Just a stab at this, there are two well known tests that one must survive.  One
is the double up, where two batteries are attached in series.  In cold areas of
the country, tow truck apes are known to perform this feat of magic on
occasion.  The second is load dump, where the junk car has bad wiring, and the
wire from the laternator somehow becomes intermittent.  When the alternator
comes back on line, the regulator responds slowly, and you can get 60V for a few
seconds.

I would design a front end that can work with 30 to 35VDC (because a 7805 and
most switchers can survive that), and simply add a Polyswitch and a good TVS
diode in front.  Just because you kill the product momentarily at over 3x volts
doesn't mean you failed the survival test.  Use a 1.5KE rated diode, unipolar to
protect from reverse polarity too.

Don't forget to make it work at low (11.0V) levels too.

Chris Eddy

Johnny Lindholm wrote:

> So what kind of circuit WILL absorb spikes and other kind of noice in a
> car-ish enviroment?