> I am designing a distributorless ignition system and have run in to
> many designs on the web, all of which did not say much about
> computing dwell time.  Some systems used fixed dwell (like a
> millisecond or two), some use proprietary chips to compute dwell.
> How does one determine the appropriate dwell time for a particular
> ignition coil?  Too much dwell time and the DIS system may consume
> too much power and may cause the coil to overheat, too little and
> obviously the spark won't be as hot.  I don't have any specific data
> on coils yet, I am going to find some dual-tower coils and see if I
> can rig up a wasted-spark DIS system with a pic, hall-effect sensor,
> etc.   I am just wondering if there is some preliminary calculations
> I can make based on the primary inductance/resistance to determine
> how much dwell time can get me in the ball park for an optimum setup.

I think you will find that the dwell time is highly related to the primary
inductance of the ignition coil. The points need to be closed long enough
for the current to ramp up to near maximum without saturating the coil, and
then open, giving the maximum energy stored in the coil. The resonance with
the capacitor then ensures the "fattest" spark for a given coil/capacitor
combination. For this reason the capacitor is also a critical component that
needs to be matched to the coil you use.

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