This is the complete design power supply circuit. This circuit has stable, clean and regulator 0-28V 6/8 Amp output voltage. This circuit is using transistor 2N3055. Although you could use this design to deliver 20 amps (with almost no modifications and with a proper transformer and a huge heat sink with a fan), it didn’t need much power. This is the figure of the circuit.
Although the 7815 power regulator will kick in on short circuit, overload and thermal overheating, the fuses in the primary section of the transformer and the fuse F2 at the output will secure your power supply. The rectified voltage of: 30 volt x SQR2 = 30 x 1.41 = 42.30 volt measured on C1. So, all the capacitors should be rated at 50 volts. Caution: 42 volt is the voltage that could be on the output if one of the transistors should blow. P1 allows you to 'regulate' the output voltage to anything between 0 and 28 volts. The LM317 lowest voltage is 1.2 volt. To have a zero voltage on the output I've put 3 diodes D7,D8 and D9 on the output of the LM317 to the base of the 2N3055 transistors. The LM317 maximum output voltage is 30 volts, but using the diodes D7,D8 & D9 the output voltage is approx 30v - (3x 0.6v) = 28.2volt. Calibrate your build-in voltmeter using P3 and, of course, a good digital voltmeter. P2 will allow you to set the limit of the maximum available amps at the output +Vcc. When using a 100 Ohm/1watt varistor the current is limited to approx. 3 Amps @ 47 Ohm and +- 1 Amp @ 100 Ohms.
Although the 7815 power regulator will kick in on short circuit, overload and thermal overheating, the fuses in the primary section of the transformer and the fuse F2 at the output will secure your power supply. The rectified voltage of: 30 volt x SQR2 = 30 x 1.41 = 42.30 volt measured on C1. So, all the capacitors should be rated at 50 volts. Caution: 42 volt is the voltage that could be on the output if one of the transistors should blow. P1 allows you to 'regulate' the output voltage to anything between 0 and 28 volts. The LM317 lowest voltage is 1.2 volt. To have a zero voltage on the output I've put 3 diodes D7,D8 and D9 on the output of the LM317 to the base of the 2N3055 transistors. The LM317 maximum output voltage is 30 volts, but using the diodes D7,D8 & D9 the output voltage is approx 30v - (3x 0.6v) = 28.2volt. Calibrate your build-in voltmeter using P3 and, of course, a good digital voltmeter. P2 will allow you to set the limit of the maximum available amps at the output +Vcc. When using a 100 Ohm/1watt varistor the current is limited to approx. 3 Amps @ 47 Ohm and +- 1 Amp @ 100 Ohms.
