In automotive systems there's a tremendous amount of electrical noise.  The
automotive specifications allow for very high voltage spikes and surges.  I
designed a system for use on transit buses in Europe.  When the air
conditioner cycle there were voltage surges in excess of 100 volts.  When
the brakes were applied and released there were also electrical surges. The
input voltage rating on a 7805 regulator is lower than the noise you are
likely to see.  Minimally you should put filtering and some means of spike
protection on the input to the regulator.  Instead of the 7805 you may want
to consider using a switch mode controller such as one of the simple
switcher chips.



At 09:41 AM 7/16/98 +0100, you wrote:
>At 12:28 15/07/98 -0700, you wrote:
>>On Wed, 15 Jul 1998, White Horse Design wrote:
>>
>>> We've found that a JW (EPROM 20MHz) PIC device fails when powered up in
the
>>> car. It is directly connected to 12V with some minor protection
>>
>>I'm surprised any of them survived at all. I've had bad luck past about 8
>>volts. You really need a regulator or something to drop the 12 volts down.
>
>Sorry should have said I'm using a 7805!
>
>(By "directly" I meant we are not using a dc-dc (isolated) converter module
>such as the Alphatronnix which we do use on the lorry systems (24 down to
>12V))
>
>Regards
>
>Adrian
>
>WWW    WWW   Adrian Gothard
>WWW WW WWW   White Horse Design
>WWWWWWWWWW   +44-385-970009 (Mobile/SMS), +44-118-962-8913/4 (voice/fax)
>WWWW  WWWW   whd@zetnet.co.uk, http://www.users.zetnet.co.uk/whd
>---
>Developers of GPS satellite-based tracking systems for vehicles/helicopters
>
>
Larry G. Nelson Sr.
mailto:L.Nelson@ieee.org
http://www.ultranet.com/~nr