Another thing to consider:
In a typical points type ignition it was common to use a 6v coil
with a power resistor in series to keep the "running voltage" around 6v.
During cranking the full potential was delivered directly to coil+
bypassing the resistor and thus providing more spark during cranking.

$.02 - KF4HAZ - Lonnie

> -----  From: "Sascha" <sascha@ 
<snip>
> First I thought of adding a emitter resistor and measure the voltage drop
> across it. But this again means power loss 1.5V across the transistor, 0.5
> across the resistor and if the battery is not very strong when I try to
> start the car it will drop to 9-10V so the remaining voltage across the
> induction coil will be as low as 7V. And this will translate as a weak  spark
> right at the moment where I need the strongest.
> I want to drive the coil with a BUT12 transistor which is driven by a PNP  or a PMOS.
> Any suggestions?
>
> TIA,
> Sascha


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