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ATX Power Specification no -5VDC Is this a problem?

Discussion in 'General Hardware' started by ralphy, Jun 1, 2007.

  1. ralphy New Member

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    Is -5V really necessary? What is it used for on the MB? What if the motherboard is older (ie not ATXV2.2) but the power supply is a v2.2 ATX specification without -5VDC? Can the MB be used with the newer power supply without -5V?

    I'm no expert at the ATX Power specifications, but V2.1 includes -5V, but v 2.2 does not. And then there the ATX12V Version 2.2 that also doesn't include -5V. Why the differences, I don't know, and what are the implications?

    I just installed a Zalman ZM460B-APS to my (now aging) Abit NF-7S motherboard and the Abit EQ monitoring software started beeping away with a 'reading' of +3.6V on the -5V rail. Now, I assume this is wrong, on the basis that the Zalman Power supply conforms to the ATX12V version 2.2 spec, which I now know, has 'no connection' on the -5VDC pin.

    But not knowing what was causing the problem at the time (I suspected my new power supply was a dud), I reverted to my old supply, but it seems as though the motherboard was otherwise operating ok, without a -5V rail.

    Abit's documentation gives no clear guidance on whether -5V is required - but it does show a negative 5VDC pin on the ATX power connector. So presumably it is used for something, but seems to work without!

    So, I'm wondering
    • what is the -5V used for?
    • is the lack of a -5VDC rail on the power supply and issue when used on my ABIT NF-7S motherboard.
    • Can I re-install the Zalman power supply with the Abut NF-7S, and disable the Abit EQ alarm for -5V without problems?
     
  2. mrw1986

    mrw1986

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    I suggest reading How A PC Power Supply Works. It will give you an answer as to what the -5v rail is. As for your other 2 questions I'm not 100% sure on. I'll dig around to see if I can find anything.
     
  3. Random Murderer

    Random Murderer The Anti-Midas

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    -5v rails are used as a trip switch. when you press the power button, on old psu's, an interrupt occurs in the -5v rail telling the psu to supply power to the motherboard. on new style psu's, the -5v rail has been eliminated and replaced by a grounding loop that serves the same purpose.

    you can use the newer psu on an older mobo and be fine. just disable that annoying alarm! lol
     
    Last edited: Jun 1, 2007
    ralphy says thanks.
  4. mrw1986

    mrw1986

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    Ya I just read that in the article I posted but you beat me to it :laugh: What he's saying is that you have no need for it with the newer power supply and should have nothing to worry about it. I couldn't imagine any problems arising from it.
     
    ralphy says thanks.

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