtained in the phase, proportional to the mode with their sum. Applying the first Kirchhoff theorem (Fig. 6b) the following relations are obtained: Example 1: Determine the gain and the output voltage for an AO inverter with the input voltage V1 = 50mV, R1 = 1 kOhm , R2 = 2,2 KOhmi. Solution: The gain is: A (-) = - R2 / R1 = -2.2 / 1 = -2.2; and the output signal represents the product between the input signal and the gain = -2,2 x 50mV = -110mV (see relation (9) and fig.5). Example 2: In FIG. 6b if we have only R 1 and R 2 and R = R 1 = R 2 = 5KOhm, then the gain for both inputs will be: 5kOhm / 5kOhm = -1. Given V 1 = + 1V and V 2 = + 2V we obtain at the output a level due to V 1 of 1 x (-1) = - 1V and an output due to V 2 of 2 x (-1) = - 2V. Therefore the total output is V = -1-2 = -3V. 4.2 Non-inverting operational amplifier In this case the signal is applied to the terminal (+). To deduce the value of the amplification, it is observed that the voltage between terminal A and ground is obtained from the circuit fed by the output voltage as follows: Since A = ∞, then V A = V = 0, so V A = V B = V 1 (V 1 represents the input voltage). In this case: The gain will be: It is observed that the output signal is in phase with the input signal. The properties of this amplifier can be deduced as in the case of the inverter, from the amplification formula. Note that he cannot divide because A +> 1, unless one of the resistors is replaced by a device with negative resistance (tunnel diodes). With ordinary physical elements, he can achieve: - Multiplication with a constant . The following condition is met : Then: - Adder. Considering the above circuit, the following relations can be established: Around the node applying the first Kirchhoff's theorem, we obtain: in which: Substituting we obtain: Assuming for simplification: R1 = ... = Rn = R, we obtain: but: so: If: It is observed that at the output the sum of the applied voltages from the input was obtained, in the same phase. In order to operate in the alternative current, the operational amplifier must be equipped with capacitors on the signal circuits or on the reaction circuits, according to the intended purpose. Obtaining a linear amplification requires the judicious choice of the values of the capacitors used. 4.3 The integrating operational amplifier In order to obtain a type I AO, the resistance in the reaction circuit will be replaced by a capacity resulting from the diagram in fig.9. It is considered that the voltage u C2 , at the capacitance terminals has the value: Respectively, considering u1i≈0 is obtained: Between the voltage u C2 at the capacitance terminals C 2 and the current I 2 passing through the respective capacitance there are the relations: Substituting in the expression (31 ) the value of u C2 from the relation (30) results: For the current I 1 the relation I 1 is preserved * К 1 = u 1 -u1i and taking into account u 1i ≈0 is obtained: By replacing the expressions (32), (33) in the relation I 1 and 2 we obtain: From the relation (34) it is found that the operational amplifier from the schematic fig.9 makes a law of type I, since the expression (34) corresponds to the definition relation of a law I of the form: From the expressions (33) and (34) it results for the considered scheme: 4.4 The operational amplifier of type PI In order to obtain an AO of PI type, in the negative reaction circuit of fig.5 we must introduce, in series with the resistance, the capacity. Thus, the scheme of Fig. 10, which represents an AO of type PI or by analogy in automatic results, we can say that the diagram of Fig. 10 represents a PI type controller with operational amplifier. In this case the voltage u C2 at the capacitance terminals C 2 is similar to the one shown in the expression (31). And if the voltage u R2 on the resistance R 2 has the expression: Then summing the voltages u C2 and u R2 the difference of the voltages from the reaction circuit terminals is obtained, respectively: And considering u 1i≈0, results: For the current I 1 the value in the expression (33) is preserved, since in the input circuit the same resistance R 1 is found , as in figures 5 and 9, it results: Substituting in (40) is obtained: The expression (41) ) attests that the diagram in figure 10 makes a regulation of type IP, since, ignoring the sign (-), which is taken into account when making electrical connections at the output of the regulator, this corresponds to the relation that defines the law of type PI: If we want to change the parameters K R and T i of the regulator with AO, then the resistors R 1 and R 2 will have to be adjustable. From relations (41) and (41) it can be seen that if the value of the resistance is modified to obtain a variation of the value, then an undesirable change of the value is obtained, so an interdependence of granting the parameters of the regulator occurs. From the relations 41 and 42, it is shown for the diagram in fig. 10: 4.5 Proportional - derivative (PD) operational amplifier In order to obtain a PD-type operational amplifier, which is characterized by the proportional derivative law, in the input circuit must be let us introduce in parallel a resistance and a capacity, as shown in Figure 11. In this scheme the current I 1 , which enters the node M, is equal to the sum of the currents I 1R and I 1C by the resistance R 1 and the capacity C 1 . For the current I 1C the expression results: Because u 1C = u 1 + u 1i and considering the relation u 1i ≈0 results: For the current I R1 and I 1 we obtain: For the current I 2 we have: I 2 = (u 1i -u 3 ) / R 2 and considering: u 2i ≈u 1i is obtained: Replacing the expressions of currents I 1 and I 2 it is obtained: This expression attests to the fact that the scheme in fig. 11 makes a PD regulation law, since this expression corresponds to the relation that defines the PD law: Result: 4.6 PID type operational amplifier In order to obtain a PID type operational amplifier, the input circuit it must have the aspect shown in Figure 12. In the diagram of Fig. 11, the current I 1 is determined by the expression (46), since the input circuits are identical in Figures 10 and 11. On the other hand, for the scheme of Fig. 11, the expression (39) remains valid because the reaction circuits in the figures 10 and 12 are identical. From the relation (46) we obtain: and by replacing this expression of the current I 2 (39) it results: The expression (51) attests that the scheme in figure 12 makes a PID-type regulation law, since this expression corresponds to the relation that defines the PID law. : 5. The influence of the negative reaction on the amplifier parameters 5.1 The influence of the negative reaction on the operational amplifier (fig.13) The negative reaction decreases the amplification but increases its stability. Indeed let us consider that for some reason (eg temperature variation) a variation ΔA << A of the unreacted amplifier occurred. In this case, in the relation A ' = A / (1-β · A), which represents the relation of the amplifier with reaction, A becomes A + ΔA and A ' becomes A ' + ΔA ' : subtracting the two relations is obtained: Dividing by A 'and taking into account that the reports ΔA / A and respectively ΔA ' / A ' are obtained , they give stability to the amplification without reaction, respectively with reaction. In the case of the negative reaction K> 1, so ΔA ' / A '<ΔA / A, stability improves. 5.2 The influence of the negative reaction on the amplitude-frequency characteristics In the case of a negative reaction, the frequency characteristic is modified as shown in Figure 13, resulting in a broadening of the frequency band. It can be shown that the upper and lower boundary frequencies become: 5.3 The influence of the negative reaction on the linear distortions Suppose that at the input of the amplifier a sinusoidal signal is applied, and at the output due to the non-linear characteristic of the transistor, the signal appears distorted. Through the negative feedback circuit, it is applied again to the phase opposition input, so with a deformation contrary to the output one. Consequently the resulting signal will be less deformed by compensation. The distortion factor in the case of the amplifier with negative reaction, is given by the formula: 5.4 The influence of the negative reaction on the input and output impedances of the amplifier In the case of the amplifier with serial reaction, the input impedance increases with respect to the case of the amplifier without reaction. Indeed starting from the formulas: and using the relation β = U 1 / U 2 regarding the reaction coefficient β and the input voltage in the reaction amplifier and the fact that I 1 = I 1 ' results: It can be shown that the output impedance decreases when using the negative reaction, according to the formula: In general, if a very strong negative reaction is used 1- βA >> 1, replacing A ' = A / (1-β · A) results in A '= -1 / β, that is, the amplification with reaction becomes independent of the amplifier parameters, thus obtaining high stability amplifiers. These consequences of applying the negative reaction in the amplifiers are justified for the simple reason that it is absent from the amplifiers. 6 Measures to protect and balance the operational amplifiers The correct and safe operation of the operational amplifiers depends on the compliance with the maximum permissible data indicated by the manufacturing company. The protective measures to be taken refer to the supply voltages, the surges that may occur at the input and output of the amplifier and the current supplied by the load amplifier. By balancing the amplifiers, it is usually understood to compensate for offset or offset currents and voltages. 6.1 Supply voltages In the catalog data, it is usually indicated the maximum values of the supply voltages, values greater than the nominal values. Although the operational amplifiers can work at voltages lower than indicated, the characteristics of the amplifiers change. The supply voltages may vary when connecting and disconnecting loads, as well as the variation of the mains voltage. The application of the voltages with the indicated polarity is obligatory, the change of the polarity could lead to the destruction of the amplifier. A good role for the proper functioning of the amplifier is played by the internal resistance of the sources that must be as small as possible. Since the voltage amplification is very high even at high frequencies, it is recommended to connect in parallel with the power sources some decoupling capacitors with values between 10 ... 100nF. 6.2 Protection against overcurrent and overvoltage For operational amplifiers that do not have internal protection, it is recommended that the output resistance R be equal to the internal resistance of the amplifier. This resistance is especially indicated when the load is capacitive in nature. If the load is inductive, dangerous voltages may occur when switching on and off. The protection against overvoltages can be done with the circuits presented in fig.15. 6.3 Limiting input voltage and output voltage Differential voltage V D = V 1 + -V 1 - may not exceed certain values indicated by the manufacturer. To limit the input voltage to small values (+/- 0.6V) use the mounting in figure 16a. If the operational amplifier supports higher voltages, Zener diodes mounted in opposition as in figure 16b are used. In order to limit the output voltage, the circuits shown in figure 17 can be used between points A and B of the operational amplifier (fig.17g). These circuits are individualized by a non-linear character, entering into action only when the output voltage exceeds a certain value. For example, the circuit in Fig. 17a limits the asymmetric voltage according to the characteristic of the Zener diode. Obviously, the diode can be connected and vice versa, it being parallel to the RN resistance (fig.17g). In the case of the circuit in Fig. 17b, the output voltage is symmetrically limited to values slightly higher than the voltages of the Zener diodes. In Fig. 17c, the voltage is symmetrically limited to approx. +/- 0.6V; high-value R resistor with the role of allowing residual currents to flow. The circuits of FIG. 17e and 17f allow depending on the choice of resistors or diodes, symmetrical or asymmetrical limitations of the output voltage. For example, in Fig. 17d, the limit voltage value for each alternation is calculated using the formulas: 6.4 Balancing operational amplifiers. Generally, by balancing the operational amplifiers is meant the compensation of the offset voltage, the resting current and the offset current. To compensate for the offset voltage, there are two possibilities: - the operational amplifier has special terminals, in which case the information in the catalog must be respected; - the operational amplifier does not have such terminals. In this case the voltage compensation is made according to the circuit. Figure 18a shows a circuit for offset voltage compensation on an inverter amplifier, and in Figure 18b a circuit for offset voltage compensation on a non-inverting amplifier. In both circuits, the input terminal (u1 = 0) is connected to the ground and the semi-regulator (or potentiometer) is operated until the output voltage is canceled. The internal resistance of the compensation circuit depends on the position of the semi-regulator and appears connected in parallel with the resistance R P (fig.18a) or R 1 (fig.18b). In the case of the circuit of figure 18b, with the modification of the resistance R 1 there is also a modification of the amplification. In order for the amplification modification to be as small as possible it is necessary that the resistance R be adopted much higher than R 1 . Although the offset current and voltage can be compensated, the compensation is effective only for a temperature range and only for a certain operating mode. For this reason, in the case of more complex circuits from which the last drop of performance is squeezed, specialized circuits are introduced in the offset cancellation in stationary and dynamic operating regime. The offset current drift is kept relatively small if the circuit is sized so that the resistors are low value. The resting currents although charging the signal sources are necessary for the operation of the amplifier, they constitute polarization currents of the transistors in the input circuit of the operational amplifier. In the ideal case, the input currents on the inverter and non-inverting terminals are equal. In fact, due to the voltage drops on the input resistors, these currents are not equal, which leads to the emergence of a differential voltage between the inverting and non-inverting terminals, which voltage is amplified and appears at the output. In order to compensate for the influence of currents, the resistors are chosen so that the voltage drops on them are equal. CAUTION! In the case of inverter mounting, the introduction of an offset current can also be done by adding a resistance to the non-inverting input connected to the ground. For optimal operating conditions, the value of this resistance, which I will note with R, will have to respect the relation: R = R 2 R 1 / (R 2 + R 1 ) , where R 2 and R 1 are the resistors in figure 5 For this reason, in different electronic schemes with AO, only certain values of the external electronic components are used, the performances are no longer 100% the same, if the type of the operational amplifier is changed. 7 Operational amplifier applications 7.1 Ideal voltage controlled current source The ideal voltage controlled current source is very useful in all applications that require ideal voltage sources with linear variation as a function of time and with controlled slope (this is the case with analog digital converters, multipliers, of triangular function generators ramp etc.). One of the efficient possibilities for realizing such a current source is shown in figure 19. If the condition of equality between the absolute values applied to the two inputs is set, that is:: | U 1 | = | -U 2 | = U and if I choose the resistors so that R 5 << R 4 and R 4 / R 2 = R 3/ R 1 , then I R << I 5 and I E = I E '+ I R ≈ I E ' ≈ U E '/ R 5 and: 7.2 Negative impedance converter (CIN) Figure 20 shows the diagram of a active dipole, with differential operational amplifier, capable of converting a Z impedance (connected as the amplifier load) to a "negative" impedance: Z J = KZ. This circuit is used, in particular, for the synthesis of active RC filters. 7.3 Active RC filters The combination of operational amplifiers with passive RC filters leads to the realization of active RC filters with transfer functions of the most diverse, suitable for certain applications, and of very good quality (efficiency). Active RC filters are used, in the case of electronic measurements, in distortion meters, audio filters, in the low frequency selective voltmeters (5 Hz to 1 MHz), in the interferential oscillators in the signal generators, etc. With the help of active filters you can obtain a wide class of frequency characteristics required in practice (such as the characteristics of the TJ, TB and TS filters) with the minimization of the active and passive elements. 7.4 Simulated inductances At low frequencies, where the coils would be too large, or in the case of integrated circuits (where the coils that is, those circuit components having the inductance circuit parameter L cannot be physically realized), the inductances (inductances) L must be "simulated" ", an operation that can be carried out with floors often called and rotating. In the drawing in Fig. 21a is shown a rotator with operational amplifier whose equivalent scheme (Fig. 21b) is that of a coil LS with the inductance CR 1 R 2 and with the losses given by the resistance series R 1 + R 2 . 7.5 The Comparator comparator is a floor which is attacked by two different signals (for example two voltages U 1 and U2 ) provides an answer when there is a certain relationship (eg equality) between the two input signals. Figure 22 shows a comparator with hysteresis. The presence of hysteresis is useful if it is desired that the two stresses are not affected by the disturbances. The two levels of voltage or comparison determine the output state: U eP or U eN . The hysteresis effect is given by the relation: The error of comparison, expressed as a deviation from the value U 1 , with which compares U 2 = const., Is: where u d1 is the voltage gap related to the input, the second term of the relationship the area of uncertainty, and the third term of the relationship, the signal on the incomplete common node rejected. 7.6 Self-propelled amplifiers at output with modulation in duration With the help of operational amplifiers, simple rectangular signal generators can be made, with a constant period, but with a duration of 1 + t 2 of the positive impulse (see fig. 23) adjustable so with a filling factor t 1 / t 1 + t 2 that can be adjusted between 0 and 1 ie a self-propelled amplifier with the output voltage modulated in duration. Thus, the presence of the positive reaction introduced by the circuit R 3 R4 , causes the voltage at the output of the amplifier to take only one of the limit values + U e or U e . 7.7 Sinusoidal oscillators With the use of operational amplifiers, sinusoidal self-oscillators can also be obtained, if a reaction assembly such as the one in figure 24 is performed, which contains a selective circuit on the positive feedback loop and a negative reaction through two resistors R 3 and R 4 . Noting with β (ω) = U N / U E the transfer function (attenuation) of the positive reaction block (network), the maintenance condition of the oscillations at the tuning pulse ω0. So, the self-oscillation condition is: In order to ensure the functioning of the auto-oscillator (sinusoidal oscillations maintained) it is necessary that, in addition to satisfying the above condition, the equality of the attenuations of the two reaction networks is maintained, which means achieving equal gains in the two reaction loops. 7.8 Amplitude detectors Amplitude detectors are electronic floors that convert a variable voltage into a continuous voltage (this being the case of the type mutator: alternator continuous converter) or a low frequency voltage (the case of the type mutator: tire converter, used for signals). harmonics with amplitude modulation). Amplitude detectors are used in alternating current electronic voltmeters (which basically consist of an amplitude detector floor, followed by a direct current electronic voltmeter), in signal generators, in electronic blocks to determine the level or modulation degree (in modulometers) etc. There are two types of amplitude detectors, named: medium value detectors (fig. 25) and peak detectors (fig. 26). If the diodes are reversed in Fig. 26, a peak detector is obtained for the negative values of the input voltages U1. 7.9 Phase and frequency discriminators There is a wide variety of phase discriminators, depending on the purpose of their use. An example of a purpose is that of phase measurement, which can be done by the so-called phase sensitive detector. Frequency discriminators are used to measure the frequency deviation in the case of frequency modulated signals or for analog frequency measurement (in analog electronic frequency meters, these discriminators produce a continuous voltage proportional to the frequency of the signal to be measured, so that in fact these discriminators frequency are numerical-analogical converters of frequency-voltage). Frequency discriminators are also used in AC generators for automatic frequency adjustment, in which case they also function as a frequency converter. 7.10 Electronic multipliers Electronic multipliers are stages that have a voltage proportional to the output of two analog voltages applied to the inputs. Floors with electronic multiplier function are used in many devices used in electronic measurements, including: effective value electronic voltmeters, electronic wattmeters, electronic meters (active and reactive energy), modulometers, amplitude modulated generators, interference oscillators, synthesizers frequency, synchronous detectors, electronic phasemeters, as well as in the phase shift circuits, in the circuits for the automatic amplification control, in the analogical calculation circuits, in the circuits for obtaining voltage-controlled (adjustable) impedances and many more. With the help of operational amplifiers, the synthesis of networks or signals in the field of frequencies or time with minimal errors can also be realized, many of them with applications being found in electronic measuring devices, such as: negative impedance converter, active filters, RC, comparator circuits with or without hysteresis, simulated inductors, analog splitters, peak detectors, mean value detectors, frequency discriminators, phase discriminators, self-tapping amplifiers, correlation analyzers, Fourier transformer samd. 8. Practical schematics with operational amplifiers Figure 27 shows a simple mounting with operational amplifier type 741 in DIP-8 capsule. For other types of capsules it is recommended to study the datasheets of the integrated circuits, in order to identify the functions of the pins. There are also two operational amplifiers in the DIP-8 capsule (see NE5532). The RS resistance represents the load resistance of the operational amplifier. Fig. 28 shows a Baxandall audio tone controller made with the operational amplifier OPA134: Bibliography : [1] http://elearning.algonquincollege.com [2] HANDBOOK OF OPERATIONAL AMPLIFIER APPLICATIONS - Texas Instruments [3] Edmond Nicoalu, Beliş Mariana - Electrical and electronic measurements "- Didactic and Pedagogical Publishing House, Bucharest 1984 [4] Theodor Dănilă, Monica Ionescu-Vlad - Electronic components and circuits "- Didactic and Pedagogical Publishing House, Bucharest 1984 [5] G. Vasilescu, Ș. Lungu - Electronic "- Didactic and Pedagogical Publishing House, Bucharest 1981 [6] Miron C. - Introduction to electronic circuits " Dacia Publishing House, Cluj Napoca, 1983.
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