0-28V 20A Adjustable Power Supply (Based on LM317 and 2N3055)

0-28V 20A power supply (LM317, 2N3055)

S2 switches between ±3 amps and full output current.

The relay is used to cut off the power supply voltage when the power supply (S1) is turned off. Therefore, there is no delay caused by the discharge of C2, thus ensuring that the output voltage returns to zero immediately.

Use the MB2504 because it is a 25-amp rectifier bridge and also requires cooling. Alternatively, you can use eight BYW29 8-amp diodes (TO220 package) mounted on a heatsink.

Install a small heatsink on the LM317 IC.

Remember to isolate the 2N3055 transistor from the chassis/heatsink! Use a heatsink of appropriate size and surface area; insulating and thermally conductive pads or at least thin mica; thermal adhesive and thermal paste.

     2 x 15 volts ± 20 amperes

     D1...D4 = Rectifier bridge MB2504 (25 amps, cooled)

     Or eight BYW29 8-amp diodes (TO220 package, cooled)

     Alternatively, 8 x MR750 (MR7510) diodes (MR750 = 6 Ampere diodes) or 16 x 1N5401 (1N5408) diodes.

     F1 = 2 amperes

     F2 = 25 amperes

     R1 2.2k 2.5 Watts

     R2 240 ohms

     R3, R4, R5, R6 0.1 ohms 10 watts

     R7 6.8k

     R8 10k

     R9 47 ohms 1 watt

     S2 micro switch

     R10 8.2k

     C1,C7,C9 47nF

     C2: Four 4700uF/50V capacitors or one 22000uF/50V capacitor.

     C3, C5 10uF/50v

     C4, C6, C9 100nF

     C8 330uF/50v

     C10 1uF/16v

     D5 1N4151

     D6 1N5401

     D10 MR750

     D11 LED

     D7, D8, D9 1N4001

     IC1 LM317

     Two 2N3055 transistors

     P1 5k

     P3 10k fine-tuning potentiometer

     Relay = 30 volt AC, 2 x 10 amp switch