ATX Power Supply – Why -12V is Needed

atxpower supply

Checking the standard ATX connector pinout it mainly gives out "standard" positive voltages, like +3.3VDC, +5VDC and +12VDC. There is one exception, pin 14, which actually gives out -12VDC:

pinout

Based on the ATX12V specification I can see that it only needs to supply 500mA of -12VDC (compared to the other voltages where even the +5V standby voltage is at least 2A), but I cannot find which component of an ATX computer requires this unusual voltage to operate.

(There is also pin 20 with -5V, but that pin is deprecated, and the deprecation note clearly say that this voltage was used to power some features in old ISA cards. -12VDC is still not deprecated and required based on the spec, so I guess it's still used somewhere in a modern computer)

Best Answer

AFAIK there are 3 type of devices on a motherboard that require -12V.

  1. RS232, RS485 ports without build-in circuitry for generating the negative voltage.
    These are starting to become very rare... Haven't encountered any for the last 5 years or so.
  2. (mini-)PCI slots. They are slowly being phased out in favor of PCI-e, but there are still a lot of new computers made that still have them.
  3. Some high-end sound-cards require -12V as a reference voltage. These are usually powered from a PCI slot, which brings us back to point 2.
    (There have been a few motherboards that had such a card integrated in the motherboard itself.)

PSU manufacturers can't know in advance if the motherboard will actually need the -12V so they will keep adding the -12V line to the plug as long as the ATX standard requires it.
And it is not only a matter or whether it is needed or not: If you don't include it, your PSU will not be ATX compliant and you can't sell it as such.

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