Simple Computer Power Supply Tester

Introduction: Simple Computer Power Supply Tester

About: BHP High Class of '08 Broadcasting Major

This instructable is a quick guide to build a 20 pin computer power supply tester out of parts from old computers and PSUs. The tester will also work on power supplies that have the 20+4 pin connecter. You can use this method to make a 24 pin PSU tester as well. Similar units sell for around $15-$20 but you can make one for pennies if you have the parts laying around like I did. The inspiration for this came when my friend gave me his old dead tester after he bought a new one.

Step 1: Gathering the Materials

This instructable is meant to help save you money and keep people from wasting resources. All the parts I used (except the heat shrink) came from an old computer. This is just one of many things that can be built entirely from recycled components. I intend to use every piece of material from this pc. Every computer that goes bad i take apart and strip out all the working components before sending the rest off to be recycled or disposed of properly.

Be careful de-soldering! If you heat up the pins in the power connector too much it will melt and deform the plastic. I have unwittingly done this before.

button or switch (as long as it is not momentary)
2 wires (same length, however long you want it)
20 (or 24) pin connector socket

Step 2: Finding and Removing the Unnecessary Pins

For this instructable you only need 4 of the 20 pins on the MOBO power connector.

The pinouts for 20 and 24 pin connectors can be found here and are very helpful:
20 pin -
24 pin -

The only pins we will use are (on the 20 pin) pins 7 and 8 which are Ground and Power OK respectively, and pins 13 and 14, Ground and Power On. The others can be removed by pushing them up through the bottom. Don't throw the unused pins away yet. If you mess up you may need them.

From here on I will be referring to the number of the pin for 20 and 20+4 pin connectors so if you need a 24 pin look up the pinout on the link.

Step 3: Attach the Switch

The switch you will use to turn on the power supply is soldered to the two wires. After attaching the wires to the switch, attach them to pins 13 and 14 of the connector.

Step 4: Attaching the Indicator LED

Finally, the LED is soldered onto pins 7 and 8. Make sure that the positive side of the LED is on pin 8 and the negative side is soldered to pin 7, Ground. After i soldered it i bent it upward so it wouldn't catch on anything and mess it up.

Step 5: Test Your Tester

Now that you have a completed PSU tester in your hand, my suggestion is to use it first on a power supply that works. Unhook everything from the power supply first (except the power cable of course). Take sure the switch on your tester is in the "OFF" position and attach the tester. Once it is attached, flip the switch. If your LED lights up, you have a working power supply! If your power supply has a fan in it and you notice the fan is spinning but the LED isn't lit, you have placed the LED on the wrong pins or it is poorly soldered (or you have a faulty LED).

And there you have it, a computer power supply tester made from recycled computer parts!



    • Metalworking Contest

      Metalworking Contest
    • Fix It! Contest

      Fix It! Contest
    • Creative Misuse Contest

      Creative Misuse Contest

    22 Discussions

    bookmarked. im gonna need one of these as i have suffered from a bad psu without even knowing it, and it will save lots of time, money, and frustration down the line

    The idea here is very good, and the environmental consideration is also excellent. However I would like to add that a more useful power supply tester would test also the voltages rails (as already mentioned in the comments).

    I would have done the following:
    Grab connectors from old PC. These must include: one female 4-pins molex, one female SATA power connector (from a dead SATA drive), one 6-pins molex connector (salvaged from old GPU with such a connector) and of course a 24 pins ATX power connector.

    Each of these connectors carry the same voltages, so along with testing the PSU rails, you will also be able to test if either a cable of connector itself is not working.

    The next step is to make a small "circuit" that collects all the similar voltages from each connector and joins them together, so that you end up with one +12V, one +5V, on +3.3V and one -12V rail. To these, hook up a resistor of appropriate value (to drop the voltage enough) so that the LED you will add does not burn. This will only however not test the PSU under load; it will merely indicate that the rail is providing right now the correct voltage (assuming the LED is lit).

    In order to truly test a PSU, you must measure the voltage on each rail of the power supply, when it is under load (important). Therefore, instead of a resistor + LED like in the previous step, I would recommend taking a high power resistor feed by the power supply. Then, make terminals (usually, simply a bare wire) so that you may easily measure the voltage using a multimeter. Be careful however about the resistor values - for example if you want to test it under a load of 25W, for the 12V rails, you would need a resistor of about 5.8 ohm, which will draw a current of about 2 amps, which is kinda a lot. The resistor will also get very hot - this has to be expected.

    Keep in mind, this (very) long comment should NOT be used as a straightforward guide to make a PSU tester, rather it should only give you the basic idea behind it. A computer PSU can provide a HUGE amount of power, which can be very dangerous if the tester is not made properly. I will probably make a proper instructable about eventually, including safety aspects of it.

    Hope this can help anyone out.

    1 reply

    One a side note, what could be a really interesting device is a power supply stresser. This would be some kind of huge load absorber on which to simply unload the PSU, to check if the PSU is able to provide the wattage it is advertised.

    To make such a device, all you need is Ohm's Law and a couple of (high) power resistor. Also, plan something to dissipate an immense amount of heat (about half of a toaster) because those thing will get HOT.

    Now let's say you bought a cheap PSU that's rated 400W. Let's assume one of the12V rails has maximum of 20 amps. According to Ohm's law, you would need a 12/20=0.6 ohm resistor to have a current of 20 amps. That's 240W, that's A LOT. But if you can have your hand on high power resistors and make a combination that is around 0.6 ohms, you could seriously test if the PSU would handle it. Again I don't expect anyone to do if you have no experience with this. Those who can do this will already know how just by reading this, the others shouldn't bother.

    Many cheap PSU available for very cheap are of that kind, and the wattage advertised is really nonsense. Sometimes, the makers imply that the advertised wattage really is the peak wattage of the device, which is useless.

    This is a good page where a qualified dude explains what happens with cheap power supplies and what's different in a good one. He also blows one, by merely loading it with something _lower_ than the rated value.

    this is a cool thing to have but i have seen psu where they work and say for example the 5volts is not but 12 and 3.3v and -12 work fine. maybe some more leds with some risistores to test all the rails would be a great upgrade to this. have different colors for each voltage rail. nice work.

    Have you got enough LEDs to modify this to test any of the other outputs? Most people would pop a resistor in there somewhere - I assume it's a 12V supply you're testing? L

    4 replies

    i have enough LED's but since i used the signal line which only works when the voltages are correct you dont need to put more LEDs on it. It is a 12v power supply but the PWR_OK signal line is only 5v on a standard psu.

    (You'd think the manufacturers could put an LED in the PSU...)
    Thanks for the information.


    Yeah, you would think that. But then again, the people who make the PSUs also make the testers so thats more money for them. You're welcome. i find this little thing quite useful.

    It happens often enough (event happened) "and now it doesn't work, please help?" - so this would be an easy way to eliminate one suspect.


    As berserk said below me Yeah it powers on BUT the wattages have to be correct etc for it not to blow a capacitator in the board and f*ck up your Video card I fix pcs for people and this is a good standard one but needs to be more advanced lol

    5 replies

    The wattages do not affect anything on the computer board, as long as they are sufficient enough to power anything. It could be 3000 watts, or 300; the motherboard only consumes as much as it needs. What really matters is the voltage, not the wattage.

    Thats what i was gonna say but i assumed stephenniall meant voltage not wattage. Voltage testing is what this really does, it just says it comes on and the voltages are safe.

    I work on them for a living too and all i know is that the simple testers you can buy work in the same way and so far i havent gone wrong with my DIY one. The PWR_OK signal line is how the motherboard tests the power input no naturally you should be able to test it in the same way with an LED. I wouldnt have posted something that i have had problems with or that didnt work like i say it does. I dont have all the answers (go see if you can find some from a manufacturer who makes these little things like Antec but i have had no luck) but i do know that it works and i have been using mine for well over a year with no harmful effects.

    That is true, but this is just a quick little tool meant to make sure the thing will at least run. It has saved me lots of time and money.

    um.......seeing the led light up isnt a good indicator of a working power supply. that voltage just has to be above 3V and it would light up. im HOPING thats not how the store bought tester works.......... a better way would be with 1kOhm resistors across the connections and measuring them with a voltmeter.

    4 replies

    As i said, the Power ON line is just a signal indicating that the voltages are correct to be working. If they are not, you wont get a green light. And yes, the one from the store works on the same concept because it is also attached to the Power ON signal.

    that led lighting up tells you nothing about the voltages exept that your getting more than 3V, i think for your led its even lower than 3V. that voltage could be bouncing between voltages and that led would still be lighting up, or you could be getting 3 times more voltage than your supposed to be getting and it would still light up. all that led shows is that youve turned your power supply on.