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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Triac low noise power controller. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Regulators of current, voltage, power Trinistor power controllers with phase control have been repeatedly described on the pages of our magazine. But, unfortunately, many of them are strong sources of electromagnetic interference, which limits the scope of devices. Foreign domestic regulators are necessarily equipped with a built-in noise suppression filter. Moreover, the level of interference they create must meet the stringent standards adopted in a particular country. The author of the article talks about one of these regulators. The scheme of the power regulator with phase-pulse control is shown in fig. 1. It is assembled according to the classical scheme on a 32V symmetrical dinistor (VD3) and a TIC226M triac (VS1). With each half-wave of the mains voltage, the capacitor C1 is charged by the current flowing through the resistors R2, R3. When the voltage on it reaches 32 V, the dinistor opens and the capacitor C1 quickly discharges through the resistor R4, the dinistor VD3 and the control electrode of the triac. Thus, the triac is controlled in quadrants I and III: when the voltage at the conditional anode of the triac (upper terminal VS1 according to the circuit) is positive, the control pulse is also positive, and with a negative voltage - negative polarity.
The power value in the load connected to connector X1 depends on how long the triac will be on during each half-cycle of the mains voltage. The moment the triac is turned on is determined by the threshold voltage of the dinistor and the time constant (R2 + R3)C1. The greater the resistance of the input part of the variable resistor R2, the longer the period of time during which the triac is in the closed state, the lower the power in the load. The ratings of the time constant elements indicated in the diagram provide an almost complete range of output power control - from 0 to 99%. To achieve a sufficiently smooth output power control, the variable resistor R2 must be with the characteristic of group B. The group B resistor is also suitable, but then it will have to be turned on in such a way that the increase in output power (i.e., with a decrease in the resistance of the variable resistor) occurs during rotation its knobs counterclockwise. The circuit formed by diodes VD1, VD2 and resistor R1 ensures smooth adjustment at minimum output power. Without it, the controller control characteristic has a hysteresis. For example, the brightness of an incandescent lamp used as a load, with an increase in output power, changes abruptly from zero to 3 ... 5% of the maximum brightness. The essence of this phenomenon is as follows. With a high resistance of the resistor R2, when the voltage across the capacitor C1 does not exceed 30 V, the dinistor does not open during the entire half-cycle of the mains voltage and the output power is zero. At the same time, by the time the mains voltage passes through "zero", the voltage on the capacitor has a zero value, and in the next half-cycle, the capacitor is discharged for a significant part of the time. If the resistance of the resistor R2 is reduced, then after the voltage on the capacitor begins to exceed the threshold of the dinistor, the capacitor will be discharged at the end of the half-cycle and will immediately start charging in the next half-cycle, so the dinistor will open earlier in the new half-cycle. The diode-resistor circuit discharges the capacitor when the mains voltage changes from negative to positive half-wave and thereby eliminates the effect of an abrupt initial increase in power in the load. Resistor R4 limits the maximum current through the dinistor to about 0,1 A and slows down the process of discharging capacitor C1. This provides a relatively long pulse duration, sufficient to reliably start the triac VS1 even with a significant inductive component of the load. With the ratings of the resistor R4 and capacitor C1 indicated in the diagram, the duration of the control pulse is 130 μs. A significant part of this time, a current flows through the control electrode of the triac, sufficient to open the triac in any quadrant - for a 32V triac it corresponds to 50 mA. A 32V symmetrical dinistor (VD3) ensures the same opening angle of the triac in both half-waves of the mains voltage. Consequently, the described regulator will not rectify the mains voltage, so in many cases it can even be used to control the load connected to it through a transformer. The 32V dinistor can be replaced with its analogue, assembled on transistors of different structures, as shown in fig. 2. Diode bridge VD4-VD7 ensures the symmetry of the triac control, and the low-power zener diode VD8 sets the analogue response threshold. Transistors VT1 and VT2 must withstand a significant (at least 0,1 A) impulse base current. The static current transfer coefficient of the base of the transistor VT2 is at least 50. The bridge diodes must also withstand a direct pulse current of at least 0,15 A. For example, diodes of the KD103 series with any letter index are suitable.
The maximum allowable voltage of diodes and transistors of the dinistor analogue must be at least 30% more than the stabilization voltage of the VD8 zener diode, i.e., at least 50 V. Two low-power zener diodes can be used by turning them on in series so that their total stabilization voltage is 25 .. .30 V. Resistors R7 and R8 provide analog high temperature stability. The TIC226M triac, whose permissible current is 8 A, allows you to control a load with a power of up to 1 kW. For loads up to 2 kW, triacs with a permissible current of 15 ... 16 A can be used. Instead of the TIC226M triac, you can use the domestic trinistor KU208G. However, it has a significantly worse sensitivity. For reliable operation, a current of at least 208 mA at an ambient temperature of -250 ° C or 60 mA at room temperature must flow through the control electrode of the KU170G trinistor. Therefore, when using the KU208G trinistor, the resistance of the resistor R4 should be reduced to 100 ohms, and the inductance of the inductor L1 to 100 μH. Accordingly, transistors and diodes in the analogue of a dinistor (Fig. 2) must withstand currents up to 0,3 A. The level of interference generated by such a regulator will be significantly higher. In addition, it will have less stability when operating on a load with an inductive component. The voltage drop across the triac VS1 is approximately 2 V, therefore, with a load of more than 100 W, the triac must be installed on an appropriate heat sink. At a lower load, the regulator circuit board itself can serve as a heat sink. To do this, the triac in the TO220 case should be placed on the foil side of the printed circuit board, screwed with an M3 screw with a nut, and a foil area of 5 ... 2 cmXNUMX should be left under the triac installation site. In amateur designs, a diode bridge and a trinistor are often used instead of a triac, which increases the cost of components and the size of the structure. This solution approximately doubles the power loss in the regulator and narrows the range of permissible loads. In addition, the storage capacitor is charged with a unipolar voltage, which, as correctly noted in the article by A. Maslov "Once again about the trinistor power controller" (see "Radio", 1994, No. 5, p. 37), leads to malfunctions regulator at low installed power. Speaking about the article by A. Maslov, it is impossible not to mention that the method he proposes to reduce the rate of voltage rise on the trinistor (dV / dt) can lead to damage to the trinistor due to its overload with pulsed current at the moment of switching on, since the discharge current of the capacitor shunting the trinistor, not limited in any way. If a high quality capacitor with low internal resistance is used, the SCR will almost certainly be destroyed by exceeding the current value or current slew rate (dV/dt). To eliminate this disadvantage, it is necessary to connect a wire or bulk carbon resistor with a resistance of at least 10 ohms in series with the storage capacitor. Metal-film and carbon-film resistors are unsuitable for this purpose, as they can fail due to the large instantaneous power dissipation at the moment the trinistor is turned on. In the described power controller (see Fig. 1), the rate of change of voltage on the triac VS1 is limited by capacitors C2, C3, and the current of their discharge when the triac is opened is limited by the choke L1. Modern triacs withstand a voltage rise rate of 50 ... 200 V / μs, and some even up to 750 V / μs, so the relatively small capacitance of the capacitors C2, C3 prevents false trips of the triac even with low-ohmic loads. It is regrettable to note that obsolete domestic SCRs of the KU208 series have only 10 V / μs. At the same time, the inductor L1 and the capacitors C2, C3 form a low-pass noise suppression filter. The inductor must withstand the load current without saturation of the magnetic circuit. As a magnetic circuit, the author used a ring with an outer diameter of 26,5, an inner diameter of 14,5 and a thickness of 7,5 mm made of powdered iron with a magnetic permeability of 75. The winding contains 58 turns of PEV-2 wire with a diameter of 1 mm. Such a choke is suitable for operation with a load of up to 1 kW. When using a KU208G trinistor, the number of turns of the inductor should be reduced to 40. Capacitors C2 and C3 must be of type X1 or X2 (this is the international designation of capacitors), specially designed to be connected between network wires; they are in housings made of self-extinguishing plastic, which prevents fires that are possible during the breakdown of capacitors. On the case of a capacitor of this type, its rated voltage of 250VAC must be indicated, which corresponds to the use in an alternating current network (AC = alternated current, i.e. alternating current). In addition, the cases must bear the symbols of the testing laboratories that have tested this type of capacitor and found it suitable for use in the AC mains. Good capacitor cases are usually littered with such marks, as they have been tested in many laboratories. In extreme cases, instead of a capacitor of type X1 or X2, a metal-film or paper capacitor with a rated voltage of at least 400 V can be used. Author: A.Kuznetsov, Moscow
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