The power supply shown in the above schematic diagram uses a Zener diode as the main regulating device, to maintain a constant output voltage in spite of changes in input voltage. The type 1N4742A diode is rated at 12 volts, with a power dissipation limit of 1 watt. Since we want to be able to deal with load currents of at least 100 mA and possibly more, we cannot use the Zener diode by itself. (Under no-load conditions, the Zener diode would have to handle that 100 mA, which would result in a power dissipation of 1.2 watts. This would overheat and destroy the diode in short order.) Therefore, we add a power transistor as the main current-handling device for the power supply. The TIP42 PNP silicon transistor (available from Radio Shack) is rated to carry up to 10A collector current and a power dissipation of 65 watts.
To construct and the -12 volt power supply circuit, you will need the following parts:
- (1) 2.2, ¼-watt resistor (red-red-gold).
- (1) 10K, ¼-watt resistor (brown-black-orange).
- (1) .01µf (or larger) disc capacitor.
- (1) 10µf (or larger) 35 volt electrolytic capacitor.
- (1) 2200µf (or larger) 35 volt electrolytic capacitor.
- (2) Silicon Rectifier Diodes.
- (1) 1N4742A 12 volt Zener diode. From a two-pack available from Radio Shack.
- (1) PNP silicon power transistor. The TIP42 sold by Radio Shack is quite suitable here, and will be used as the example.
- (1) 2N3906 or 2N4126 PNP silicon transistor. Radio Shack sells a package of 15 PNP transistors that will perform well here, and provide transistors for experimental circuits as well.