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[SOLVED] Concern about ATX12 PSU with low-voltage readout

This is a discussion on [SOLVED] Concern about ATX12 PSU with low-voltage readout within the RAM and Power Supply Support forums, part of the Tech Support Forum category. Hello-- I ran into an anomaly when replacing the power supply on an older (10+ years) system. It is not


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Old 07-01-2012, 02:33 PM   #1
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Hello--

I ran into an anomaly when replacing the power supply on an older (10+ years) system. It is not obviously harmful but it's definitely disconcerting, so I thought I would ask for advice.

The machine has an ASUS P3B-F motherboard (700 MHz Pentium III). The current power supply is 7-8 years old (ATX12, version unknown). I replaced it with an Antec 350W ATX12 2.0 PSU, which worked fine except for one problem: the BIOS readout showed the -5V level as -6.14V and the system would not boot. I tried a more recent ATX12 power supply (purchased within the last year) and got exactly the same result (to the second decimal, even). Looking at a very similar computer I have (slightly older motherboard), I discovered that, although the -5V number looks OK (-5.1), that readout was set to "Ignore"; I have no idea why. Greatly daring, I tried this on the system with the replacement PSU, and it seems to work fine--at least no smell of smoke or anything.

Question, therefore: What is the significance of the -5V number? Is ignoring it acceptable or foolhardy? (I'd hate to lose the computer; it's almost like an heirloom by now.)

Thanks much.
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Old 07-01-2012, 02:42 PM   #2
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some BIOS have it wjere it will show +5 +3.3 +12 and then -5 -3.3 -12 aslong as the figures dont drop or go high than 5% of 5,3.3 and 12 then your ok.

Your older system may not have accurate sensors, so the only way to truly test is with a multimeter
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Old 07-01-2012, 05:22 PM   #3
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OK, thanks. Looks like I need to do a little study and testing. I long ago concluded fans and batteries are the twin curses of modern civilization; guess we'll have to add power supplies too. In the meantime, I have stuck in a not-too-old salvaged PSU that has some life left, so I'm OK for the time being.

Thanks again.
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Old 07-02-2012, 12:32 AM   #4
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The newer ATX spec no longer has the -5v circuit.
Does the old supply in fact have a white wire in the pin 18 slot of the 20 pin connector?
Does the replacement supply have it?
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Old 07-02-2012, 02:31 PM   #5
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By golly, it doesn't! The original PSU has a white wire in either 5/6 or 15/16 (I don't know where to start counting), whereas that position is blank in the new one. This would clearly seem to imply the readout can be ignored, yes? Thank you.
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Old 07-02-2012, 02:42 PM   #6
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Does the new PSU work alright? Some of the older PII, PIII boards needed the -5v.
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Old 07-02-2012, 02:57 PM   #7
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It seemed to; I tried running the system on it for perhaps 5-10 minutes and noticed no anomalies or loss of functionality.
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Old 07-02-2012, 03:00 PM   #8
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You'll be good then as I remember they would not boot on systems that had the requirement for it.
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Old 07-02-2012, 03:01 PM   #9
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Aha, that's conclusive. Thanks again. Problem solved.
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Old 07-02-2012, 11:05 PM   #10
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good luck.

Most newer power supplies will work on older boards but if they were very old boards (talking about 10 years) then you would probably have issues. This was more true for intel and via chipset boards.
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Old 07-03-2012, 12:12 PM   #11
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Right--thanks. We'll see what happens and hope for the best.
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