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

[Matthew B. Watts]

-----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
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