Using transistorized regulators with NPN and PNP transistors we can create a symmetrical source of better performance, for output currents up to 500 mA. The source shown in the figure has an output voltage that is determined by the zener diodes. The resistors R1 and R2 have their values dependent on the voltage of the zener diode according to the following table:
Output voltage Transformer (secundary) R1, R2
Zener diode
6 + 6 V 7,5 + 7,5 V 150 ohms x 1/2 W
6,6 V x 400 mW
6 + 6 V 9 + 9 V 270 ohms x ½ W
6,6 V x 400 mW
6 + 6 V 12 + 12 V 390 ohms x ½ W
6,6 V x 400 mW
9 + 9 V 12 + 12 V 220 ohms x ½ W
9,8 V x 400 mW
9 + 9 V 15 + 15 V 470 ohms x ½ W
9,8 V x 400 mW
12 + 12 V 15 + 15 V 270 ohms x ½ W
12,6 V x 400 mW
The recommended secondary current for the transformer is 500 mA but if smaller transformers are used, the source will also work but will provide a lower maximum current. Transistors must be equipped with heat radiators and their position must be carefully observed. The circuit can be mounted in a small plastic box, with different colored terminals for the output. An LED indicator in parallel with the outputs can be added to indicate operation. The resistor in series with this LED will be 470 ohms for the 6 V source, 560 ohms for the 9 V source and 1 k ohms for the 12 V source.
