Forgot to mention connecting a line from the "start" position of the ignition switch to the coil+ to provide more spark during cranking (when the battery was being pulled lower than normal)

Better still, use a 6 volt coil with an automotive "coil resistor" in series with coil+
add a "condenser" (small high voltage cap) from coil- to Gnd. and a 2.5Amp 1Kv diode from coil- to 12V.
Connect Drain lead to coil- with source grounded.
I ran this setup on a VW for many years, gateing the mosfet with a pnp pull-up with the base grounded by the points (through a resistor), not only did the mosfet hold up, but the points no longer burned-up so frequently. I did have to bump the timing up a degree or 2? to compensate for the switching times of the transistor and mosfet.

KF4HAZ - Lonnie

----- From: "Russell McMahon" <apptech@

> > 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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