ATX PSU to bench PSU conversion

I originally posted this on my (essentially defunct) website, but I see enough search engine hits and refer back to it often enough that I’m going to transcribe it here, on my main web platform, with minimal editing:

Modding a PC PSU to a bench PSU

In order to test and run my projects, I need a regulated power supply unit. Now, to buy a lab PSU it might cost several hundred pounds, and even the type that high schools have aren’t cheap. So I’m going to utilise a cheap source of power supplies- PCs- and modify them to suit my purposes.

Materials required:

• ATX PSU (preferably scrounged from an old computer, no need to unnecessarily spend money)
• LED
• 330Ω resistor
• 10Ω, ≥10W resistor
• terminal posts (4mm sockets)
• toggle switch
• heatshrink tubing.

Removing the PSU from the PC

This part’s easy; there are usually 4 screws holding the unit to the ATX case. Unscrew these, and open the case. Trace the cables from the PSU to the motherboard, and disconnect them. It should just then just slide out of it’s compartment and come straight out. Cut the connectors off the cables, leaving about 3 or 4 inches on the connectors for reuse. You now have a PSU ready to mod.

Inside the PSU

Inside the PSU case.

Unscrew the four screws on top of the PSU, holding the case together. This stage is probably the easiest to sort the wiring out, so sort the wires into colours; I had 4 orange wires, 13 black wires, 6 red wires, 5 yellow, and 1 white, 1 purple, 1 blue, 1 grey, and 1 green. You’ll probably get about the same, with possibly a pink replacing a red, or a brown replacing an orange. These are feedback wires, so just make sure they’re always attached to the correct color, and you can safely ignore them. Tape or rubber band each colour together, so they’re easier to deal with later.

The wires sorted into colours.

If it’s an old PSU, unscrew the PCB from the case and clean it off, with a blower brush, canned air or something. Then screw it back in.

Installing the new Hardware.

In a free area of the case (check inside as well, for heatsinks, fan casing, etc), drill holes that will take the terminal posts, the LED indicator, and the power switch. Line them up first by centrepunching the holes, then enlarging them with a drill.

The finished holes

Now insert your components into the correct holes, and you should have (depending on your design) something a little like this:

The face plate

Soldering it all up

Ok, now it’s time to start thinking about those wires again. here are the colours and their purpose:

• Black: 0V (Ground)
• Orange: +3.3V
• Red: +5V
• Yellow: +12V
• White: -5V
• Blue: -12V
• Grey: Power on indicator
• Green: On/off switch
• Purple: Standby indicator

And a picture, for you visual learners out there:

Bear in mind, the 5V (red) supply is the only one that is properly regulated, if you want the others to be also, you’ll have to do that separately. There are ways to do it, but I don’t think it’s necessary for mine yet. On V.2, perhaps.

When you have them all soldered up and heatshrunk, it should look somewhat like this:

Almost ready to put the lid back on

I accidentally cut the wires too short, so I extended them using mains rated wire. This was so I could just use one wire per terminal, and it can still take any high current. You may also notice that I haven’t used all the wires of some colours. I decided that only 4 per terminal was necessary. The two black terminals are for convenience, rather than to use up wire. One black wire went to the switch, another to the two LEDs.

The power resistor

It goes between a black wire and a red wire, and provides a small load for the PSU, if it requires one to stay switched on. Fix it to the PSU case with cable ties, as it can get quite hot and the case acts as a heatsink. You could use a thermal compound for increased heat transfer, but that’s probably unnecessary. Personally I didn’t use one, and just didn’t turn the unit on until the load was connected. You may feel safer having the flexibility that the resistor gives. It’s up to you.

And now you’re done

Now you’re finished, and once the case is on, it should look something like this:

Complete!

Sources

• WikiHow
• Instructables
• Chris Shadwick

© Robert Taylor 2007

So if you don’t want to keep buying batteries for multiple electronics projects, need a constant power source for high-drain applications like electrolysis (my current reason), or whatever- this should solve the problem.

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