The +12V and the -12V is present on the actual power supply only for respect
the specifcation of the ISA BUS (The old AT 80286 and the old PC 8088 that
have on its pin the +12V and the -12V that was used for the first dinamic
RAM and for the RS232 card (1488 and 1489 RS232 front end chip) and for the
floppy drive old Tandon 5,1/4 drive.

From my point of view and for my experience, actually for a standard PC is
not  needed the +12V and the -12V.

You can supply the motherboard only with only a 5Volt.

The only device that need 12Vdc is the fan over the CPU and the FAN into the
Power supply.

Regards.

Leo




I need to build a power-supply to run a PC in a car, so I'm trying to figure
out why I really need +/- 12V, which may be tough to produce with the
ignition off (not enough overhead for the regulator).  I could build a
DC-to-DC coverter that would raise the voltage, but that means $$$.

Thinking of a regular desktop PC with a MB, audio card, video card, fan, and
IDE drive only (no floppy or CD)...

- MOTHERBOARD:
    - 5V around board, which may be regulated to lower for
      processor, etc, but that should not affect me.
    - The motherboard has a 12V requirement (standard ATX
      PS), but does it really need it, or does it pass on
      the 12V to other devices?
    - AFAIK, the parallel port does not require 12V.
    - AFAIK, USB does not require 12V.
    - Does the built-in serial require a +/-12V supply?  Or
      does it generate +/-12V (like a MAX232 for example)?
      If I need to supply it, and decide not to use serial
      comms, then can I just ignore it?  Or can I just supply
      it with +/-5V (as an RS423 device) and make sure the
      other end handles that level?

- COOLING FAN:
    - I can always get or modify the fan for 5V use, or I
      could generate 10-ish volts for that.  So no prob
      here.

- AUDIO CARD:
    - I believe line-out sound levels go to 10V, so +/-12V
      is probably needed there.  Any way around this?

- VIDEO CARD:
    - No idea?  Anyone know anything about these?  Or would
      it vary by manufacturer?  (Which I doubt).

- HARD DRIVE:
    - Have an IDE laptop drive which says 5V at 500ma, so I
      should be okay here.

Anything else I'm missing?

Cheers,
-Neil.

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