Turning an old PC ATX Power Supply into a bench PSU

Old ATX power supplies are an excellent and cheap way of getting a power supply that can generate 3.3, 5 and 12 Volts in a stable form and with high currents. It’s also really easy to convert them into bench power supplies or to power anything that requires 3.3, 5 or 12 Volts. You can either obtain them from an old PC you have lying around, or one that a business is throwing out. Alternatively, they can be bought really cheaply from places like eBay.

Before you open up the PSU unplug it and leave it unplugged for several days prior to opening up the case. Yes, you heard me right. The power supply has great big capacitors inside that can store charge for a long time after you disconnect the power. Fail to do this and you risk getting a nasty jolt. Don’t say I didn’t warn you. If it’s a brand new one or been unplugged for some time then you’ll be fine. However, I take no responsibility for you getting a shock. Take precautions first.

Open up the case. Now chop off all of the plastic connectors. Now, the red wires are 5V, the yellow are 12V, the orange are 3.3V and the black is ground. You also have -5V on the white wire and -12V on the blue wire. If you wish to use these then fine. Check out the WIkipedia entry for ATX for the full diagram of outputs and wire colours: http://en.wikipedia.org/wiki/ATX

You will also need the green wire. This is PS_ON and needs to be shorted out by connecting it to one of the blank (ground) wires. Shorting the green wire powers up the device (when plugged in and turned on).  Alternatively you could connect this via a switch.

Optionally you can also use the grey PWR_OK cable to power up an LED (via a current limiting resistor and ground) to show the power is on.

For my conversion I used banana plug sockets. Banana plugs are usually rated for at least 10 amps which is more than enough for my usage but check the ratings for your own specifications. Drill some appropriately sized holes in the case for your banana sockets and push them through.

Bundle as many of the red wires together as you can and solder to the base of a red banana socket. This will be your 5V output. Any red wires unused can be cut off as close to the circuit board as possible.

Do the same with the yellow wires for 12V, orange wires for 3.3V (I didn’t require these on my PSU as I am using it to power my astrophotography equipment which only needs 5V and 12V. I did however leave some orange wires inside, just in case.) and then black wires for ground. If you wish you can also use the white for -5V and blue for -12V.

Optionally solder a switch and LED to the grey PWR_OK wire. Also solder the green PS_ON wire to ground.

Make sure all of your connections are solid and then wrap electricians tape around any exposed wires. Screw your case back on and power up. Voilà, you now have a stable bench PSU able to provide nicely regulated voltages at high current. Check the outputs with a multimeter prior to use.

I’ve heard of people building the outputs into their desk for easy access, which is a great idea if you do a lot of electronics. My conversion is going to be used to power my astronomy equipment such as the motorised mount, filter wheel, camera, dew heaters, USB hub and so on. I can get rid of the numerous power supplies I need to power these and use this single PSU for them all. As I have used banana plugs, which can be daisy chained by inserting into each other, I can power as many 12v or 5v devices as I wish from the one PSU.

The following advice about this project has been offered from Brad Levy:

Using an ATX supply as an inexpensive powerful lab supply is a good idea, but there are a couple of things you need to watch out for.

Many ATX power supplies are not designed to be operated with less than a certain minimum load. If run with a load lighter than that minimum load, the supply may shut down or may output voltages outside the regulated range. The minimum load specification varies depending on which version of the ATX spec the supply adheres to.

The early ATX supplies (circa 1995) had a minimum load requirement of 1 amp on the 5V line and 0.3 amp on the 3.3V line. This can be achieved by loading the 5V line with a 5 ohm 5 watt power resistor, and the 3.3V line with a 10 ohm 2 watt power resistor.

The ATX12V spec v1.1 (circa 2000) specifies a minimum load of 0.1 amp on the 5V line and 0.3 amp on the 3.3V line. This corresponds to a 50 ohm 1 watt resistor on the 5V line, and 10 ohm 2 watt power resistor on the 3.3V line.

The ATX12V spec v2.2 (circa 2006) specifies minimum load currents of 1 amp on the +12V line, 0.3 amp on the 5V line, and 0.5 amp on the 3.3V line.  A 12 ohm 16 watt power resistor (such as TE Connectivity p/n 7-1625966-9 ) would suffice on the +12 line, with a 16 ohm 2 watt resistor on the 5V line and a 6.2 ohm 2 watt resistor on the 3.3V line.

For other versions of these and other supplies, check the supply manufacturer’s specification for the minimum load ratings.

I’ve rounded down resistor values and rounded up the resistor power ratings above to give a little bit of margin and reasonable life. Don’t forget that a 12 ohm 16 watt power resistor is going to get warm in the continuous operation, so should be mounted in air flow.

The other caution is to add a fuse or circuit breaker in series with each of the output jacks.
The ATX supplies are designed to be able to provide a lot of power – 20 amps or  more on the 5 volt line, for example, which is likely far more than your intended load. This is enough to allow a miswired circuit being powered to seriously overheat or start a fire. (Think how hot a 100 watt light bulb gets – some of the ATX outputs can double that power.) You should use a fuse or circuit breaker value in comparable to a little more than your expected maximum load.


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