I have been struggling with this for a week or two.
It is called a high side switch and it is used
to isolate the 300 supply when the 1 mfd cap
discharges through the coil.

The single chip half bridge drivers that are supposed
to do this are not designed for a cap discharge ignition
system that must run from a dozen rotations of the engine
when it is starting to 7500 RPM or 125 PPS.
Also the cap discharge ignition system duty cycle
must deal with pulses only a few hundred micro seconds
wide in a period of 8 milliseconds. Most high side
switch chips expect a 50% duty cycle.

This may look simple as frequently good ideas do
but this doubles the voltage available for firing
the high side FET. Works so well I was surprised.

It relies heavily on opto isolators
and CMOS CD4049's which are 50 cent chips.
Six to a package so that makes each amp less
than 10 cents.

Here is the schematic and a table of traditional
approaches to this problem. Note; this trick is
not in the table :)

It took me a half hour to mock it up and test it
once I had the idea.

I'll be using a cheap ATmel single chip computer
to handling the timing of turning off the PS and
discharging the cap. It is easier to do that
than provide the necessary delay with logic gates.
The Atmel chip is programmed in ZBasic just
like the main computer.

The schematic is one of four identical circuits for each
leading plug. The leading plugs will fire four times, 10 degrees
apart, of eshaft rotation. The trailing plug will use
some other entirely different ignition system
as a safety feature. This is a highly redundant system.
If one of the these circuits fail you may not
notice the difference except for a slight increase in
fuel burn.

Paul Lamar

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