Two types of Homebrew 12/20/30 Ampere 13,8volt power supplies

revision 3

By Guy, de ON6MU

This is an easy to make power supply which has stable, clean and protected output voltage. The overal dimensions can be kept (relative) small by using TO220 darlington BDX-33 transistors. Using 3 BDX-33 darlington transistors is almost 3 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. Second reason was the size of the alu box I happen to have spare HI. There was simply not enough room for the transformer, and surely not enough space to mount a huge heat sink, as the BDX33 transistors can get very hot, and they do not like that so much.

Although the 7815 power regulator should kick in on shortcircuit, overload and thermal overheating, I build in a very simple secondary overvoltage protection that's made out of 12 volt relay. The rectified voltage of: 15 volt x SQR2 = 15 x 1.41 = 21.15 volt measured on C1. This is the voltage that could be on the output if one of the transistors should blow. We need a little calculation to get the exact voltage (or higher) to power the 12volt relay which should disconnect the output. In this example we use for diode Zd 9v/5watt -> 21v - 9v = 12 volt. To allow the relay to disconnect the output on lower voltages, use a lower voltage for diode Zd. You could use a different voltage relay too, but diode Zd should be calculated to allow the relay to work just when the output voltage rises over 16 volt + (Zd in the schematic).

P1 allows you to 'trim' the output voltage exactly to 13.8 volts.

Remember to isolate the transistors from the chassis/radiator! This is very important! 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. Use thick wires.

Just to be sure to prevent HF entering (or going back to the mains) use a ring core to turn the mains a few times around it (see insides pics).

Revision 3
. Zd was wrongly connected after the relay switch instead of before
. C5 changed to 330uF to improve ripple rejection and stabelisation
. primary side of the transformer added 250v/2n2 decoupling cap
. P1 = 500Ohms or 1k trimmer is sufficient

Power supply Schematic 1

Part list for 12 Amp BDX33-based power supply:

• 2 x 15 volt 6+ amps

• 2 times two MR750 (MR7510) diodes (MR750 = 6 Ampere diode) or 2 times 3 1N5401 (1N5408) diodes.

• F1 = 1,5 (2) Amp

• F2 = 15 amp

• R1 2k2 1 Watt

• R2 10k

• R3 1k 0.5 watt

• R4,R5,R6,R7 0.1 ohm 10 watt

• R8 4.7

• R9 6k8

• C1 two times 4700uF/35v

• C2,C5 330uF/35v (revision 2: C5 = 330uF -> improved ripple rejection and stabelisation)

• C0',C3,C4,C6,C10 100nF

• C7 330uF/25v

• C8 47nF

• C9 47uF/25v

• D1 1N5401

• D2 LED

• D3, D4, D5 1N4001

• IC1 78L15

• relay 12 volt 2x5 amp switching

• 3 darlington transistors: T0,T1,T2 = BDX-33 NPN TO-220 transistor

• Zd 8 or 9 volt, 5 watt

• P1 1k trimmer

If using a bridge rectifier (like in schematic 2) you do not need 2 x 15 volts 6 amps, but 1 x 15 volt 10+ Amps

Part list for 20 Amp BDX33-based power supply:

• 2 x 15 volt 12+ amps

• 2 times 3 MR750 (MR7510) diodes (MR750 = 6 Ampere diode) or 2 times 5 1N5401 (1N5408) diodes.

• F1 = 3,18 Amp

• F2 = 25 amp

• R1 2k2 1 Watt

• R2 10k

• R3 1k 0.5 watt

• R4,R5,R6,R7 0.1 ohm 10 watt

• R8 4.7

• R9 6k8

• C1 22000uF/35v

• C2, C5 330uF/35v (revision 2: C5 = 330uF -> improved ripple rejection and stabelisation)

• C0',C3,C4,C6,C10 100nF

• C7 330uF/25v

• C8 47nF

• C9 47uF/25v

• D1 1N5401

• D2 LED

• D3, D4, D5 1N4001

• IC1 7815

• relay 12 volt 10 amp switching

• Four darlington transistors: T0,T1,T2,T3 = BDX-33 NPN TO-220 transistor

• Zd 8 or 9 volt, 5 watt

• P1 2k trimmer

If using a bridge rectifier (like in schematic 2) you do not need 2 x 15 volts 12 amps, but 1 x 15 volt 20 Amps

The power supply insides

20/22 Ampere or 30/32 Ampere 13.8 volts power supply
de ON6MU

Power Supply Schematic 2 (new design)

Remember to isolate the 2N3055 transistors from the chassis/radiator! This is very important! 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.

Specs

• Heavy duty power supply 13.8 volt, 20 or 30 amps continue

• low ripple

• short-circuit protection

• HF-immunity

• Voltage can be set between 12,3 and +/- 15 volts

• only 0.35v drop at full load

• parts widely available and calculated way over the maximum load

Of interest

Parts (30 amp version in blue)

• transformer capable of delivering 20 amps @ 15volt (30 amps)

• 4 x 2N3055 (6 x 2n3055)
  Use a large 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.

• IC 1: 7812 (small heatsink)

• D1: MB2504 is used as it is a 25 ampere rectifier bridge and should also be very good cooled. Or you could use 3 times four BYW29 8 amp diodes (TO220 pinning, cooling).

• D2,D3: 1N1004

• D4: 1N5401

• C1: 22000uF (+ 10000uF) 35volts

• C2: 47nF

• C3: 100uF/35volt

• C4: 100nF

• C5: 4.7uF/35volt

• C6: 4.7uF/35volt

• C7: 100nF

• C8: 220nF

• C9: 220uF/25volt

• C10: 47nF

• C11, C12: 100nF

• R1: 2k2 / 1W

• R2: 10 1/2W (2,2 1/2W)

• R3: 6,8 1/2W (2,2 1/2W)

• R4': 1 Ohm 1/2W

• R5': 0.1 Ohm/10W

• R6: 2k2

• R7: 10

• R8: 2k2

• R9: 22

• R10: 1k

• R11: 10

• P1: 500

• F1: 2A (3.18A)

• F2: 22A (35A)

P1 allows you to 'trim' the output voltage exactly to 13.8 volts.
Just to be sure to prevent HF entering (or going back to the mains) use a ring core to turn the mains a few times around it (see insides pics).
Be sure to use thick braid wires!!! They need to handle 20 (30) amps continues!

Total revision changes:

. highly improved voltage stabilisation
. output voltage feedback circuit stabilisation
. BDX-33 removed
. Cap after 7815 changed from 1uF to 4.7uF
. Resistors before the input of the 7815 changed
. reverse diode (between collector and emittor of 2n3055) removed
. voltage can be set exactly to 13.8v (P1)
. minor HF-immunity changes

Links of interest:
.ON6MU Homebrew projects
.Radioamateur related projects

.ON6MU 78h05_powersupply
.Versatile 7805 based 5Amp powersupply

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