Such a power supply allows to smoothly change not only the output voltage but also its polarity. It can be used, for example, to control a model railroad or for other similar purposes.
The circuit is a conventional bipolar power supply between closed with an output of each "arm". As a result, at the middle position of the variable resistor of the engine, each "arm" the transistors are closed and the output voltage is absent.
When the engine rotates in one direction or another opening corresponding transistors and appears at the output voltage of a particular polarity, the magnitude of which is controlled from the "zero" to "high".
The maximum value of output voltage and output power of the applied depend on the circuit parameters of the transformer and other electronic elements.
For example, to obtain an output voltage from "negative" to "plus" 12 volts required transformer with a secondary winding with a tap from the mid-voltage of each of the halves of the windings 12 volts (AC voltage).
Electrolytic capacitors C1 and C2 may be a capacity of 500 uF and above, their values depend on the magnitude of the output voltage ripple.
Rectifier diodes can take the type KD226, KD105, 1N4001 - 1N4007 or similar, depending on the voltage and power load, or use the appropriate parameters of the diode bridge.
Zener diodes VD1, VD2 - D814D type (for the output voltage +/- 12 volts) or similar import. For other values of the maximum output voltage to apply the appropriate power transformer and stabilizer. The variable resistor may be resistance from 3 to 15 ohms.
The transistors in the circuit can be replaced by any similar corresponding structure and capacity. For example KT502, KT503 transistors can be replaced by KT361, and KT315 respectively. Instead, use the KT817 KT805, KT814, V1274, the D400, but instead KT816 - KT814, KT837, A1658, V1366, and other equivalent.
Selection of the parameters do not require transistors. Also, the scheme does not need any configuration and defective parts and correct assembly starts working immediately.
Powerful regulating transistors (KT816, and KT817) should be set to radiators, heat sinks, the size of which depends on Applied on transistors and the output power supply unit.