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Testing a Power Supply

Posted on 2015-01-13
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Last Modified: 2015-01-14
Any advice on how to test a power supply for a desktop?
To check if all the cables are delivering the proper voltage?
Sometimes everything looks OK with a voltage meter but then you put a load on the Power Supply and it fails.
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Question by:scottjnorris
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by:John Hurst
ID: 40547981
As you note, you need to test a power supply under load.

Look at the power supply connections on the board and see if you can find a measurement point for each power supply lead.

Then turn on the computer and see if the supply is putting out the required voltage at each lead.
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by:Dave Baldwin
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I just took a look at the Power Supply Testers listed on Amazon.  While they may be helpful, they are not 'load testers'.  Most only put the load of a single LED on each supply line.  That is not enough because a voltage may be too low to run the PC and still turn the LED on.  The last time I built a 'load tester' for a power supply, I used resistors that were rated for 25 to 100 watts on a metal plate.  I should re-wire that to test PC power supplies.
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by:garycase
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ID: 40548260
Power Supply testers indeed don't do a good load check; but they do confirm that the rails are reaching stable output with a reasonable length of time, and measure the time to a "Power Good" signal ... which is a key indicator of whether or not a power supply is okay.    If the power-good time is outside of spec, the unit will indicate a failed PSU ... and it will also alarm if any of the power buses is outside of the correct range.

Basically, if a PSU tester says a unit is bad, it's bad;   but the converse isn't true -- a "good" power supply may or may not actually be good under load.    For that you need, as already noted, to actually measure the voltage when the system is drawing a load.

I've often wondered why they don't make "in-line" testers ... i.e. a tester that provides  Y-cable adapters for the key ATX connections, so both the tester and the system could be connected at the same time.    this would let the tester measure the outputs under real-world loading.
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by:garycase
garycase earned 800 total points
ID: 40548263
... Note:  If you DO want to get a tester as a confidence test for power supplies, a fairly simple inexpensive unit like this is fine:  http://www.newegg.com/Product/Product.aspx?Item=N82E16899705002&cm_re=power_supply_tester-_-99-705-002-_-Product

The more expensive units just add fancier LCD display outputs ... they still don't test under load.
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by:nobus
ID: 40548376
i keep old mobo's around for this
just connect both, and a display, and you see if it actually boots into bios
you can also connect more loads; disk - cd drives etc..
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by:rindi
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ID: 40548599
All those PSU's that still output voltage, but were still bad which I have seen had one reason for the failure, and that was bad electrolytic capacitors. As you can identify those bad caps easily by just looking at them (they usually show bulges or cracks, or are leaking), all you need to do is to remove the PSU's cover, clean out the dust so things get visible, and look inside...

If there is no output voltage at all, it can be other causes, but just to measure the voltages you only need a volt meter.
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nobus earned 400 total points
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