Igniter and IGBT quetions

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razzbarry
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Igniter and IGBT quetions

Post by razzbarry »

I am trying to rebuild a coil driver for a Geo Metro I have in a garden tractor. The max RPM of the tractor is 3600 RPB but the typical speed is 3000 RPM. The Geo Metro 993cc engine has a distributor with vacuum advance. I blew the driver and wanted to design a circuit to replace it using an IGBT I got from Allied Electronics. I broke the old module apart and milled the old support bracket to I could put a small circuit board in the distributor to amplify the cam lobe sensor signal using a Darlington transistor to get a very fast rising signal. I plan to feed that signal to a driver for the IGBT. The IGBT driver is a single shot multi vibrator what will turn the IGBT on for 4 or 5 ms after the sensor signal to charge the coil and then turn off to cause a spark.

What Id do not understand is how the original circuit worked. Since the IGBT must be on to charge the coil I assume, we all know what that means, the cam love is cut to give the proper spark time after that IGBT has time to charge the coil. The IGBT will charge during the time the multi vibrator is turned on and cause a spark when the period ends.

The other part I don't understand is how the timing will work. With points the coil is charging for variable periods of time depending on the engine RPMs, and fires when the points open. With a sensor and IGBT the sensor starts the coil charge time and then causes the spark later. Is this delay already built into the sensor cam profile? Does the engine already expect a charge time after the sensor signal? I also want to replace the IGBT with a CDI later for better cold starting in MN. It gets really confusing how to time the IGBT or MOSFET used to discharge the capacitor into the coil. The IGBT when used to charge the coil there is a charge delay that is not needed with CDI. With CDI the capacitor can be charged starting at the end of the spark and be ready to fire again. I have also seen topics for multiple spark systems using CDIs since a capacitor can be charged very quickly, in micro seconds.

From what I have read it is not acceptable to turn the IGBT on for long periods of time. It will over heat and also over heat the coil. If it is turned on from 3-5 ms then it can charge the coil and cause a good spark. But a 5 ms charge time when the engine is running at idle will cause the spark to be way advanced, 5 ms when the engine is running at 3000 rpm will cause the spark to be retarded. Maybe this is already designed into the distributor since it already has a sensor and probably used an IGBT for the spark as I outlined above and I can simply build a drive that will turn the IGBT on for 5 ms. It seems the vacuum advance will also have a play in how far the spark advance is. Please help me understand the dwell and spark timing when using a sensor in a distributor with an IGBT driver and CDI systems?

Thanks,
Barry
nimblemotors
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Re: Igniter and IGBT quetions

Post by nimblemotors »

you still around barry?
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Fred
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Re: Igniter and IGBT quetions

Post by Fred »

Send him a PM and if his email address is still the same he should get a notification. If you use the PM button in his post, bottom left, then it will contain a link to this thread by default. :-)

This post must have come up when I was very busy for some reason. I usually try to make sure everyone gets an answer fairly promptly. Yes, I was in Spain with a girl who is now my wife! :-)

Fred.
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