> IF the spark plug fires then it will clamp the primary to a lower
> voltage based on plug voltage and transformer turns ratio (and other
> 2nd order factors).  If the plug does NOT fire then primary bvoltage
> may rise VERY high. In an automotove circuit you MUST design for
> somethng to dissipate the energy when the plug mis-fires. This may be
> an auxilliary sparkgap, a zener (as Dave mentions) or the FET can be a
> type designed to take repetitive avalanche energy. The energy is
> usually severe and few FETs would stand it indefinitely.  The FET MUSt
> be specifically avalanche rated if used this way - ones that aren't
> will conduct unevenly across the die and fry parts of thermselves
> which leads to total failure.
> 
> The failed FET has probably been destroyed by avalanche energy. What
> precautions are taken in the circuit to prevent this?

You bring up some excellent points, I never considered effect of the
coil not firing. This is probably the reason I have a dead FET.

Righ now I don't have anything in the circuit to handle avalanche
energy. I simply whipped up the circuit to test an idea.

Although I'm using an ignition coil, this little experiment has
nothing to do with automotive ignition. I'm trying to whip up a device
that will aid in the striking of a plasma. The ignition coil is a
cheap and easy method to get HV arcs across a gap.
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