On Fri, Aug 30, 2002 at 10:14:18AM -0400, Pic Dude wrote:
> I need to build a power-supply to run a PC in a car,

Join the club.

> 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).

You'll need a Low DropOut regulator.

>  I could build a DC-to-DC coverter that
> would raise the voltage, but that means $$$.

And extra hardware.

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

Cool. That's my base system for my card MP3 player.

>
> - 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?

It passes them on to other devices. The MB itself doesn't need 12V.

>     - AFAIK, the parallel port does not require 12V.

Right.

>     - AFAIK, USB does not require 12V.

Right.

>     - Does the built-in serial require a +/-12V supply?  Or
>       does it generate +/-12V (like a MAX232 for example)?

The former. This is one spot where the +/- 12V is critical.

>       If I need to supply it, and decide not to use serial
>       comms, then can I just ignore it?

Yes.

>   Or can I just supply
>       it with +/-5V (as an RS423 device) and make sure the
>       other end handles that level?

Yes. Or even better a +12/-5 system.

>
> - 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.

Right.

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

I haven't had any problem with my sound card running on only +5 and +12.

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

My onboard video worked find with just +5 and +12.

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

Interesting. Good idea.

>
> Anything else I'm missing?

It's ATX right? So what about 3.3V? isn't it required. That's one reason why
I've remained with Baby AT MB for projects like this.

Also I'm using full sized 3.5" HD so I'm supplying 12V via a LDO regulator
and using a 7AHr 12V Gel cell as a micro UPS so that the system doesn't reboot
when the start is activated.

BAJ

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