Power Supply Failure

I noticed when most power supplies fail on computers it is because the -5vdc goes out in them. I've even had new ones do the same. Seems there is no schematic available to service them and you end up buying a new one. What I would like to know is why the -5vdc goes out and not the +12vdc for example.
I have boxes full of these power supplies with that problem. Any suggestions?

 

I understand what you are saying Arie and appreciate your response. These psu are in new computers, older computers when they come in for service. I imagine its lightning or such that is taking them out. I was just wondering why the only part of the power that is out is the -5vdc. Every one we test has that problem. Just curious. They are original equipment power supplies.

 

Ummm, there is NO -5VDC used in ANY ATX Form Factor computer which is basically 100% of the PCs sold in the last nearly 17 years!!!!!!! This has been true since the ATX Form Factor "standard" replaced the old AT Form Factor Standard. ATX computers use +12VDC, +5VDC and +3.3VDC, and +5Vsb standby voltages. They do not use -5VDC (or -12VDC for that matter, with a few unique exceptions that most users will never encounter).

Therefore, if you have been tossing these supplies because they had no, or incorrect -5VDC reading, you probably tossed some perfectly good supplies.

See the ATX12V Form Factor PSU Design Guide, Ver 2.2, March 2005 which clearly states on page 3, Revision History notes for Ver 1.2

Note in Section 3.2.1, Table 2 on page 13 the following table for ATX Form Factor PSU voltage tolerances. See any -5VDC listed?

 

All my testers have a -5vdc led on them and when the computers come in they are not working at all. When tested they all test bad with the -5vdc led out in them. Replace the power supply and they work fine. If its not the -5vdc circuit that is bad, then what is wrong with them? If you test a working power supply, then the -5vdc tests fine. I appreciate all the response to my question but still my question is not answered. Maybe I should buy a quality tester. Anyway its a throwaway society, not concerned with component troubleshooting. Seems a waste though. I can also guarantee that these power supplies are bad, for whatever reason. I know you are doubting my ability to test them. I withdraw my question. Have a great day.

 

No. That is not necessarily so at all. As noted by THEE "standard" that governs the electrical and physical characteristics of ATX PSUs, motherboards, cases, and accessories, I say again, -5VDC is not used in PCs.

But you should also understand that ATX PSUs are NOT limited to being used in personal computers.

It does not matter if the -5VDC tests bad. The motherboard does not use it. If the other voltages are good, within required tolerances, including ripple specifications, then the PSU is good - assuming it has ample power. You cannot test a PSU properly with a plug in tester - you need the proper test equipment (e.g., an oscilloscope or a power supply analyzer) to conclusively test a PSU - this is why swapping in a known good one is often the best method for most users.

And btw, you also cannot properly test a PSU unless it is under a realistic load. The little 10Ω resistor in those testers do not do that. And of course, multimeters are designed to not influence the circuit at all so you cannot properly and conclusively test a PSU with a multimeter either.

I keep a good PSU Tester in my tool bag when I am "in the field" and don't have a good spare power supply to swap in. While not a certain test, they are better than nothing.

 

On the original IBM PC, XT, AT standard there was a -5 V rail for peripherals on the ISA bus but, as Bill pointed out, it was not actually used by the motherboard.

 

Right - but to reiterate, that was with the AT standard, not ATX. Big difference.

 

Correct.

 

It is so nice when everyone is on the same page!

 

I agree.