DIY Power Supply

pc power supply tumaker
pc power supply finished tunmaker

we recondition an ATX power supply to power our electronics


Salvaging parts from computers can be very useful especially the ATX power supply as it can be converted to a lab bench power supply giving us +3.3V  +5V   +12V -12V -5V outputs with lots of power ,mine a 300W unit gives up to 30A on the 5V output which can be very useful.

Let’s gather the parts and start making our power supply , forget about batteries and cell phone chargers to power our circuits :

About The Circuit

What we need?

  • 5x Binding posts (red)
  • 1x Binding post (black)
  • 1x Toggle Switch
  • 1x 3mm Green LED
  • 1x 3mm Red LED
  • 2x 220Ω Resistor for the led’s
  • Dummy Load

Hers an image explaining the color code of the wires of a power supply :

power supply color code
Power supply color code

the dummy load on the supply output is required to keep it stable even when we only draw small amounts of current. Old power supply’s use the most amount of power on the 5v rail, so i connected the dummy load on the 5v if you have a newer power supply you’ll need to put them on the 12v rail.

Assuming the dummy load should draw at least 0.5A.

  1. Here is the calculation if you have most of your power on 5V/3,3V rail:



2. Here is the calculation if you have most of your power on 12V rail:



If you have most of your power on the 12V rail then you need to connect a 24Ω resistor to 12V instead of 10Ω to 5V.

  • 5v  => 10Ω draws 2.5W
  • 12V => 24  draws 6W
Power supply schematic
Power supply conversion schematic diagram

My Power Supply Finished


  1. Hi i have a question about the resistor.What do you think i should use on this one …….SPEC are (+3.3v -30a) (+5v-30a) (+12v1-22a) (+12v2-20) (-12 )…Would i need 1 resistor each on 3 and 5v?

    1. you need the dummy load only on the 5v output even though it will work without it its only used to protect the circuit

  2. 2 Questions, if my psu has more amps on the 12v rail and I plan to only have the PSU on while hooked up to my 3d printer do I need to worry about dummy load since there will always be a draw?

    Second question I need 2 12v connections. Can I just split the yellow 12v between 2 binding posts and use each at the same time? One source is just a few amps. The other is higher but still under my Max of the PSU. I appreciate any help you can give.

    1. for the first one the dummy is there only for protection ( when you turn off the Power Supply it acts like a charge to consume all the remaining energy left faster than with out having them) not that crucial to have them.

      for the second questions all the power can go in one or two wires i keep the wires together so they don’t heat up with all the AMPS passing through.split them in groups as many as you want just keep in mind if your drawing higher amps you need more wires.

  3. I think the green led should be in parallel with the 5ohm resistors, only in serie with its own resistor.

    1. yes you are right but there is no difference only it will dimm the a led a bit (by 2.5 ohms more) and one less wire to solder =D

Comments are closed.