Two
types of Homebrew 12/20 Ampere 13,8volt power supplies
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 will 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.
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).
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 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 330uF/35v
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 2k 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 = 2 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 330uF/35v
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
More
Schematic 2
A MB2504 is used as it is a 25 ampere rectifier bridge which also should be cooled. Or you could use 2 times four BYW29 8 amp diodes (TO220 pinning).
Mount a little heatsink on the BDX33 transistor
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.
Links of
interest:
.ON6MU
Homebrew projects
.Radioamateur related projects
.ON6MU
78h05_powersupply
.Versatile 7805 based 5Amp powersupply
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