I design automotive products over 5 years I went thru this
Long time ago and I learned my lessen a few things I will suggest.

1. use DC to DC converter to isolate 5 volts (if you can or you have
room)
   this is for best result other wise use 100mh inductor with 10 ohm
series
   resistor on 12v line.

2. use opto isolator to drive your replay never connect direct

3. any glitch on crystal pin or on 5v line effects internal
functionality
   and PIC may start doing wrong things. Some hackers use this method
   to disable code protection bit.

4. use at list 4 layer board with power and ground plane.

5. any input or output must be optically isolated.

6. input of each opto isolator add 39 pf cap to filter some noises.
Because
   of leds are in noisy area.

do not use SCR wont work use IGBT transistor with optoisolator

hope this helps

Andre Abelian





-----Original Message-----


I apologize in advance for posting this message... my flame shield is on
:)

I'm embarking on a automotive project using a PIC that will control some
electric fans.  The "parts" involved with this portion of the project
are
the PIC, somet interface, and an automotive relay.  I should probably
find
some specs on an automotive relay but I'd assume that it only needs a
few
volts to engage and a very small constant voltage to keep it on.  I
highly
doubt that a little output port on a PIC micro is going to be able to
drive
a relay, so I'm wondering what kind of intermediary device I would need.
I
was thinking something along the lines of a power transistor, but
someone
suggested to me using an SCR(?).  What are your
thoughts/ideas/experiences?

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