What's happened to the -5volt supply on PSU's

  wile e 18:07 PM 13 May 13
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Hi, I recently bought two EVO Labs PSU's one for a PC that would freeze and after checking and replacing the memory, the processor, and the graphics card the computer still froze. As a last resort I decided to replace the PSU as well, I own a PSU checking device and noticed that the checker did not show the -5volts. I contacted EVO Labs who suggested I return the PSU's and obtain replacements, I duly returned to my local computer store and explained about the missing -5volt. They checked the rest of their stock which had the same problem, I got a refund and tried another store this time two ACE psu's and lo and behold they have exactly the same problem (no -5volt) I have four pc's and all of them on testing do show -5volts. Is this a design fault or is the -5volts not necessary any more?

  Fruit Bat /\0/\ 15:03 PM 14 May 13

As far as I know the ATX power Supply Unit has never had a -5v line/pin

-5v was used on the old AT PSUs

Perhaps a read here may help.

  bumpkin 21:47 PM 14 May 13

The point is does the PSU work on your machine. -5v I would have thought was just the neg return.

  onthelimit1 22:08 PM 14 May 13
  bumpkin 22:53 PM 14 May 13

Anything with a negative - sign is ground or neutral. -5v is not an output. Any basic ATX PSU should work fine.

  chub_tor 08:03 AM 15 May 13

bumpkin "Anything with a negative - sign is ground or neutral. -5v is not an output. Any basic ATX PSU should work fine."

Sorry but that is incorrect, you can have voltages both positive and negative with reference to neutral or ground. In my transistor circuit design career they were both essential especially if you were mixing PNP and NPN transistors. -5V is just as much an output as +5V is but in the case of modern semiconductors is much less required.

  onthelimit1 08:54 AM 15 May 13

Sorry - no idea what happened to my link - it usually works OK!

  bumpkin 10:41 AM 15 May 13

Sorry wile e, my error. Thanks chub_tor.

  wile e 17:06 PM 21 May 13

I have tried using two new PSU's (no -5volt) on an ABIT NF7 motherboard AMD Athlon XP2600+ and the computer crashes requiring a restart. I've been playing with the bios settings and checked all connections today in an attempt to eradicate the problem But I swapped the PSU's for an old 350W with -5volts and the computer is now stable. Clearly the -5volt pin has a function but still no explanation as to why manufacturers have omitted this on new PSU's.

  chub_tor 18:01 PM 21 May 13

I downloaded the manual for the NF7 from here and sure enough amongst the many connections for power it clearly shows a pin to connect a -5V supply. And so does my Gigabyte mother board (Pin 20) but the Abit board uses a 20 pin ATX connector while my board uses a 24 pin connector. I suspect, and maybe someone else can confirm this, you need an adapter to use a modern 24pin ATX power supply with an old board that has 20 pin power socket.

  onthelimit1 19:56 PM 21 May 13

from a PSU connector webpage

'The ATX specification requires the power supply to produce three main outputs, +3.3 V (±0.165 V), +5 V (±0.25 V) and +12 V (±0.60 V). Low-power −12 V (±1.2 V) and 5 VSB (standby) (±0.25 V) supplies are also required. A −5 V output was originally required because it was supplied on the ISA bus, but it became obsolete with the removal of the ISA bus in modern PCs and has been removed in later versions of the ATX standard. '

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