On Fri, Aug 30, 2002 at 06:59:29PM -0400, Pic Dude wrote:
> 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"? :-)

I haven't worked it through to completion. But I've already given you the
basic jist:

* Use a 12V gel cell as a temp UPS
* Use an LDO.

>
>
> > > 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).

Closer to 0.2-0.3V.

> So I need 13V min at all times in the
> car.

This usually isn't a problem for 3 reasons:

1) You normally have 13-14V when the vehicle is running.
2) The gel cell is usually above 12V
3) and the 0.2-0.3V is less than the 5% variation that most computer items
   require for regulation (i.e. it'll run fine at 11.8V. I have emperical
   evidence.)

>  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.

Not necessary. Alternator + car battery + small gel cell means that the input
voltage will never be below 12V. With an LDO you can get 11.7 to 11.8V
consistently with that input.

This part I have tested. I've booted an ordinary PC from a 12V gel cell using
an LDO regulator that produced 11.7V. It worked fine.

> 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.

Why not give my idea a run first? If it works, you're done. If not then you
can look at the more complicated solution. The only single advantage that your
original solution has is that it'll work when the vehicle is starting. But
with my proposal you can test without the gel cell and it'll work just as long
as you are not trying to start. That's the whole purpose of the gel cell: to
carry 12V input while the vehicle is starting.

I went this route because a gel cell + 1 steering diode is a lot less
compilicated than a buck boost regulator/doubler.

>
>
> > >     - 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.

The it should be a problem.

>
>
> > > - 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.

I'm not sure about that. My 12V requirement was for the hard disk. So I
never tested the audio/video with 5V only. You can quite easily simply by
removing the 12V line from your PS and trying to boot. Since you have a 5V
only HD, it should boot fine. You can then test the audio (because the
video test will be instanteanous at boot. ;-)

>
>
> > > 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.

I find it to be exactly the opposite. a 12V LDO linear regulator is no problem.
Generating a multiamp 3.3V or 5V regulator from a 13-14V input is the
challenge. I'm using a variation of Russell's inspired switching regulator
from a year or so ago. I simplified it by using an opamp as the control
element. It give 5V output and boots my MB just fine.

I found that ATX added too many compiliations in terms of starting, persistant
5V, and the like. AT is a much simpler beast.


>
> > 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.

Well since it carries for such a limited time, I'm sure that a smaller
battery could do the job. 1.2AHr's are really small and probably could carry
the board for the 15-30 seconds that the main electrical system is in dropout.

It's an untested theory though.

>
> What did you do for the 5VSB signal?

5VSB?

BAJ

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