signal from coil and back emf......

Clare Snyder snyder at snyder.on.ca
Thu Mar 5 02:39:36 GMT 1998


Matthew B. Watts wrote:
> 
> -----Original Message-----
> From: Sandy <sganz at wgn.net>
> To: diy_efi at efi332.eng.ohio-state.edu <diy_efi at efi332.eng.ohio-state.edu>
> Date: Wednesday, March 04, 1998 6:00 PM
> Subject: Re: signal from coil and back emf......
> 
> >I'm not exactly sure of your setup, but I guess that you are using the
> >points or something similar to trigger the 555. EMF will be a problem, the
> >kick back can be upwards of 400volts. What you want to do is something
> >similar to a tach input, attach a resistor, and use the zener/tvs to clamp
> >the voltage. You may also need a small cap to eliminate some noise. I'm not
> >sure where but you can find a simple circuit in some magazines or
> >electronic sites that have digital tachs and they should have a sample
> >circuit.
> 
> You'll definitely need a clamping diode between a home-built driver
> and any inductive load to suppress kick-back.  For an injector or even
> an ignition coil, I'd recommend a 1N5408 silicon rectifier diode.  It's
> a 32 cent part that you can connect between the driver output and
> ground; it will save you a lot of grief.  The striped side (cathode)
> hooks to your output to provide a current path for the inductor to
> discharge once the output signal is dropped.  You should try to get
> the diode as close as you can to the injector to keep the current
> snap during discharge from causing any false triggering in your
> circuit.
> 
> Hope this helps...
> 
> Matt Watts
> University of Utah
> Formula SAE Racing
> _________________
I think you guys are reading this all wrong - he is popping his INPUT
from what I read. This is due exclusively to the fact that the primary
of the ignition system does NOT run on 12 volts, but something closer to
400 volts due to the self generated EMF in the coil - look at a coil
primary trace on a scope sometime. It is this that is popping the 555.
Just clamping it will definitely solve the problem - the engine will not
fire. You MUST use a high value resistor - something in the order of
470K should do the job, to limit the current, then a varister or zener
rated somewhere about 5 volts to keep the trigger voltage low enough it
will not blow holes through the junctions in the 555. You could use  up
to 12 volts with a 555c. The coil back-emf from a 2 ohm injector is
negligible - I would use the low impedence units with a limitting
resistor to start with, then go to a peak and hold driver if necessary.

just my $.02 worth - and .02 of a $.70 dollat at that!!


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