Instructables

Simple Computer Power Supply Tester

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.
 
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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.

Materials:
button or switch (as long as it is not momentary)
LED
2 wires (same length, however long you want it)
20 (or 24) pin connector socket
solder
zack2474 years ago
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
matroska2 years ago
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.

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.
Blackice5044 years ago
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.
lemonie5 years ago
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
cwebsterlusk (author)  lemonie5 years ago
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.

L
cwebsterlusk (author)  lemonie5 years ago
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.

L
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
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.
cwebsterlusk (author)  Padlock5 years ago
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.
cwebsterlusk (author)  stephenniall5 years ago
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.
Well it will work im just saying because it puts output doesnt mean its 100% working
cwebsterlusk (author)  stephenniall5 years ago
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.
Berserk875 years ago
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.
cwebsterlusk (author)  Berserk875 years ago
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.
No it doesn't, Pin 8 is PWR_OK, a signal generated inside the PSU to say that the voltages being made are safe. On most PSUs I have modified/messed with it guarantees the voltages are within about 1v of what they are supposed to be.
Check the "Signal Lines" section of http://www2.mmae.ucf.edu/wiki/ATX_Power_Supply_Mod
ahhhhh ok. that makes sense then.
cwebsterlusk (author)  Knucklehead5 years ago
Knuckelhead you are correct. I tested this on a power supply that i knew was overloading on voltage (actually giving 20v on the 12v line) just so i would know what would happen. The LED DID NOT, i repeat DID NOT come on. The PWR_OK signal line is 5v on a normal PSU which is enough to light up the LED but not enough for it to overheat. My LED is actually a 3.4 volt BTW.
cwebsterlusk (author) 5 years ago
You could make similar testers for the other outputs (i have in the past) and put as many LEDs on the 20 pin block as you want but you will need resistors for the LEDs on the 12v and 5v lines. However, the Power ON pin i attached the LED to is a signal line that lets your computer know the voltage it is putting out is within range. Otherwise, the LED will not light up.
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