>
>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.

Or when the mechanic pulls that plug wire to see which cylinder is 
misfiring. Wouldn't THAT be a tragic design? :)


>  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.

I've seen SMPS designs that also limit the turn-off speed a bit, so that 
the collapse is slower, but I'm not sold on the technique.

I do know that the destructive spikes can be so fast that you can't see 
them with a 100 MHz scope and proper probe placement.


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