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

[Clare Snyder]

Sandy wrote:
> 
> 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.
> 
> Sandy
> 
> At 11:44 AM 3/5/98 +1300, you wrote:
> >Hi All
> >
> >I'm using a 555 timer set up as a one shot firing an injector. To trigger
> it I want to
> >use the negative of the coil. The circuit works great on the bench
> >but the once I connect it it fries the input.... I think this is
> >because of the back emf from the coil so I put a 12v zener across the
> >input.
> >Can anyone help me with a filter/clipper to stop this.
> >
> >Cheers
> >
> >Simon
> >++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
> >  Simon Quested (E-mail questeds at lincoln.ac.nz)
> >  Computer Technician, Silicon Graphics & Windows NT Support
> >  Centre for Computing and Biometrics
> >  LINCOLN UNIVERSITY OF NEW ZEALAND
> >  Phone (64)(03) 3252811 Ext. 8087
> >++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
> >http://www.lincoln.ac.nz/ccb/techs/simon/default.htm
> >++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
> >I drive way too fast to worry about cholesterol
> >++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
> >
> >
Do like on a Bosch D-Jet - use a second set of points to trigger the
EFI. Either that or use a pulse transformer or a high value resistor and
a 5 volt zener to clamp the signal. Might just want to install 2, back
to back, to clamp the negative ring as well.
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