Construction details of A Complete 13.8 Volt, 50 Amp Power Supply


Active User
Dec 29, 2012
In part-1 Ioutlined the basic needs of an high current power supply and dealt with the protection circuits needed for safe operation. In part-2 I will deal with the construction of the unit itself including PCB layout, wiring diagrams and testing.
Construction can be split up into five parts:

1. Construction and testing of the PCB.
2. Making the case
3. Pass transistors and heatsinks construction.
4. Wiring and testing of the transformer, rectifier and smoothing capacitors.
5. Final wiring and testing.

The Printed Circuit Board (PCB) Construction

Figure 1 (components side)

Most of the smaller components are mounted on a single sided printed circuit board. Figure 1 shows the component side of the PCB and the position of each component, Figure 2 (bottom) show the copper track pattern.

Figure 2 (copper side)

The problem most people have is getting the track pattern off the paper and onto the board and still maintaining a reasonable amount of accuracy. To make the PCB you will need a piece of single sided board, a printout of the copper track pattern, PCB pen and etch. The board was made using the following method:
Print the copper track pattern on paper. Clean the PCB with wire wool and then wash it in hot soapy water. Fix the printed paper on the copper side of the PCB and drill all holes. When finished remove the tracing paper and start to re-draw the pattern onto the board with a PCB pen (or your wife’s red nail varnish). Be careful when drawing the IC pads and thicken up the lines as shown on the layout. My wife’s red varnish was used on the prototype board but other colors should work just as well.
When the varnish has dried etch the board following the instructions supplied with the acid. You will know when the acid is working as the un-covered copper starts to turn pink and slowly dissolves. When the board is etched clean off the varnish with acetone, then re-drill the holes (as they are filled with varnish as well) then clean it with wire wool and the pcb is ready.
Next photo shows the component overlay of the board.


Mount and solder all the components as shown.
Take care to mount the diodes, electrolytic capacitors and the semiconductors the correct way round. You may have to enlarge the mounting holes to take RL1. Instead of one R13 resistor 2,2 Ohms 25 Watts (expensive) I have used 4 resistors 10 Ohms 5W (cheap) as you can see on top of the PCB.

Testing the Control Board

It is recommended to test the board before finally wiring it into place. To test the board you will have to supply it with about 20 to 25 volts D.C. BUT UNDER NO CIRCUMSTANCES you must use the section already wired to supply this voltage. When testing the board you will simulate several fault conditions and at the moment there is no way of disconnecting the mains when a fault occurs; failure to remove the supply will result in the possible destruction of R13 and TH1. I used a 0 to 30 volt, 1.5 amp variable power supply.

First set the preset resistors as follows, VR1 centre, VR2 centre and VR3 fully anti-clockwise. Next connect a wire between the sense output and the emitter of TR1. Bring in the supply wires from your external variable PSU and wire the positive to the “+” volts in and the negative to “0 volts.”

Turn the supply voltage on and if possible set the output to about 23 volts. The on board relay should “pull-in” and if you check the output voltage at the emitter of TR1 it should read 13 to 15 volts. Rotating VR2 should adjust the output voltage. To test the under voltage stage monitor the output and adjust VR2, when the output voltage reaches about 12 volts the relay should “dropout”. Under a fault condition the positive input voltage is almost shorted to ground via R13 and TH1 so make sure your variable supply is short circuit proof. Switch off and reset VR2 to about centre.

To test the over voltage stage set the output voltage to 15 volts using VR2, then adjust VR3 until the relay “dropsout”. Switch off your supply and re-adjust VR2 to its centre position again. Switch back on and monitor the output, start to increase the output voltage via VR2, when it reaches about 15 volts the relay should “dropout” as the over voltage protection trips in. Switch off and re-adjust VR2 to its centre position. Remove the link between the sense and TR1 emitter. This completes the testing of the PCB.

The relay has a coil voltage of 24 volts but under operating conditions it has been found that a voltage as little as 13 volts will “pull” the relay in. The operating voltage at which the relay will open or close can be increased by placing a resistor in line with the coil. The board has a link close to the relay and this can be replaced with a resistor, the value of which can be between 100 and 560 ohms. A good starting point is about 220 ohms. You may not feel that this resistor is necessary, but if you think that the relay is operating in a sluggish manner then add it. If the value is too high then the relay will not pull in when SW2 is pressed.


Enclosure for the PSU.

A power supply like this uses large and heavy parts, so physical construction should be strong. The 800VA transformer is heavy, and even the heat sink is not small. So, a good, solid, sturdy cabinet is needed. The cabinet should be very well ventilated.
The transformer, the bridge and the four heatsinks take considerable space. Measuring the dimensions of each part and making a sketch I found that a box 45 x 26cm needed.

I tried to design an appropriate box (with the help of e-machineshop) to make it out of metal sheet but I needed a sheet bender that I do not have (yet), plus all these parts are pretty heavy so I needed something stronger.


Then I thought “why shall I reinvent the wheel?” I could use something strong and readymade instead of designing a new one. What’s the most appropriate strong box I have? An old computer case!
Dimensions are ok except the height.
Some cutting and riveting later I had a strong box that I could use.

A New case strong and elegant was ready.


Plenty of space for parts! The PCB, the Rewired Microwave Oven transformer, bridge rectifier and Even a 230 V microwave fan could fit in the back.
On the front side all switches and instruments placed on a small metal panel bolted in place. I even made an opening to fit a ventilation grill salvaged from the same computer case.


Panel bolted in place, ventilation grill riveted, front side of the case ready!


To layout the main components in the case was no problem.

All 10 capacitors were soldered on two copper rings one for (+) and the other for (-)


The inner circle have the pins bent upwards at a right angle and the outer ring has the pins bent downwards at right angle also.


This way the capacitors form a circle under the outer ring and have their positive ends soldered at the outer ring and the negative ends soldered to the inner circle.


Those rings were bolt on the bottom of the case using as insulation, a thick yellow piece of polyethylene gas pipe.
All instruments were already placed and all low voltage connections were made using copper strips instead of thick wire.


Transformer, bridge rectifier and fan also secured inside the case.


Pass transistors and heatsinks unit

The bigger part of the PSU was the 4 transistors together with their heatsinks as they had to be well ventilated and isolated from the metal case.
I used a 3mm aluminum sheet to bolt on the transistors and the heatsinks. I used computer CPU thermal paste and made all wire connections.


To keep it electrically isolated from the case I used the best isolator available a 1 inch …garden hose.


The whole transistor-heatsink part fits nicely under the top lid of the case


Finally the printed circuit board was placed and connected to the transistor-heatsinks unit.



Wiring and Testing

Fig 4. Wiring of the PSU

We will start by wiring the secondary of the transformer, bridge rectifier and smoothing capacitors, but we will not be wiring the primary side yet. The secondary of the rewired microwave oven transformer produces 15.6 volts at 60 amps. Two copper strips are then taken to the A.C. side of the bridge rectifier BR1, these are then bolted on bridge connectors. Capacitors C1x to C10x must be wired in parallel and this can be achieved by welding the terminals of the capacitors together on 2 copper strips. MAKE SURE you weld POSITIVE to POSITIVE and NEGATIVE to NEGATIVE.

The positive and negative sides of BR1 can be wired to the respective sides of the capacitors. I used bolts to make the connections to the capacitors and to BR1. Remember to connect R1 across smoothing capacitor’s C1x … C10x. so when you switch off the device the capacitors will discharge through it. All the wiring for this section is shown in Figure 4. That completes the wiring of the first section and it can now be tested.

Testing the First Stage (transformer & rectifier)

To test this section you will have to temporarily connect 240v A.C. to the primary side of Transformer. First connect a voltmeter across C1 and C2, turn the power on and check the reading on the meter, this should read about 22 to 24 volts D.C. If all is well then switch off and allow the smoothing capacitor’s to discharge through R1. NEVER be tempted to discharge these capacitors by placing a screw driver across them. A word of warning, you are now dealing with 240v A.C. which if touched could kill you.

Pass Transistors

The next photo shows the pass transistors mounted on an aluminum plate where the heatsinks are bolted as well. As the whole plate-heatsink part will be isolated from the rest of the box the transistors will be bolted straight on the plate without using any isolation (i.e. mica) just some thermal compound grease (the one used to computer CPUs).
The number of needed transistors depends on the final rating of the supply and the diagram shows the four transistors needed for a 40 to 60 amp supply.


Start by marking the position of each transistor on the plate, (you can use a mounting kit insulator as a template). Drill all mounting holes and bolt all transistors to the heatsinks. Using an ohms meter check there is good connection between the case of each transistor and the heatsink but no short circuit between any transistor pin and the heatsink as the pins pass through the plate.
You can use some isolation sleeves here. I used some wire insulation I took stripping a thick wire.


Fig. 5 Heatsink with Pass Transistors and connections

Next wire the transistors as shown in Figure 5. As already stated the 0.1 ohm current sharing resistors could be made up of 0.22 ohm paralleled pairs as shown in Figure 5. Use heavy current wire for the collector and emitter wiring.

Main Wiring

You are now ready to couple together all the parts of the power supply to make one complete unit, the main wiring diagram is shown in Figure 4. A good idea here is to use colour coded wire for the mains supply i.e. BROWN live, BLUE neutral and YELLOW for the earth. If you use thick wires for the PSU, instead of copper strips, it is a good idea to use colour coded wire also to identify the voltage rails, perhaps ORANGE for the un-regulated supply, GREEN for the 13.8 volt regulated output and BLACK for negative rail.

The first stage of the supply should already be wired and working, so it is just a matter of wiring the mains input, PCB and front and back panels. Follow the diagram carefully and double check all wiring. Put insulated sleeving on both sides of R15 and F1. If you are in doubt get a friend with a little more experience to check your work.

Final Testing and Setting Up

Give all wiring a final check and if satisfied you are now ready to test the full supply. Fit the mains plug with a 5 to 7 amp fuse (10A for 115V AC). If the PCB has not already been tested then set the pre-set resistors to the following positions:- VR1 centre, VR2 centre and VR3 fully anti-clockwise. Connect a voltmeter to the output, plug in and switch on. Pressing SW2 should cause the relay to pull in and the front panel LED to light. If the relay pulls in and quickly drops out again then this could indicate that the output voltage is below 12 volts and firing the under voltage circuit. If this happens then turn VR2 up slightly. If all is well then the output voltage should read between 13 and 15 volts. To test the supply the procedure is similar to that all ready described in “Testing the PCB” but I will go through it again in a little more detail.

Under Voltage: To test the under voltage stage monitor the output and adjust VR2, when the output voltage reaches about 12 volts the relay should drop out and the mains supply switched off. Re-adjust VR2 to its previous position and re-start the supply by pressing SW2.

Over Voltage: To set the over voltage trip the output voltage needs to be increased to about 15 volts or to the level at which the trip is needed. For a 13.8 volt supply this is usually about 15 volts. First make sure VR3 is fully anticlockwise and then adjust the output voltage to the required trip level, say 15 volts. Adjust VR3 until the relay “drops-out” and the supply is switched off. Re-adjust VR2 to about its normal operating position and restart the supply by pressing SW2. Monitor the output and start to increase the output voltage again, at about 15 volts the over voltage should trip and the supply switched off.

The under and over voltage can be checked again if wished by simply turning VR2 down to about 12 volts or up to 15 volts. At each level the protection circuits should trip and turn the power supply off. Don’t forget to re-set VR2 to its normal position before re-starting the supply again. Finally set the output voltage to the required level i.e. 13.8 volts.

Current Limit: The supply can now be tested on load and the current limit set. To do this you will have to find a resistive load which can draw the required current.
To do this I used a 250 watt car kettle and a car lighter.



Each of them draw about 20 amps when supplied with 13.8 volts. If you are going to set the current limit properly you will need a current meter that can measure at least 60 amps.
Turn VR1 fully clockwise. Switch the power supply on and then connect the meter and load to the output. When the supply is drawing full load adjust VR1 until the output current drops back to about 40 amps. A suitable current meter was not available when setting up the prototype so I placed a voltmeter across the output and on full load I set VR1 so that the output voltage fell back to 13.5 volts. If you set the output to fall below 12 volts this will turn the supply off.


While I was testing the current I connected the kettle and the Ammeter indicated 20A. As soon as I connected the car lighter the PSU failed miserably. No output voltage!
I tested the first stage and it was supplying 22 Volts to the board.
I tested the board it was working as it should.
I tested the TIP 120 transistor it was ok.
Then I tested the 2n3371 power transistors. All of them had gone!!!
I was very surprised of the failure as the four transistors can provide 4 x 30 = 120 Amps peak current! There is no way they failed out of 4 x 10 = 40 Amps.

I decided to open a transistor to see the guts of it.
I had a nasty surprise while I opened it. Instead of a heavy duty power transistor, it was a fake power transistor that could not supply more than a couple of amps.



As I could not find the particular transistors locally I decided on putting four 2n3770 transistors that can handle 20A each.

After that substitution, the PSU was in order again and passed all tests successfully.

Lesson learned: DON’T TRUST e-bay sellers promising cheap parts

Short Circuit: To test the short circuit protection you will have to place a short across the output terminals. Momentarily shorting the output should have no effect but leaving the short on for more than half a second should cause the supply to shut down.

The power supply is now set-up and ready for use. Needless to mention that any equipment powered by the PSU must have its own in-line fuse as the power supply has no internal fuse of its own.
Before putting it into full operation I left the supply switched on for about two hours and every so often I drew about 20 amps using either the kettle or the lighter to make any faults show up before declare it as “READY” and give it to my friend.

A few photos of the PSU on my friends garage bench.



The prototype has been in daily use now for two months powering fault diagnosing equipment. It has been used to fast charge car batteries as well and my friend is very happy with it.

APPENDIX 1 COMPONENTS & cost (summer 2017)

1. 5PCS LM723CN LM723 DIP-14 IC Adjustable Voltage Regulator 2-37V
Price: US $1.09 Approximately EUR 1.01

2. 1 PC 100Amp 1200Volts 4 Pins 1 Phase Diode Bridge Rectifier QL100 Type
Price: AU $8.98 Approximately EUR 6.41
Shipping: AU $0.59 (approx. EUR 0.42)

3. 5PCS TIP120 120 NPN Darlington Transistors TO-220 S
Price: AU $1.17 Approximately EUR 0.84

4. 8PCS 0.1 ohm 0R1ΩJ 20 watt resistor
US $4.80 Approximately EUR 4.44

5. 10PCS 2N3771 Transistor THAT PROVED TO BE FAKE!!
Price: EUR 8.45
4 pieces 2N3770 transistors to replace the above fake ones
EUR 12

6. DC 100V 100A Dual Red LED Digital Voltmeter Ammeter Amp Volt Meter Current Shunt
Price: GBP 5.11 Approximately EUR 5.90

7. 10PCS 25v 10000uf 105°C Electrolytic Capacitor
Price: US $5.74 Approximately EUR 5.31

8. 2PCS AC 0-300V Voltage Analog pointer Gauge Panel Meter 91L16 Voltage Voltmeter
Price: US $0.99 Approximately EUR 0.92

Price: US $2.16 Approximately EUR 2.00

10. 5 PCS 250 Ohm 10Watt 5% Axial Ceramic Cement Resistors
Price: AU $1.82 Approximately EUR 1.30
+ EUR 0.28 shipping

11. 10Pcs 5/10W 10 OHM (10R) 5% Ceramic Cement Power Resistor White Through Hole
Price: US $1.27 Approximately EUR 1.17

1. EUR 1.01
2. EUR 6.83
3. EUR 0.84
4. EUR 4.44
5. EUR 20.45 (8.45 for the fakes + EUR 12 for the substitutes)
6. EUR 5.90
7. EUR 5.31
8. EUR 0.92
9. EUR 2.00
10. EUR 1.58
11. EUR 1.17
Total EUR 50,45

Thank you for reading

Last edited:


Sep 4, 2019
Hproyecto es fabuloso yo también lo he fabricado pero solamente la PCB una pregunta vr1,vr2 y vr3 de cuántos kilos vienen siendo los potenciometros saludos desde mexico


Active User
Dec 29, 2012
Hproyecto es fabuloso yo también lo he fabricado pero solamente la PCB una pregunta vr1,vr2 y vr3 de cuántos kilos vienen siendo los potenciometros saludos desde mexico
Sorry I partly understand what you write!
All details about Vr1, vr2, and vr3 are here to the schematic

saludos desde Greece


H-M Supporter - Gold Member
H-M Supporter Gold Member
Feb 1, 2015
A well thought out and executed project!


H-M Supporter - Diamond Member
H-M Lifetime Diamond Member
Feb 28, 2016
Ditto! An excellent project and REALLY WELL documented!


Jan 10, 2019
Not to rain on the parade, but more about offering alternatives: If you're building one for fun/education/a hobby, fine, otherwise, a switcher supply with the same specs is $35 on Amazon.
Last edited:


Sep 19, 2019
Not to rain on the parade, but more about offering alternatives: If you're building one for fun/education/a hobby, fine, otherwise, a switcher supply with the same specs is $35 on Amazon.
Some switching power supplies produce a lot of RF noise. particularly inexpensive ones. I think this power supply is being used for a HAM radio and the operator is probably sensitive to the cleanliness of the power supply.
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