In practice, very often to power a variety of devices require voltage from 3 to 12 V. the Described power supply allows to obtain the voltage of the next some: 3; 4,5(5); 9; 12 In load current up to 300 mA. There is a possibility quickly changing the polarity of the output voltage.
Adjusting the output voltage is stepped through switch SA2 (see figure).
Switch SA3 is possible to change the polarity the output voltage. Diode VD2 type AL 102 is part of a chain stabilization the reference voltage R1, VD2, VD3 and signals its presence. The stabilized voltage (selected by switch SA2) is supplied to the base transistor VT1. The transistor VT2 for reliable operation must be installed on a small radiator.
The power supply is not protected against short circuit output. But practice has shown that the transistor is able to withstand short CT the circuit for 15...17 C. And with a load current of more than 300 mA led VD2 reduces the brightness, which indicates overload.
The Transformer T1 has the core Sh, the primary winding of 3000 turns of wire sew 0,1, secondary - 260 turns of wire sew 0,31.
The dimensions of the block 90x70x45 mm.
Author: O. G. Rashitov, Kiev