At 13:41 4/11/99 -0500, you wrote:
>> > i need to connect a pic to my car, using the 12v car battery...umm i
>assume i can't just wire up the car battery to like a 7805 to control the
>voltage..or maybe im wrong...Suggestions?
>
>
>It works for hobby projects.
>Add a series fuse before the reg, and a 15V or maybe 18V zener across power
>in front of the reg.
>This protects you from overvoltage surges, and reverse voltage transients.
>If you have more than about 100uF on the 05 output, then add a diode from
>input to output on the 05, such that it's normally reverse biased.  Taking
>the 78XX or 340-XX reg inputs to ground suddenly (like during an alternator
>spike) can kill the reg otherwise. They aren't designed to withstand any
>voltage on the output larger than the input.
>
>The auto industry has standards for such things, but this is a practical
>minimum.
>Don't forget the input and output caps on that 7805 or you may end up with
>an AM broadcast station for free!
>
>


Delving back into my memory (Most brain cells past on quite a few years
ago). But the Automotive environment is not a nice place to be. A simple
7805 will die in this situation! National Semiconductor had a great app
note no this, as you will find, not all voltages in a car a positive with
respect to ground, and you will find some nasty spikes up to 60V, and then
there is the cranking current voltage drop (GM require operation down to
3V), and the air conditioner etc.
I suggest that you use one of the automotive 5V regs, as it will survive.
There is automotive parts manufacturer that make cruise controllers that
had 7805 in them, many have failed within 5 years of operation.

Dennis