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

[T Hergen]

Simon,
 
filtered +12 ---/\/\/\----------|
                  r2            |
points---/\/\/\----o----| |-----o-----pin2 of 555
          r1       |     c1     |
       -----|\|----|            |              
       |    |/|                 |
       |     z1                 |
       o------------/\/\/\------|
       |              r3
      gnd

r1: 1k
r2: 5k
r3: 5k
c1: 0.1uF
d1: 1n5231 (5v zener)

A circuit I saw for a 555 tachometer had the
circuit shown (view with fixed font).  In any case,
try to avoid taking the 12v for your circuit from
the 12v terminal on the coil.

Tom

Standard disclaimer, use at your own risk etc....

On Thu, 5 Mar 1998, Simon Quested 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
> ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
>