atxpower supply
There are some rumurs around the web that ATX power supplies should not be turned on with no load. I have a partially unstable PSU that I want to turn it on out of PC case and check the voltages with multimeter.
Can you tell me having no load is dangerous or not? is it a myth?
It's theoretically possible. Switching power supplies (like a PC PSU) all have a minimum load requirement, typically on the +5V rail. Perhaps there was a device you forgot to plug in on the +5V/+12V rail?
Yes, I'm doing it right now. Just make sure you don't choose a board that's designed for use with a 12V wall-wart as they have a round DC power jack (like many laptops) on the back.
Best Answer
There should be no danger in turning on an ATX power supply at zero load.
That being said, the outputs may be out of regulation, hunting (overshooting and undershooting their setpoints), or absent. None of these things are 'dangerous' per the definitions of safety used by regulatory agencies like UL.
What this means is unless you apply some load, you won't be able to determine anything useful by probing it while disconnected from the PC. How much "some load" is will likely vary from model to model, but if you don't want to shell out for a full-blown PSU tester as Kruug suggested, try loading each rail with power resistors to 10% of its label-rated maximum, then test with your multimeter. If the behaviour is still bad, you may indeed have a defective power supply.