AC voltage stabilizer. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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AC voltage stabilizer. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Surge Protectors

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When powering electronic equipment from the mains, it is often necessary to stabilize the AC voltage. A great difficulty in designing such stabilizers is to obtain a sinusoidal output voltage with low non-linear distortion. From the point of view of the practical implementation of this requirement, as well as increasing the speed and stabilization coefficient, stabilizers with a transistor control element are most preferable.

In the described stabilizer (Fig. 9.1), the control element is composed of transistors VT1 and VT2, diodes VD2, VD3 and resistors R1 ... R5. When the value of the direct current flowing through the diagonal of the rectifier bridge VD1 changes, the value of the alternating current flowing through section 1.1 of the autotransformer winding changes. As a result, the value of the alternating voltage on section 1.2 of the winding changes. This inclusion of the regulating element reduces its influence on the shape of the output voltage sinusoid. Resistors R1...R4, shunting the control element, reduce the power dissipated by transistors VT1, VT2.

(click to enlarge)

The main technical characteristics of the stabilizer:

supply voltage, V.....220 ±22;
AC output voltage, V....220;
load power, W.....130.....220;
instability of the output voltage with the specified changes in the mains voltage and load power,%, not more than ..... 0,5;
coefficient of non-linear distortions, %, no more.....6.

Transformer T2 serves to power the DC amplifier and at the same time enters the negative feedback circuit. Winding voltage II, rectified by the diode bridge VD5, is supplied to the divider R12 ... R14. With an increase in the mains voltage or a decrease in the load current connected to the output of the stabilizer, the voltage at the base of the VT5 transistor increases, and hence its collector current. Approximately to the same extent, the collector current of the transistor VT4 also decreases.

The voltage drop across the resistor R10 remains virtually unchanged, since the voltage at the base of the transistor VT4 is stabilized. In this case, the voltage on the collector VT4 increases and the current flowing through the transistor VT3 decreases. Due to a decrease in voltage at the base of transistor VT2, it begins to close, the voltage on its collector increases. This leads to the closing of the transistor VT1, since the voltage at its base is fixed by the divider R1, R2, R3, R4, VD2, R5. Diode VD3 excludes the influence of this divider on the base of the transistor VT2.

As a result of the increase in the resistance of the transistors VT1, VT2 of the regulating element, the direct current in the diagonal of the rectifier bridge VD1 decreases and, consequently, the alternating current in section 1.1 of the winding of the autotransformer T1, which is equivalent to an increase in the voltage drop in section 1.2. Therefore, the output voltage retains its original value. When the mains voltage decreases or the load current increases, the current through the transistor VT3 increases and the transistors VT1 and VT2, on the contrary, open even more. Diode VD2 in this case is closed by voltage from resistor R7. Diode VD3 provides full opening of the transistor VT1.

Transistor VT6, resistor R11 and capacitor C2 form an electronic filter that delays the supply voltage to the DC amplifier. The delay is necessary to eliminate the output voltage surge at the moment the stabilizer is turned on. The limitation of the minimum load power to 130 W is due to the fact that at a lower power and a mains voltage of more than 220 ... 225 V, the initial voltage rises above the established tolerance due to a decrease in the voltage drop across the inductive resistance of section 1.2 of the mains transformer.

Rectifier. KTS405A (VD1) can be replaced by four diodes with a reverse voltage of at least 600 V and a rectified current of 1 A; KD906A (VD5) - diodes with a direct current of at least 30 mA; transistors KT809A (VT1, VT2) - similar to them, for example, KT812A, KT812B.

Transistors VT3.VT6 can be any low-power corresponding structure. Resistors R1...R4 are mounted on a separate board, which is located under the switch SB1. The power dissipated by each of the transistors VT1, VT2 is 8 W, so they are installed on separate heat sinks with a surface area of 500 cm2. The overall power of the autotransformer T1 is about 22 watts. You can use an autotransformer from the Mayak-202 tape recorder (ShL20x20 magnetic circuit, winding section 1.1 contains 1364 turns of PEV-2-0,31 wire, section 1.2 - 193 turns of PEV-2-0,63 wire). Transformer T2 is made on a magnetic circuit. SHL16x16. Winding I contains 2560 turns of PEV-0,1 wire, winding II - 350 turns of PEV-2-0,2 wire with a tap from the 70th turn (to power the HL1 indicator lamp).

The stabilizer casing is best made of insulating material. Ventilation openings must be provided in the casing panels. If the casing is metal, care must be taken to reliably isolate all current-carrying parts and wires from it.

When establishing, first, by selecting the resistor R11, a voltage of 12 V is set at the emitter of the transistor VT6 (the negative terminal of the VD5 diode bridge serves as the common wire of the device). At the same time, a voltage of about 4 V should be established on the basis of the VT8 transistor. A load is connected to the output of the stabilizer. It can serve as an incandescent lamp with a power of 150 ... 200 watts. A voltage of 220 V is supplied from the laboratory autotransformer to the input of the stabilizer, and the nominal mains voltage of 13 V is set at the output with resistor R220.

The voltage drop on each of the transistors of the regulating element should be 80....100 V. When the input voltage changes by ±22 V, the voltage at the output of the stabilizer should remain practically unchanged. The lack of stabilization indicates an error in the installation or a malfunction of one or another part. The excitation of the stabilizer is eliminated by selecting the capacitor C1. The power of the stabilizer can be increased to 450 W if its regulating element is mounted according to the scheme shown in fig. 9.2. For this case, the autotransformer T1 must be performed on the magnetic circuit. SHL20x25. Winding section 1.1 should contain 1300 turns of PEV-2-0,36 wire, section 1.2 - 180 turns of PEV-2-0,9 wire.

AC Voltage Stabilizer

The most important advantages of the described stabilizer compared to a ferroresonant one are small non-linear distortion of the output voltage and the almost complete absence of a magnetic field that adversely affects the operation of color TVs.

Author: Semyan A.P.

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