Author Topic: -12V on tiny ATX power supply is way off  (Read 1485 times)

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Offline dentaku

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-12V on tiny ATX power supply is way off
« on: January 28, 2015, 07:34:06 pm »
I have a small FSP270-50SNV ATX power supply that has a -12V output that's way off, it measures around -10V.
http://www.aerocooler.com/images/PWSP5SNVS_m.jpg

All the other rails seem fine. I know it's not much information to work from but nothing seems burned or bulged and it's so cramped in there that it's impossible to read the writing on any of the parts.

QUESTION: Is there anything obvious I should look for to try and figure out why the -12V is messed up but the +3.3V, +5V and +12V seem fine?
« Last Edit: January 28, 2015, 07:43:16 pm by dentaku »
 

Offline Phaedrus

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Re: -12V on tiny ATX power supply is way off
« Reply #1 on: January 28, 2015, 08:11:05 pm »
The -12V rail is considered legacy and unimportant by all modern ATX PSU manufacturers (source: am in the ATX PSU industry). They'll typically use a really shitty inverting boost circuit off the +5V rail. The ATX12V spec allows +/-10% voltage and up to 120mVpk-pk ripple, but it's not uncommon to see +/-20% and 200-250mV, even on $100+ PSUs, because no one cares. The only thing -12V is used for in a modern PC is the RS232 port, and most motherboards and serial I/O cards will have their own power supply for it. It also used to be used as the negative op-amp supply rail in some cheap sound cards about 10-15 years ago, but they stopped doing that specifically because the -12V rail was usually so noisy.

-12V is expected to be removed from the ATX12V spec in the next major revision, the way the -5V rail was removed in 2003. The only thing that kept it on there so far is that the PCI bus specified a -12V rail. But now that PCI is largely obsolete, and PCIe requires only +12V and +3.3V, the -12V is gonna be up for the chopping block soon.

The PSU is probably fine.
« Last Edit: January 28, 2015, 08:19:19 pm by Phaedrus »
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Offline dentaku

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Re: -12V on tiny ATX power supply is way off
« Reply #2 on: January 28, 2015, 09:57:36 pm »
The -12V rail is considered legacy and unimportant by all modern ATX PSU manufacturers (source: am in the ATX PSU industry). They'll typically use a really shitty inverting boost circuit off the +5V rail. The ATX12V spec allows +/-10% voltage and up to 120mVpk-pk ripple, but it's not uncommon to see +/-20% and 200-250mV, even on $100+ PSUs, because no one cares. The only thing -12V is used for in a modern PC is the RS232 port, and most motherboards and serial I/O cards will have their own power supply for it. It also used to be used as the negative op-amp supply rail in some cheap sound cards about 10-15 years ago, but they stopped doing that specifically because the -12V rail was usually so noisy.

-12V is expected to be removed from the ATX12V spec in the next major revision, the way the -5V rail was removed in 2003. The only thing that kept it on there so far is that the PCI bus specified a -12V rail. But now that PCI is largely obsolete, and PCIe requires only +12V and +3.3V, the -12V is gonna be up for the chopping block soon.

The PSU is probably fine.

I was wondering what the -12V rail was for and why this PSU didn't have a -5V pin.
 


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