> An ignition coil is essentially a pulse transformer with a very high 
> output
> ratio.  You want to let the primary go as high as you can tolerate 
> to force
> most of the energy out the secondary.  It seems to me that a simple 
> diode to
> +12V in series with a resistor would be adequate.  The maximum 
> possible
> current in the primary is easily known, so the resistor can be sized 
> so that
> the primary voltage is just below the tolerable limit at maximum 
> current.
> The voltage will be less in all other cases.

Yes. Using a zener has the advantage of taking no energy until 
misfiring occurs. It then however transfers the dissipation problem 
from FET to zener. The resistor has the advantage of better ruggedness 
per $.

    RM



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