Two types of multi-functional Homebrew 6/8Amps - 20 Ampere
regulatable power supplies

rev4.1.1

By Guy, de ON6MU

This is an easy to make power supply which has stable, clean and regulatable output voltage. By using two 2N3055 transistors we become more then 2 times the amount of amps then the power supply delivers, making it real though to brake ;). Although you could use this design to deliver 20 amps ((with almost no modifications and with a proper transfo and a huge heat sink with a fan), I did not needed such much power. Make sure you mount them on a huge heat sink, as the 2N3055 transistors can get very hot at full load.

Although the 7815 power regulator will kick in on shortcircuit, 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 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 varistat the current is limited to approx. 3 Amps @ 47 Ohm and +- 1 Amp @ 100 Ohms.

Power supply Schematic 1

Part list for 6/8 Amp regulatable power supply:

• 2 x 15 volt (30volt total) 6+- amps

• D1...D4 = four MR750 (MR7510) diodes (MR750 = 6 Ampere diode) or eight 1N5401 (1N5408) diodes.

• F1 = 1 Amp

• F2 = 12 amp

• R1 2k2 2,5 Watt

• R2 240 ohm

• R3,R4 0.1 ohm 10 watt

• R7 6k8

• R8 10k

• R9 47 0.5 watt

• R10 8k2

• C1,C7,C9 47nF

• C2 two times 4700uF/50v

• C3,C5 10uF/50v

• C4,C6 100nF

• C8 330uF/50v

• C10 1uF/16v

• D5 1N4151

• D6 1N4001

• D10 1N5401

• D11 LED

• D7, D8, D9 1N4001

• IC1 LM317

• Two 2N3055 transistors

• P1 5k

• P2 47 Ohm or 100 Ohm 1 watt * (be sure you can reach 0 ohms as any resistance limits the current)

• P3 10k trimmer

The power supply insides

Heat sink

Remember to isolate the transistors from the chassis/radiator! Use a radiator (heat sink) of appropriate size and surface area; insulating and heat-conducting spacer or at least a thin mica; hot adhesive and thermal paste.

Finished power supply:

Today's highlight!

Mark, PA4M, made a 3 ampere version by using just one 2N3055 and build into a Zetagi power supply box. He reported a very good linearity of the output voltage over the entire range.

Thanks Mark for the feedback and pictures!

Schematic 2

Part list for 20 Ampere regulatable power supply:

• 2 x 15 volt 20+- amps

• D1...D4= Bridge rectifier MB2504 (25 amps cooled)
  or eight BYW29 8 amp diodes (TO220 pinning cooled)
  or 8 x MR750 (MR7510) diodes (MR750 = 6 Ampere diode) or 16 x 1N5401 (1N5408) diodes.

• F1 = 2 Amp

• F2 = 25 amp

• R1 2k2 2,5 Watt

• R2 240 ohm

• R3,R4,R5,R6 0.1 ohm 10 watt

• R7 6k8

• R8 10k

• R9 47 Ohm 1 watt (or 4watt potentiometer for adjustable output amps, but be sure you can reach 0 ohms as any resistance limits the max. output current)

• S2 mini switch

• R10 8k2

• C1,C7,C9 47nF

• C2 four times 4700uF/50v or one 22000uF/50v

• C3,C5 10uF/50v

• C4,C6 100nF

• C8 330uF/50v

• C10 1uF/16v

• D5 1N4151

• D6 1N5401

• D10 1N5401

• D11 LED

• D7, D8, D9 1N4001

• IC1 LM317

• Four 2N3055 transistors

• P1 5k

• P3 10k trimmer

• relay = 30 volts AC, 2x10 amp switching

S2 switches between +- 3 Amps and full output current

The relay is used to switch off the power supply voltage when the mains (S1) are/is switched off. So no delay do to the discharge of C2, and so preventing output voltages from not return to zero immediately.

A MB2504 is used as it is a 25 ampere rectifier bridge which also should be cooled. Or you could use eight BYW29 8 amp diodes (TO220 pinning) mounted on a heat sink.

Mount a little heatsink on the LM317 IC

Remember to isolate the 2N3055 transistors from the chassis/radiator! Use a radiator (heat sink) of appropriate size and surface area; insulating and heat-conducting spacer or at least a thin mica; hot adhesive and thermal paste.

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