Power Supply

How to make a very simple power supply using an LM317 and a 2N3055(15A) or a 2N5686(50A).

** This text was originally made as a guide for making a power supply using a PC PSU. However I now believe that

if one wants to make a supply with higher current capabilities then the PC PSU is not sufficient.

A transformer is a far better solution. **

I always wanted to make a power supply since its as important as a soldering iron(on a lab bench).

Commersial solutions are too expensive for me. I can afford an 100euro supply(0-12, 0-3A maybe) but

why not BUILD ONE that has 0-12V 0-20A(at least) at HALF the cost. 

The finished supply can go up to 30Amps at least.


The concept is SIMPLE!!

Get a PC PSU!   

  • They are cheap(some are bad quality though) [25-30 euro]
  • Switching => light weight vs AMPS [we add some capacitors if the word "switching" makes you uncomfortable]
  • Output all the typical voltages [3.3 , 5, 12]
  • Have fuses and many other types of protection built-in 

Get a tranformer 240V to 12V or 24V  **edited

  • A bit more expensive solution
  • Less ripple will propagate through the system
  • More current More current More current!!! 
  • The user has to manually add a diode bridge rectifier and capacitors to convert AC to DC
  • No extra safety features, if you short it the wires melt!

How to make a variable one!

You need:

>> A power source. A PC power supply or a transformer+diodes+caps. 


>> A power transistor [not a darlington] such as the very famous 2N3055(15A) or the 2N5686(50A)


>> A heatsink for the transistor. There are ready ones drilled to the TO-3 package. Aim for the lowest Heat Resistance.


>> A Low Dropout Regulator(LDO). The LM317 is alright. I would suggest going for a 3-5Amp ones!

       I went for the LD1084V from ST. 1.3V dropout at 5A so its goood! 


>> Some Digital/Analog panel meters.


>> A case maybe


[optional] Add a microcontroller to do some neat things such as driving an LCD for everything,

                   rotary encoders instead of potentiometers, current cut-off, etc.





>> For the PSU to turn on. The EN wire must be connected to GND.


>> Be careful so that the case of the PSU is not touching any part of your circuit, even ground.


>> Why do we use the configuration on the LEFT instead of the one in the right?


In the event of dust going in the potentiometer(or if we overscrew in on the case) the "needle" might not touch the carbon resistive trace.

This makes PIN 2 to disconnect.

If we use the RIGHT configuration this will cause our circuit to have very high resistance[open circuit] and this will probably cause the regulator to output maximum voltage.

If we use the LEFT configuration the part's resistance will still be within limits we KNEW when designing the circuit!


>> If in doubt, put more capacitors.


>> What if the supply shuts down?

Typical PSUs have Over voltage/current and under voltage protection. 

Over current/Under voltage

If we try to draw 10Amps in bursts, the temporary voltage drop might trigger the under voltage protection mechanism.

Since I had problems like this, I added some capacitors to the output and that fixed the issues I had,