Basic electronic stuff

[dzorde at erggroup.com]

Dave, haven't been following the thread but I wouldn't suggest trying to connect
the PIC directly to the coil negative as you'll fry it.  As soon as you ground
the PIC output the coil will try and charge through your PIC at a rate of 8A or
so, basically your PIC will let out all the magical smoke that makes it work.
There are two options depending on what you want to do.
1.   if you want to use the PIC for actually operating the coil with proper
electronic advance and retard, you can use the PIC to drive a nice big
transistor (my ignition system uses a BUX80 driven by a smaller 2N3055) or FET
to drive the coil.
2.   if you just want to cut ignition pulses (rev limiter style), use the PIC to
drive a decent size SCR (probably C122E) and just sample boost and revs and
short coil negative to ground at the appropriate time.
In either case you will need some spike protection on the devices but that's a
different matter.

Dan  dzorde at erggroup.com


Date: Wed, 1 Dec 1999 07:17:40 -0500
From: "Posea, David G, SITS" <dposea at att.com>
Subject: Basic electronic stuff

Some of you may remember I am working on a boost retard device based on a
PIC controller for the Ford DIS ignition. I found a good trigger circuit
that goes to logic 1 when the EEC grounds the trigger wire for a coil. My
question is how do I handle the "open" state on the output side of the
controller? This is the negative side of the coil. If I attach it to an
output pin out on the PIC, won't I be feeding Vbat (+13V)into that same pin
from the coil side? Can I just set the output pin high until time to fire
the coil, then ground it? Or do I need to use the output as the input to a
transistor to switch the negative coil wire to ground?  Something seems
wrong here, and I'm not sure what it is. I'm a programmer by trade with
limited electronic knowlegde. Thanks in advance for any help.

David Posea