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331 posts

Ultimate Geek


# 139481 10-Feb-2014 21:50
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If the -5v light on my pc psu tester doesn't light when psu is plugged in and switched on, does that indicate a faulty psu?
All other lights are green.


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3385 posts

Uber Geek


  # 983720 10-Feb-2014 22:06
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When i used to work in a repair shop, our tester also showed no -5v (pretty sure that was the one)
Anyhow when a PSU was bad, it would either not light up any, or only two would come on.
There was always one light that never worked. I think it had something to do with if you were connecting a 20pin or 24pin mobo cable.

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Ultimate Geek
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  # 993528 24-Feb-2014 18:47
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Searching the great google tells me that the -5v rail was used exclusively by ISA cards back in the day, and that with ISA being obsolete it's now optional in the atx standard. Apparently, most modern PSUs don't include it. edit: 20pin is likely to have -5v, 24pin is not.

-12v is apparently used by serial ports, and is a pin in the PCI standard. Apparently some sound cards use it, but since it's often not well regulated it's can cause noise and/or distortion. Not sure how long until PCI becomes considered obsolete, but I figure that'll be about the time -12v becomes optional too.

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