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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, 2012 at 1:50

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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, 2012 at 0:48
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    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, 2012 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, 2012 at 1:31
  • I've taken your suggestion but invite you to edit my answer to improve it.
    – Nicole Hamilton
    Nov 19, 2012 at 1:41
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    I think the original question implicitly states that its a ATX-style PSU.
    – Journeyman Geek ♦
    Nov 19, 2012 at 1:49
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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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Just a word of caution: I just made some thick white (and stinky) magic smoke by paperclipping an old ATX power supply, with a sound effect clearly intended to illustrate runaway loops...

  1. This item was marked "OK" by a previous test (unfortunately, I didn't make a record if that test was loaded or unloaded).
  2. It has sit on a shelf unused for at least 3 years, after that test.
  3. It's an embarrassingly old (2001) & rather weak (250W) Chinese model ("Mercury").

So, what does this mean?

If this was a medical test, with the control test "what if there was some load?" being impossible to perform, the medical staff would certainly warn the other patients in similar condition, but still alive, not to do this. It may not have been directly linked to "no load", but you know the drill: avoid getting the flu if you're 90.

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Power supplies are power supplies. I've operated and built everything from linear phase to switching to class a... The load is not going to affect the psu itself in a negative way unless the voltage or current going to a specific compontent such as a capacitor grossly exceeds the spec for the component. The most problematic time this will occur is during the period when the psu is initially powered. This is called inrush current and can be quite high and quite damaging down stream of a toroid. Since all CSA approved apparatus are designed to handle this inrush spike you will have no issues running it without load...unless you have built this yourself and didn't build a buffer into the design. The only time you may have trouble is (especially with switching psu's) is if you have a low power device connected alone-there is a chance that the voltage at the terminals may be too high, and may damage the attached device. Use a voltmeter to ensure that your 12V rails don't exceed about 14.4VDC

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