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No -5V output, yet still working

  • Power Supplies
  • Computers
  • Components
Last response: in Components
August 17, 2009 11:47:21 PM


I have a power supply tester that tests each line of a PSU for the correct rating (voltage). From all the lines, all LEDs are lighting up, except for the -5V. Yet, the computer is still working correctly. I double checked the PSU tester with another PSU, and this one had all lines working correctly including the -5V, so the PSU tester is working well.

What is the -5V line used for? Do I really need it for the computer to run correctly? What can go wrong if I use the PSU which is not outputting -5V on that line?

Thanks in advance!

More about : output working

August 18, 2009 12:19:58 AM

If your using a mobo with 24-pin atx connector then there would be no -5v input. If you would check the specs of the 24-pin atx, you would see that the -5v is already omitted from the pins. This is due to the fact that -5v is obsolete and no longer needed by newer mobos which uses 4-pin 12v atx connector. This is also the reason why the 12v rail wattage of a PSU is more important today because it is the one who powers up almost everything in the PC. -5v pin is usually used by older mobos with 20-pin atx. Hopefully that may give light to your question.
August 18, 2009 6:52:29 AM

Thanks a lot for your reply! So the -5V not working does not indicate a faulty PSU? Are there any other 'obsolete' lines, so that I can keep that for future reference?

Thanks again
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August 18, 2009 6:59:52 AM

There might be some pins aside from the -5v but they are for grounding purposes. The main indication of a faulty PSU is not delivering power (busted).
August 18, 2009 7:04:10 AM

I thought it might be reason, as the computer is behaving very unstable and most of the times hangs at bios (post code 0075).

However I tried a perfectly working PSU (with even the -5v line), and it still behaved the same. I'm assuming it's the mobo faulty. Thanks once again!
August 18, 2009 7:08:01 AM

You're system freezes (post code 0075). Have you tried running memtest? Do you overclock your system?
August 18, 2009 7:20:09 AM

can't come to the point where i can run MemTest :/  It hangs exactly after it posts the total amount of RAM in the system. I tried changing ram, still doesn't work. I've posted an appropriate thread about this prob in the mobo forum. No, the system is not overclocked.
August 18, 2009 1:43:52 PM

Sounds like bad ram or wrong voltage. Read the stickys about ram.
August 18, 2009 2:09:11 PM

I tried 3 different sticks of RAM, and also resetted the BIOS to factory settings, still nothing :/ 
August 18, 2009 9:29:11 PM

I went through the whole list. This is not a new build, it was working perfectly and now after about 2 - 3 years it's having this problem.

I've changed everything except the CPU and Motherboard, and only have the graphics card connected (no HD, CDs/DVDs or floppys).

Do you think its a bad motherboard? I'm assuming that is the reason, but I would like to be sure its the motherboard before changing. Personally, every fault that I have ever seen on a bad computer never resulted in it being a bad CPU. Maybe it's just by coincidence, but I always rule out the CPU being the problem, since I've never seen a bad one in quite a good amount of computers
a c 144 ) Power supply
August 19, 2009 4:07:25 PM

Keep in mind that the PSU tester does not put any load on the PSU. If you strip everything out except the PSU and CPU & HSF - including memory and the video card, when you boot, you get a series of long beeps indicating memory problems. If you get silence, one of the three parts left (PSU, motherboard, or PSU) is the problem.
August 19, 2009 4:16:07 PM

Yes, I get the series of beeps if I strip everything out. Placing the RAM and GFX again, I can get up till the RAM check, and there is a post code 0075 in the bottom right corner.
June 24, 2012 1:51:30 PM

just give a heat on SIO chip might help

a c 243 ) Power supply
June 24, 2012 1:56:34 PM

Woohoo, 3 year old thread