> 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?

My first reaction is a NPN transistor with emitter to ground, base to the
PIC output pin via a 1Kohm resistor and emitter to the relay coil.  The
other end of the relay coil goes to +12V with a reverse diode accross it to
snub the kickback when the relay coil is switched off.  That will support a
relay coil current of up to 200mA.  Use a smaller resistor if you need more
relay current.

A logic level FET doesn't require the resistor, but will be more expensive.

An SCR is inappropriate here because there is no way to actively switch it
off.  It will also cost more than the "jelly bean" transistor.


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