Four types of Homebrew 12/20/30 Ampere 13,8volt power supplies:
RE PSF14A12D, PSF14A20D, PSF14A20 and PSF14A30

Homemade 20 ampere power supply
RE-PSF14A12D
revision 4

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.
It is obvious, but I would like to mention that you could make this power supply with less BDX-33 transistors if you do not need high power.

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).
Remember that the relay needs to be able to switch 12 amps (or more). If the relay offers multiple switches then please use them. The more the better (also less resistance hence voltage drop when loaded).

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
. reversed diode over IC1 removed

RE-PSF14A12 Power supply Schematic 1

homemade power supply using BDX-33 transistors

Part list PSF14A12D
12 Amp BDX33-based power supply:

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 PSF14A20D
20 Amp BDX33-based power supply:

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
My Homebrew powersupply inside

Homemade power supply

 

 

A simple temperature controled fan:

20/22 Ampere or 30/32 Ampere 13.8 volts power supply
RE-PSF14A20 or PSF14A30
de ON6MU

 

RE-PSF14A20
Power Supply Schematic 2 (new design) revision 2014

 

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.

PSF14A20 Specs

Of interest

PSF14A20 Parts (30 amp version PSF14A30 in blue)

P1 allows you to 'trim' the output voltage exactly to 13.8 volts.
Just to be sure to prevent HF entering through the mains use a ring core and 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
. reversed diode over IC1 removed

Revision 2016:
. R10 changed 1k5, and P1 to 1k (thanks Goran 9A6C)
Goran reported that he could not reach 13.8 volts using a 500 Ohm potentiometer(P1) parallel with the 1k resistor(R10).
Replacing P1 & R10 with fixed resistor of 680 Ohm would give approx. 13.6 volts.

Revision 2017:
. added ampere meter without using the meter in series (P2 trimmer to calibrate the meter that will be used to measure the amps)

 

Overvoltage protection:

A crowbar circuit is an electrical circuit used to prevent an overvoltage condition of a power supply unit from damaging the circuits attached to the power supply. It operates by putting a short circuit or low resistance path across the voltage output (Vo), much as if one were to drop a crowbar across the output terminals of the power supply. Crowbar circuits are frequently implemented using a thyristor, TRIAC, trisil or thyratron as the shorting device. Once triggered, they depend on the current-limiting circuitry of the power supply or, if that fails, the blowing of the line fuse or tripping the circuit breaker.

 

Pictures of people who made the PS
This is how Dan, YD1BWB made it:
click on the images to enlarge

Thanks Dan!

Links of interest:
.ON6MU Homebrew projects
.Radioamateur related projects
.ON6MU 78h05_powersupply
.Versatile 7805 based 5Amp powersupply

 

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