Quick build: DIY bench power supply from an old computer’s PSU

I have taken apart several old computers this weekend and so I had several old PSU laying around afterwards. I opened one up that was known to be faulty, that’s why I got the computer for free. So I took a quick look inside and several caps were leaking. I decided not to throw it away, instead I repaired the PSU and gave the internals a new life. I actually had this idea years ago but I didn’t have a spare PSU I could take apart. Here is a rough guide on how to build your own power supply for almost no cost at all!

Quick warning — Read!

Working with a PSU can be dangerous if you don’t know what you’re doing. Several parts inside the metal case can hold a deadly charge for several hours after disconnecting the PSU. Don’t touch any of these! You’ll have to release the charge, from example from the big chunky caps inside the PSU, by shortening out the pins with a metal object. Just make sure that the metal object has a non-conducting piece where you can hold it.

Simply don’t try this at home if you are not sure what you’re doing. You’ve been warned.

Get a PSU

Simply get one. Everything will work just fine, it’s not even necessary that it’s from an old computer, but they are handy because they have 5V and 12V rails and usually have a relatively high power output. Mine is rated 350W.

Now take a look at the sticker on the PSU. Most of the time they look like one of these:

Figure 1: The model I used. I marked the P-ON line with a red square.

Figure 2: Another sticker, just so you can see a different type with multiple models listed

Figure 1 has only one model listed, figure 2 has multiple models listed in the chart. Look for a table column labeled “P-ON” or “PS-ON” or similar. You’ll have to pull this line low, to activate the PSU, otherwise it won’t turn on, even if it’s connected to power. All the PSUs I’ve seen in my life had a green wire that needed to be connected to ground, but this can vary from manufacturer to manufacturer. In fig. 1 and 2 it’s the green cable.

Disassemble it

Most of the time the PSU is held together by 4 screws, either at the bottom or at the top of the case. If it has fans in it, these are usually hold in by separate screws. Open up the metal case and be careful not to touch any delicate parts on the inside, as they can hold their charge for a long time, even after they are disconnected from the mains voltage.

After opening up the case, you should see something like this:

Yours might look different, depending on the model you have. But the basic structure remains the same. Usually you should see a big heatsink, an optional fan, a big chunk of wires and big capacitors (located at the top in fig. 3). Make sure to short these big caps, to discharge them.

To discharge them, unscrew the logic board and take it out of the case, make sure not to touch the caps (I usually try to avoid the bottom side and pick it up by the cables and turn it around). Now shorten the pins of each cap individually. You should hear a zapping noise as they discharge. After you’ve done this, it should be safe to work with the board.

Now just desolder all the wires (except for the green one). But before you do so, take a look at the board. Most boards have the areas labelled in which the cables are soldered in, so you know what voltage is delivered by each group of cables. If your’s is not labelled, try to write it down somehow.

After you’ve done this, solder a resistor to the green wire, put a heat-shrink tube above it and connect it to ground. It basically doesn’t matter what resistor you use. If you want to, you could also wire this line to a push button at the front of your case, so you can give the power supply a manual power-on-signal:

Get the case ready

From here on you have two choices: You can either keep the metal case or you can buy a new one and fit all the electronics in it. I decided to go with the second option. Either way you’ll have to thing about the outputs you want your power supply to have. I decided to get four 5V, two 12V and one GND output that can be daisy-chained.

After you thought about your outputs get some female banana-plug connectors and mark the spots on your case where you want these to be. Afterwards you’ll have to drill the mounting holes for the connectors.

If you decided to go with a new case, disassemble the original case completely. So get the fan, the power-in and the power switch out and put them aside. You’ll obviously also need to make holes for these in your new case.

Mount all the banana-plug connectors and solder in the cables from your PSU. Just use whatever configuration you need. If you only want 5V outputs, just use these. You can also mix them. Just be aware that there are limitations (see the sticker on your case) and also make sure you use cables and plugs that are rated high enough, so they don’t get damaged.

My build looks like this from the inside:

Note that my PSU didn’t use a fan. Unfortunately I also had to use a lot of hot glue, to hold the board in place. Only one edge could be secured with a screw. The hot glue isn’t necessary on the connectors, but I added it for extra strength.

I also think it’s good practice to keep all the cables organised after you tested the functionality.

Put the case together

At the end the power supply looked like this:

Unfortunately I messed two holes for the connectors, so it doesn’t look that nice, but it’ll do the job just fine. The upper power supply is the one that I usually use for testing. I added it as a size reference.

I swapped out the default switch with a button that lights up, when the power supply is turned on.

Conclusion

This was a really quick and cheap build. It took me around 2 hours and the only thing I had to buy was the case (15€). And I really like it, because it can output a much higher current than the power supply I already had.

So if you have some PSU laying around, two hours and 15€ to spend and are experienced/smart enough not to kill yourself during the process, you should absolutely try this out. It’s economic and cheap!