Adam,

You should really be concerned about "load-dumps".  Spikes can get =20
into the 3-digit range, but those are quick enough to not be a problem =20
with decent filtering.  I design for 9-15V, with the lower end of that =20
happening during starting.  Also, reverse protect (a simple rectifier =20
diode is fine).  Older cars can be worse IMO, though newer cars have =20
more problem sources.  Avoid placing a PIC near high-power spark plug =20
wires.

Cheers,
-Neil.




Quoting Adam Field <adam@badtech.org>:

> I have a hobby project I've been developing that I want to turn into a
> self assembly kit, eventually. So far google hasn't answered all my
> questions about the hostility of an automobile environment. How stable
> is the voltage coming off the alternator? How bad are the transients?
> Are older cars worse? I assume you would see anywhere from 12-15VDC
> normally depending on engine speed / alternator load, say nothing of
> transients. If I was using a voltage regulator with a max 35VDC input,
> is it safe to assume I would rarely see a condition to damage the
> regulator? If not, what sort of input filtering should I use? Would a
> "pi" http://en.wikipedia.org/wiki/Capacitor-input_filter  filter be a
> good idea? Or simply a 16-30V zener diode may be enough?
>
> My other question is temperature extremes. If I stuck with the
> Microchip Industrial range, -40C to 85C, I would also assume I would
> not worry about the lower limit, as cars probably have trouble
> starting below -40C. The upper limit (85C, 185F) though may be
> approached during summer, in the interior, in full sun (on dashboard)
> and probably with a black plastic case to make the situation worse.
> Does anyone know what to expect?
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