Tach input from spark voltage (BEWARE!!!)

[hoss karoly]

Johnny wrote:
> 
> 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?
> 
wow ! don't try it unless U wish to fry a few components
the good 'ol points & coil sys the primary runs up to 3-400 volts
the spikes are MUCH higher so BEWARE !
if you wish to use some impulse use a hall sensor it's smaller than a
teardrop
or cut the pulses with a big resistor and zener diodes but it can affect
the ignition

bye
charley