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My computer powered off the other day on its own, and now when I push the power button, nothing happens. My assumption would naturally be that the power supply is done (possibly well done) but is there any good way to test this before I buy a new one?

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Please tell me your Hard drives were "hot swappable"? – KronoS May 1 '11 at 22:27
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Power supplies have a short-circuit cutout which resets after being disconnected for 30 seconds or so. Disconnect the mains supply to the computer completely and wait for at least 30 seconds. Reconnect it and try again. – Majenko May 1 '11 at 23:58
I can tell you that my hard drives survived. I can't tell you that they are hot-swappable (or else I'd be lying). – Sam Hoice Nov 17 '11 at 23:27
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4 Answers

up vote 18 down vote accepted

Unplug the power supply from any of the components inside the computer (or just remove it from the computer completely).

USE CAUTION HERE (Though you'd only be shocked with a max of 24 volts)

  1. Plug the power supply into the wall.
  2. Find the big 24-ish pin connector that connects to the motherboard.
  3. Connect the GREEN wire with the adjacent BLACK wire.
  4. The power supply's fan should start up. If it doesn't then it's dead.
  5. If the fan starts up, then it could be the motherboard that's dead. You can use a multimeter to check if there is power output from the power supply.
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+1, I've done this a few times. Looks like this tallahasseelights.com/tech/fm_transmitter/fm-004.jpg – hyperslug Aug 17 '09 at 17:58
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Yup, I used to do this all the time when I worked at GEEK SQUAD. The managers didn't like it because the managers though the inside of the computer was ugly and customer's didn't want to see it. I did it anyway because it's a quick check to see if the mobo/power supply is dead and then they don't have to check in their computer for a week. – Grant Aug 17 '09 at 18:03
@ Grant - And that is why you no longer work for GEEK SQUAD... your'e too good for them. – J. Polfer Aug 19 '09 at 13:09
@sheepsimulator: Man, the stories I got from that place... – Grant Aug 19 '09 at 14:55
Awesome! Thanks, I'll check it out and see what happens. If you smell something burning, call 911... – Sam Hoice Aug 25 '09 at 16:25
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up vote 11 down vote

Most well-stocked geek-stores sell a "power-supply tester" that has all the appropiate connectors to plug each part of your PSU into, with spiffy LEDs indicating status of the various rails, connectors for IDE/SATA/floppy power cables, etc. They run ~$20 US.

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I recommend this if someone is not confortable with a multimeter or connecting wires as suggested. Otherwise, Grant's suggestion is the free way to do it without another computer to test it on. – Troggy♦ Aug 17 '09 at 18:47
It's much quicker using those, they show you in a second if all of the connectors are fine; doing it by hand takes much longer, and can be a bit fiddly with something like a SATA power connector. – Dentrasi Aug 17 '09 at 19:14
Great point! I should have guessed that there was a device for testing this. If I had this problem more often, I'd be more inclined (and I'd be sad). – Sam Hoice Aug 25 '09 at 16:25
Does that mean I should be sad for having need for one? – Adrien Aug 25 '09 at 23:45
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up vote 3 down vote

You could try using a multimeter to check the +/-5V and +/-12V rails. Due to the switching nature of the power supply, you'll want to put a resistor in series with your probes while measuring.

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Getting/learning how to use a multimeter with electronic products is a good thing to do, and part of becoming superuser-awesome. You can pick up a multimeter suitable for this work for less than a trip to McDonalds, and sometimes free after rebate. That said, the above answers are better. But don't forget the multimeter and its awesome. – J. Polfer Aug 17 '09 at 20:56
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Using a multimeter can sometimes give a false positive too. In my experiance, the most common failure mode for a PSU is the capacitors on the primary side. In this case the multimer shows the voltage to be just fine in an open circuit condition, but as soon as you draw any current, it drops to an absurdly low level. So you need a multimeters to measure voltage, and a resistance as a linear load and an accurate estimate of the loading characterstics of the mobo in order to draw any conclusions. – Vaibhav Garg Sep 1 '09 at 11:30
Typo: Multimeter – Vaibhav Garg Sep 1 '09 at 11:30
@Vaibhav - good point, I've added that to my answer. – J. Polfer Sep 1 '09 at 21:26
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up vote 2 down vote

To test the power supply unit, follow these instructions from Corsair in the Power Supplies FAQ:

My Machine doesn’t start up when I hit the power button, is the PSU faulty?

Disconnect everything from your PSU except for one single fan that should be connected directly to one of the Molex connectors. Then, get a small piece of wire, paper clip, or suitable object and short the green pin and a black pin on the 24-pin connector on the power supply. The voltage present is a very low signaling voltage so no worries of being shocked. Your PSUs fan should spin along with the fan you have connected to it. If this is the case, your PSU may not be receiving the power on signal from your motherboard and you should consider other causes of the problem you’re having.

When a computer is turned on, the motherboard electronically connects the green pin (power on signal) to any one of the black connectors (ground). This signals the PSU to start supplying power. By shorting these two connectors, you are testing the PSU to determine if it can turn on when signaled to do so. If this doesn't work, then the PSU has probably failed. More information what each pin does on an ATX power supply unit is available on Wikipedia.

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