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

[Sandy]

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
>++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
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>++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
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