Filmoscope power supply. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Power Supplies

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An old filmoscope that is no longer intended to be used for its intended purpose can be easily converted into a regulated power supply with protection against overloads and short circuits. For this purpose, a housing with a transformer and a power cord is used from the filmoscope. Instead of a reflector and a bracket with a cartridge and a lamp, a board with a rectifier and a voltage stabilizer is installed. Instead of a lens, you can install a voltmeter to monitor the voltage at the output of the source.

The power supply circuit is shown in Fig.1.

(click to enlarge)

It consists of a step-down transformer T1, a voltage doubling rectifier (VD1, VD2, C2, C3) and a stabilizer on transistors VT1 ... VT3 with adjustable output voltage, which can be changed from 0 to 12 V at a load current of up to 0,7 A The stabilizer is developed on the basis of the devices described in [1,2]. The reference voltage is taken from the zener diode VD3 and then from the engine of the resistor R2 (voltage regulator) through the resistor R4 is fed to the base of the transistor VT1, which controls the transistors VT2 and VT3. Diodes VD4 and VD5 protect the stabilizer from overloads and output short circuits.

The current at which the protection is triggered depends on the value of the resistance of the resistor R5. As long as the load current does not exceed a certain threshold value, the VD5 diode is open and forms a voltage divider with the resistor R5. At the same time, the VD4 diode is closed and does not affect the operation of the stabilizer. In the event of a short circuit or an excessively high current consumption, the diodes VD4 and VD5 are connected to the negative conductor through a low load resistance. The diode VD5 closes, and the diode VD4, on the contrary, opens at this time and connects the base of the transistor VT1 to the negative wire. At the same time, all transistors are closed, and the current through the load drops to a few milliamps. Resistors R3 and R4 reduce the short circuit current. Capacitor C1 serves to suppress interference from the network.

In addition to those indicated in the diagram, the following parts can be used in the power supply: transistors KT312, KT3102 (VT1); KT3107, KT814, KT816 (VT2); KT835, KT818 (VT3); diodes D242, D243 (VD1, VD2); KD503, D220, KD105 and other low-power silicon (VD4, VD5); Zener diode KS512A or several other zener diodes connected in series with a total stabilization voltage of about 12 V (VD3); variable resistor with a resistance of 10 ... 100 kOhm (R2); resistors with a resistance of 150 ... 200 kOhm (R3); 5,1...10 kOhm (R4); capacitor with a capacity of 0,01 ... 0,1 μF (C1).

Instead of a voltmeter PU1, an LED voltage indicator can be connected to the output of the power supply (Fig. 2).

This indicator allows you to set the voltage at the output of the block with an accuracy of 0,2 V, and can also serve as an indicator of overload or short circuit of the output.

Transistor VT3 is installed on the device case. Between the VT3 transistor and the case, a metal plate 3 ... 5 mm thick and approximately 30 x 50 mm in size should be placed. This will increase the area of ​​thermal contact of the transistor with the case. The power supply output, voltmeter and variable resistor are located on the front wall of the case.

Adjustment of the stabilizer is reduced to the selection of the resistance of resistors R1 and R5. Selecting resistor R1, set the current through the zener diode VD3 equal to 15 mA, and resistor R5 - the current of protection operation.

Literature

1. Klyuev Yu., Abashev S. Voltage stabilizer//Radio.-1975.-№2.-C.23.
2. Popovich V. Improving the voltage stabilizer//Radio.-1977.-№9.-C.56.

Author: L.D. Bogoslavets, Cherkasy region

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