Byron A Jeff scribbled:

> 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.

On piclist, doesn't "club" have an implication of a "solution"? :-)


> > 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.

Among other things, since a low dropout still needs overhead
voltage above 12V.  Figure 1V (just a guess since I haven't
yet investigated this).  So I need 13V min at all times in the
car.  If the current requirement for the 12V portion is low
enough (to be determined), I could regulate 12V to 8V, run that
thru a doubler to get 15-16 volts, and regulate that to 12V.
Complicated, but the best I've thought of so far.  I've searched
for 12V-output DC-to-DC converters with inputs of up to 10V
max, but no dice yet.


> >     - 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.

I'm sure I can do without serial.  I was planning on building
a controller/keypad for it, but I could do this thru the
keyboard port, parallel port, low-voltage serial, etc.


> > - 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.

Both audio and video still leave the 12V requirement, and with
on-board audio/video on the Mini-ITX board I'm looking at, it
will be tougher to split the power rail.


> > 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.

But 3.3V is no problem to generate in an automobile.  Anything up
to 9V should be a simple regulator design.  It's the 12V that's
kicking my butt.  Doable, but not simple.

> 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.

Hmmm... that's a good idea, but still space consuming.  One of
the cars that this is going in has very, very limited space, so
I'm being very picky here.

What did you do for the 5VSB signal?

Cheers,
-Neil.

--
http://www.piclist.com hint: PICList Posts must start with ONE topic:
[PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads