Tach input from spark voltage (idea)

[Johnny]

The voltage on the primary side of the coil varies with the type of
ignition. On a good ol' points & coil ignition it is: battery voltage minus
the ballast resistor. On the coil it will say 12 volts, or 24 volts in some
applications. The primary coil voltage on a CD ignition or various
electronic ignitions (depending on configuration) can vary greatly. I have
one box that uses coils that see about 400 volts on the primary side. Some
of these systems use a standard 12 volt coil, some do not. I know that this
answer isn't as cut and dried as you had hoped, but they are not all the
same. If you are looking to do a "standard" ignition, the primary voltage
is less than 12 volts. Are your concerns for an inductive pickup about
saturation or calibration or?

-j-

----------
> From: Grant Beattie <grantb at nait.ab.ca>
> To: diy_efi at coulomb.eng.ohio-state.edu
> Subject: Tach input from spark voltage (idea)
> Date: Sunday, October 20, 1996 10:29 AM
> 
> 
> I've been working on various inductive tach schemes with some success.  I

> want to try a new approach which would be to take the input to the tach 
> from the "points" or "tach" signal from a car distributor (or whatever).
> 
> In case my description is lacking, from the input or low voltage side of 
> the coil (or cap or whatever).  What voltage range should I expect this 
> signal to be in?  Are we talking tens of volts or hundreds?
> 
> I understand that the spark plug sees kV, but the input to the spark 
> circuit should be quite a bit lower, right?
> 
> My application is data acquisition for cars/karts/motorcycles.
> 
> Thanks,
> GB
> http://ourwolrd.compuserve.com/homepages/grantb