On Mon, Jun 17, 2002 at 01:18:51PM -0400, Erik Jacobs wrote:
> I apologize in advance for posting this message... my flame shield is on :)

????. WHy the apology in advance?

>
> 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.

OK.

>  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.

May not be correct on that. You really need to look up the specs as it may
take a significant fraction of 12V to engage and some 10's or 100's of
milliamps to hold.

> I highly
> doubt that a little output port on a PIC micro is going to be able to drive
> a relay,

That's right on the money.

> so I'm wondering what kind of intermediary device I would need.

A transistor and a catch diode to take care of the collaping magnetic field
energy of the coil when the relay disengages.

> 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?

You can get both the transistor and diode in a TIP series Darlington transistor
package. I used automotive relays in my homebuilt thermostat project. The
basic config is as such:

* Connect one end of the relay coil to +12V
* Connect the other end of the coil to the collector of a TIP120 transistor
* Connect the emitter of the transistor to ground.
* Finally connect the PIC I/O port pin to the base of the transistor through
  a small valued resistor. In my project 1K worked fine for me.

That's it. When the PIC pin is raised to 5V the transistor will engage and the
relay coil will pull the relay in. When the I/O pin is dropped to 0V the
transistor will turn off and the mag field energy will conduct through the
builting diode of the TIP 120.

Hope this helps,

BAJ

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