Build an electronic workbench power supply using an old PC power unit

Publishing a post after a while since I was busy with my work and some projects that were going on last few months, Anyway for some time I had an old mini form factor PC supply lying around and decided to make some use out of it by converting it to an electronic workbench power supply.

The power supply was from an old Small Form Factor Dell Optiplex 390 and the first thing I did was open it up, cutoff the plastic connecters at the ends and sort the wire strands according to their color. The small gauge yellow, orange and red wired were also sorted with the normal gauge wires.

  • Black                : 0V
  • Orange            : +3.3V
  • Red                  : +5V
  • Yellow             : +12V
  • Blue                 : -12V (at low amperage)
  • Green              : to the on switch, connected to Black (0 V) thru the switch
  • Grey                : +5V when the power supply is on. Used to power the “On” indicator LED.
  • Purple             : +5V when the power supply has power. Used to power the “Stand By” indicator LED.

Apart from these standard 3.3V, 5V, 12V outputs I’m uisng a step up/down converter (LTC3780 DC 5V-32V to 1V-30V 10A) to provide an adjustable voltage.

I’m also uisng a Voltmeter/Ammeter module with an LED display

The inbuilt voltage and current adjusting variable resistors in the board are removed and they are replaced with external variable restores with the same value that can be mounted on the casing that we use below. (500K and 200K) These come with knob.

Also, most of the time the computer power supply is turning off automatically if no power is drained through it (if other words if it’s idle). So we use a resistor to create a power drain as a solution. Here I’m uisng two resistor in parallel and they are mounted on the metal casing to dissipate heat easily.

For the casing I used an Aluminum Electronics Box (8”x6”x3.5”).

This is the complete circuit diagram, as it looks starting from the output wires from the PSU

And some steps during the build process:

A 12V fan was powered uisng the 12V line and used for heat dissipation from inside the box by cutting some holes.

This is how the unit face looked like after it’s done.

I measured the voltages against the multimeter, and they were not completely on the dot but close. These were the voltages measured from the connectors with static voltages:

3.3V –> 3.36V

5V –> 5.05V

12V –> 12.24V

-12V –> -12.43V


When the measured/variable voltage was set to 5V using the display it measured 5.1V.

As always thanks for reading!