How to Make a Desktop PSU from an Old ATX PSU

A desktop power supply is an extremely handy kit for electronics enthusiasts, but it can be expensive when bought new. If you have an old ATX PSU computer, you can give it new life as a desktop power supply. Here’s how.

Like most computer components, power supplies (PSUs) are outdated. When you upgrade, you may find that you no longer have the right connectors — or that your shiny new graphics card needs a lot more power than your little old PSU can handle — a dual GPU setup can easily draw up to 1000W. And, if you’re anything like me, you have a stash of old power supplies tucked away in a closet somewhere. Now is your chance to use one of them.

A desktop power supply is basically just a way to supply a variety of voltages for test purposes — perfect for those who constantly play with Arduinos and LED strips. Conveniently, that’s exactly what a computer power supply does too — just with a lot of different connectors and colored wires.

Today we’re going to strip the PSU down to its basic needs and then add some useful sockets in case we can plug projects into it.

Warning

Ordinarily you would never open a power supply. Even when the power is off, there are large capacitors that can hold deadly electrical current for weeks and sometimes months after being turned on. Be extremely careful when working with the power supply and make sure it hasn’t been used for at least three months before opening the case, or make sure you wear heavy gear gloves when poking around in there. Proceed with Caution .

Also note that this will permanently damage the power supply, so you will never be able to use it on your computer again.

Required Components

Two 2.1mm barrels and a connector — I will power the Arduino directly. Two plug connectors will be used to make a male-to-male power cable.
Variety of 2mm colored sockets such as this one (can be used with banana plugs). You may prefer terminal messages.
Heat shrink tubing, 13mm x 1m (less if you can afford to buy more).
SPST (Single Pole Single Throw) rocker switch. I used illuminated to do double duty as a light source.
10 W 10 ohm wire wound resistor.

construction

Unscrew and remove the top part of the power supply case. You may need to remove the plug from the main circuit to completely separate the caps.

DIY Bench PSU - Separate Upper Half

These are nasty capacitors that contain a huge amount of electricity:

diy bench psu - nasty capacitors

Remove the plugs and run the wires through the hole in the case.

diy bench psu - sliding plugs

Then tie them together with cable ties to match the color to keep things a little more organized. As a general rule:

Black: ground
Red: +5V
Yellow: +12V
Orange: +3.3V
White: -5V
Blue: -12V
Purple: +5V standby (not used)
Grey: power indicator
Green: switch

do it yourself psu bench - flower bouquet

The exact power lines you choose to connect are your choice, but I chose to work with only 3 positive lines — 3.3, 5, and 12V. I will also not use purple or gray wires, instead I will connect a 12V illuminated switch.

Use HSS drills to cut appropriately sized holes in the metal — 2mm plugs and DC cylinder required 8mm holes. Secure the body with a piece of wood underneath. Making the hole for the rocker switch was a lot harder, but you should be able to use a smaller drill to cut out as much as possible and then feed the rest with a hobby drill and grinder.

diy bench psu - drill holes

Pulling the wires through the appropriate holes and soldering the connectors before inserting them into the case is probably a good idea; I didn’t do it.

DIY Bench PSU - components installed

The GND, +3.3V, +5V and +12V connectors should connect easily. Don’t forget to cut a small piece of heat shrink tubing and thread bundles of wires through it, before solder them to the terminals!

The DC plug is a bit more complicated. Since this will be used to power the Arduino, which is positive at the center, you should connect some of the yellow cables to the center pin. You may have heard that the Arduino can be powered from an external 9V supply, but the built-in power regulator actually provides 9-12V, so 12V from a desktop power supply should be fine. The stem jacks have 3 pins, but only one of which is clearly connected to the center. You should see a metal circular bit, but check where you bought it if you’re not sure. The other two pins are GND and both should be connected. Again, use heat shrink tubing to avoid accidentally joining the center and outer pins.

Power switch and indicator

The green wire acts as a power switch — just ground it to turn on the power supply. This is unlike a conventional power switch, which will actually cut off power from the source. The addition of lighting makes this the hardest part of the project.

Illuminated SPST switches should have 3 terminals: one will be marked with a different color or labeled as GND. The terminal opposite will typically be connected to 12V, then the rest of your circuit will be powered from the center pin. Turning it on will provide power to the circuit and also attract some light. However, this will not work for us. Instead, swap the GND and 12V lines. Use one 12V cable (yellow) on the colored terminal of your key switch (or the cable labeled GND). Pull the black wire (GND) to the pin opposite; and connect the green cable to the center pin.

rocker switch wiring

Now when the switch is pressed, the LED will still be on, but instead of 12V being returned to the center pin, GND will be shorted when the PWR is on, causing our power supply to activate.

Squeeze their tubes!

Finally, when the heat shrink tubing is gently pulled down to cover the switches and solder points, use a local heat gun to shrink them. This bit is actually quite interesting to watch.

Before:

diy bench psu - heat shrink tube on sockets

And after:

diy bench psu - Pipe shink - SHRUNK

Finally Fake Load

Many power supplies require a load to stay on — in this case we can use a 10W 10 ohm resistor to do the job. Connect it between the 5V (red) and GND lines. This will produce a small amount of heat, but should be fine with the fan on.