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-5V reading question

Last response: in Components
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April 12, 2010 2:59:47 AM

Hi all,
I received 2 OCZ PSUs and both read nothing on -5V so were sent back - eventually for a refund. I then ordered an Antec EA650 and it had the same reading. I then found a technical page regarding PSU testing that indicated that the -5V is not used on ATX12v PSUs after version 2.01. My questions are thus:
1) How do you know if your mainboard needs the -5V?
2) If needed, how is that supplied to the mainboard in the newer PSU versions if it is not used any longer (e.g. if the original PSU was prior to that version and thus supplied -5v)?
3) Could the newer version then (supposedly better) cause inconsistencies such as USB keyboard and mouse inconsistently functioning, and why would PSU manufacturers create such a problem in the PSU world?


Does anyone have the skinny on the -5V question???
:( 

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a c 272 ) Power supply
April 12, 2010 3:08:26 AM

How old is your motherboard? Version 1.2 allowed the omission of the pin on the main power connector that provided the -5V, and version 1.3 prohibited putting that pin on the main connector. No power supply made within the last few years even has a -5V rail so i find it highly unlikely that your motherboard needs it unless its very old, most motherboards create whatever voltages they need through voltage regulators on the board itself powered off the 12V rail.
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April 12, 2010 3:25:12 AM

The -5v rail used to power the long obsolete ISA bus. If your board doesn't have any ISA slots then you don't need a -5v rail on your PSU.
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April 14, 2010 9:10:33 PM

Ahh - now that explains alot. Not sure how old this MOBO is, but I am guessing about 3-4 years. It certainly has no ISA slots - only PCI and PCIe. It is not my PC. So, that would explain why out of the box a new PSU will not illuminate the -5V led on the tester.
Thanks to both of you!
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April 14, 2010 9:13:48 PM

You know what? That begs the question, what would someone with an old ISA MOBO do if their PSU failed? Buy a new board? If the new PSUs do not even support -5V I wonder what else they could do :)  Interesting... but I don't imagine there are many of those around yet?
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