0-28V 6-8A Power Supply (LM317, 2N3055)

This is an easy to make power supply which has stable, clean and regulator 0-28V 6/8 Amp 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 transformer 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 LM-317 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.

Note:
The collectors of the finals needs to be soldered with a wire all together if the transistors even if they are isolated from the heatsink or not. If you do not isolated the finals from the heatsink, then please make sure the heatsink does not make contact with the chassis (metal casing where you plan to build the PS into).
Remember to use thick wires suitable for transfering the current needed according to the power supply you tend to build.

Part list for 6/8 Amp regulatable power supply (PSR28A68):

2 x 15 volt (30volt total) 6+- amps

D1...D4 = four MR750 (MR7510) diodes (MR750 = 6 Ampere diode) or 2 x 4 1N5401 (1N5408) diodes.

F1 = 1 Amp

F2 = 10 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

C11 22nF

C2 two times 4700uF/50v

C3,C5 10uF/50v

C4,C6 100nF

C8 330uF/50v

C10 1uF/16v

D5 1N4148, 1N4448, 1N4151

D6 1N4001

D10 1N5401

D11 LED

D7, D8, D9 1N4001

IC1 LM317

Two 2N3055 transistors

P1 5k

P2 47 Ohm or 220 Ohm 1 watt * (be sure you can reach 0 ohms as any resistance limits the current)

P3 10k trimmer
Calibration:
- Get your hands on a calibrated digital meter or a good analog meter and measure the voltage at the output of the power supply.
- Turn P1 to maximum (maximum voltage of our power supply).
- Adjust P3 till the meter needle shows maximum result (end scale)
- If you want to calibrate the scale, turn P1 to several voltages (like every volt) and confirm each time with your calibrated voltage meter. Make a mark on your power supply meter-scale to calibrate the meter.
- You should see equal spaced voltage marks on your home-made scale if your meter is a linear type.

Less maximum output voltage needed?
In operation, the LM317 develops a nominal 1.25V reference voltage, VREF, between the output and adjustment terminal.
The reference voltage is impressed across program resistor R1 and, since the voltage is constant, a constant current I1 then flows through the output set resistor R2, giving an
output voltage of

Since the 100 uA current from the adjustment terminal represents an error term, the LM317 was designed to minimize IADJ and make it very constant with line and load changes.
To do this, all quiescent operating current is returned to the output establishing a minimum load current requirement. If there is insufficient load on the output, the output will rise.

Less amps needed?
Well, without much modification you can:
- only one 2N3055, will give you 4...5 amps.
- the bridge rectifier (D1...D4) only needs 4 x 1N5401 (any +/- 3 amp diodes as only halve of the max. amp is needed, so we have some room when short-circuited)
- one 4700uF (C2) is sufficient
- F2 = 6 amp
- D5, D10 = 1N4001

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:
Note: You can add a 3th 2N3055 and use for R9=27 Ohm, to make a 10/12 ampere power supply of the design stated above.

[Scema-Electronic]