Combined power supply, 220 / 0-12 and 0-215 volts 0,5 amperes. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Combined power supply, 220 / 0-12 and 0-215 volts 0,5 amperes. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Power Supplies

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This device consists of two independent power sources for radio equipment: a constant voltage regulated within 0 ... 12 V, and an alternating voltage regulated within 0 ... 215 V. The first of them is designed to power devices and devices based on transistors and integrated circuits , the second - for smooth regulation of the rotor speed of network electric motors, the brightness of incandescent lamps, the temperature of the tip of an electric soldering iron or heating element, lowering the mains voltage of 220 V to 127 V (instead of LATR) and other similar purposes. At the same time, both sources can be used to power measuring instruments and devices on digital microcircuits with high-voltage gas-discharge indicators.

The maximum load current of each of the sources is 0,5 A. The voltage of the variable component (ripple) of the DC source is not more than 0,2 V. Each of them has its own primary power circuit switch, a protective fuse and a voltmeter showing the output voltage.

The schematic diagram of the block is shown in Fig.1.

Fig.1 (click to enlarge)

In the AC voltage source, a powerful transistor VT1 is used as a regulating element, which acts as a kind of semiconductor variable resistor connected in series with the load. This technical solution provides a number of advantages compared to a thyristor regulator or LATR, for example: it does not create interference penetrating into the power grid, it has small dimensions and weight. The transistor regulator allows you to control devices with both active and reactive loads. It is also relatively simple and does not contain scarce parts.

Of the shortcomings, one is the most serious - a large amount of heat is released on the regulating transistor, which creates certain difficulties with its removal.

The diode bridge VD1-VD4 provides direct current through the transistor VT1 for both half-cycles of the mains voltage. Reduced by the transformer T1 to b V, the mains voltage is removed from its winding II. It rectifies its diode block VD5 and smoothes the capacitor C1. The variable resistor R1 regulates the base current of the transistor VT1. Resistor R2 - current limiting. Diode VD6 prevents negative voltage from entering the base of transistor VT1. The output voltage is controlled by a voltmeter PU1.

The load current operating with such an AC voltage source depends on the value of the control voltage based on the transistor VT1. By changing this voltage with resistor R1, it is possible to control the collector current of the transistor, and, consequently, the current through the load. At the lowest position of the engine of the resistor R1 according to the scheme, the transistor VT1 turns out to be completely open and the voltage at the load will be maximum. In the extreme upper position of the engine of this resistor, the transistor will be in the closed state and the current through the load will stop.

Transformer T2, which supplies a DC voltage source, lowers the AC voltage to 12 V. This voltage rectifies the VD7 diode block, and the voltage ripples are smoothed out by capacitors C2, C3. Zener diode VD8 and resistor R3 form a parametric voltage regulator, and transistor VT2 amplifies the output power of this source. The voltage taken from its output is regulated by a variable resistor R4. Capacitor C4 serves to filter high-frequency interference when powered by a block of devices on digital microcircuits. The output voltage is controlled by a voltmeter PU2.

Powerful diodes VD1-VD4 are installed on the board without heat sinks. Transistors are installed on heat sinks with a useful dissipation area for the VT1 transistor - at least 300 cm2, and for VT2 - 30 cm2. All controls, voltmeters and connectors are placed on the front panel of the unit, and fuse holders are placed on the rear or one of the side walls. All necessary connections are made with pieces of thin mounting wire in reliable insulation.

In addition to those indicated in the diagram, transistors can be used in the power supply: VT1 - KT812A, KT812B, KT824A, KT824B, KT828A, KT834A - KT834V, KT840A, KT840B, KT847A, KT856A; VT2 - KT805AM, KT807A, KT807B, KT815A-KT815G, KT817A-KT817G, KT819A-KT819G. Diodes VD1-VD4 must be rated for a voltage of at least 250 V and a current of at least 1 A - for example, KD202Zh-KD202S or from the D245, D246, D247, D248 series with any letter index. Rectifier blocks VD5 and VD7 - KTS405, with any letter index; diode VD6 - D237. Zener diode VD8-D811, D813, D814G.

Oxide capacitors C1-C3 - K50-6, C4 - small-sized ceramic KM-5 or KM-6. Fixed resistors R2, R3 - MLT, OMLT, S2-23 or any others. Variable resistor R1 - wire for a dissipation power of at least 3 W, for example, PPB3 or PPB15; R4 - SP, SPO with a power of at least 0,5 watts. Fuses FU1, FU2 - VP1-1. Toggle switches SA1, SA2-TB1-1, TP1-2, MT1, MTD1, T1-T3, T3-S. Voltmeter PU1 - Ts4203 or any other, designed to measure AC voltage 250 ... 300 V, and PU2-M4231.40 or any other DC voltmeter for voltage 12 ... 15 V. Connector X1 - standard power plug, X2 - mains socket, X3 can be of any type.

The mains transformers T1, T2 used for the combined power supply are TVZ-1-6 from obsolete tube TV receivers. Mains voltage is applied to their terminals 1 and 3. In transformer T1, an alternating voltage of 6 V is removed from terminals 4 and 5. In transformer T2, two secondary windings connected in parallel (terminals 4 and 5) must be reconnected in series-accordingly. In general, you can use any other transformers with a power of 6 ... 10 W, lowering the mains voltage to 6 ... 10 V (T1) and 12 ... 15V (T2), for example, transformers TS-25 or TS-27 from TVs "Youth".

The power supply unit does not require adjustment. If no errors were made during installation and serviceable parts were used, it starts working immediately after connecting to the network.

In conclusion, a few recommendations regarding the increase in the output power of the AC voltage source. If its regulating transistor (VT1) is selected from the KT856 series, then the power consumed by the load from the network can reach 150 W, with a transistor from the KT834-200 W series, and KT847 - 250 W. If it is necessary to further increase the output power of the source, the regulating element is made up of several transistors connected in parallel, connecting their outputs of the same name. These transistors are selected with possibly close coefficients h21e and, in addition, individual equalizing resistors are included in their base circuits. Diodes VD1-VD4 will have to be replaced with more powerful ones, designed for a current not less than consumed by the load. The VD6 diode will also need to be replaced with a more powerful one, capable of passing current up to 1 A. The PU1 fuse must also be designed for a larger current. But in this case, you may have to install a small fan to intensively remove heat from semiconductor devices.

When working with the power supply, do not forget about safety measures. Remember that the AC source is galvanically connected to the mains and is under high voltage!

See other articles Section Power Supplies.

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