On 22/10/2010 22:39, Adam Field wrote:
> 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?
Car Battery is 10V to 14.2V. Lower during starting.
Peaks are 16V
Spikes can be big.. But if you design for 16V and have a filter, then=20
you should be fine. A zener is not a good idea.

A 20V regulator should be OK. 35V is reasonable margin. An LC low pass=20
PI (coil to take the DC current without saturation) should be good.

Spikes are not really from Alternator, it may have AC ripple if battery=20
is gone bad or poor connection to it (terminals or chassis) . But Wiper=20
Motor, Internal Heater Fan Motor, Radiator Fan motor and  Starter motor=20
make spikes and noise.

Intermittent battery connection is fairly nasty. Used to wreck=20
alternators. maybe still does.

polystrene on dashboard in direct sun may melt! Matt Black might be a=20
bad idea.

Low Temperature:
Most stuff can take very low storage temperature compared to operating.

High Temperature:
This is the one you have to be careful about. You often may not exceed=20
it even when off.

Older cars are only worse if they are so old that battery is poorly=20
connected or that it uses Dynamo rather than Alternator.
--=20
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