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I'd like to test a PSU using the paper clip method connecting the green wire with one of the black ones, but I wonder if I can power it on, without applying a load, or if this could damage it.

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Beware of false positive results if you test your PC's PSU without a load. If the expected voltage is not present, then there is probably a fault. If the expected voltage is present, then you have to do another test with a load on the line. Voltages present without a load do not prove that a PSU is OK. –  sawdust Nov 19 '12 at 1:50

3 Answers 3

up vote 3 down vote accepted

A properly-functioning consumer PC power supply should not be damaged by turning it on without a load. It should regulate the voltages on the various outputs to the proper levels (so you could measure them with a meter) but there won't be any current.

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This may only be true for PC power supplies, since these are bought by uninformed customers. This is not a correct answer for power supplies in general, since switch-mode power supplies typically specify a minimum load. Even if there is no damage, an unloaded PSU will typically have a voltage that is out of spec (e.g. too high), and is no guarantee that the voltage will hold when a load is applied. So it is not a meaningful test. –  sawdust Nov 19 '12 at 0:48
    
Researching, I agree that you're right and wondering if I should delete my answer. A switching supply does typically need a minimum load and some can be damaged if operated no load. For this reason, switching supplies sometimes have a minimum internal load. –  Nicole Hamilton Nov 19 '12 at 1:22
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Power "bricks" and the wall "wart" style of PSUs are examples that can be operated without a load. No need to delete (I didn't bother down voting it), but instead of posing as a general purpose answer, make it specific to the instance of the OP's situation, i.e. PC power supplies. –  sawdust Nov 19 '12 at 1:31
    
I've taken your suggestion but invite you to edit my answer to improve it. –  Nicole Hamilton Nov 19 '12 at 1:41
    
I think the original question implicitly states that its a ATX-style PSU. –  Journeyman Geek Nov 19 '12 at 1:49

Its pretty common to convert desktop PC power supplies to desktop lab power supplies, and they recommend adding a 10 ohm, 10 watt load between the 5v and ground connectors to ensure proper operation. You're unlikely to damage it from a quick test, but you could always connect an old HDD or CD rom drive to provide a load (and to check if it actually works).

Its also apparently a good idea to connect the orange 3.3 volt wire to the brown sense wire to enable the PSU to sense what voltage its giving out to self adjust.

Considering I've not seen warnings about running a laptop type PSU without a laptop attached to it, I guess those would be safe.

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Not that I know. I've powered up several PSUs manually (shorting the green lead to any black lead) without any issues. For one thing, most larger AC adapters, such as those for laptops, are switched-mode power supplies and never have any problems with no-load scenarios.

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