Pulse Width for Ignition Coil

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Steve & Jackie Young by way of Terry Fritz <twftesla-at-uswest-dot-net>" <youngs-at-konnections-dot-com>

Hi,

I need some empirical data on the "best" width of a 12-15 volt DC pulse to a
normal automobile ignition coil (not HEI) that should be used.  I am
building a triggered spark gap driven by an ignition coil as the trigger
source.  I am using a power transistor to switch filtered DC to it - much
like an ignition circuit in a car.  Power transistor is driven by a one-shot
which sets the pulse width.

I believe as pulse width increases, the output spark energy will increase
until the ignition coil core starts to saturate.  Futher increases of pulse
width will just waste power and not produce further spark energy.  Roughly
what would be the pulse width at the point of diminishing returns (core
saturation)?

==============================================
For extra credit, please answer this:  How would one use an SCR run by a few
hundred volts of filtered DC to fire an ignition coil repetitively
(capacitor discharge type circuit)?  I have done it using an inductor in
series with the DC supply so the voltage across the SCR can go to zero and
unlatch.  But this has not proven to be fool-proof, and the SCR stays
latched-up often, putting a short across the DC supply.  Are there SCR-like
devices which can be turned off while they are still conducting current?

I think SCR based capacitor discharge electronic ignition coil circuits work
OK because the conducting SCR causes the power oscillator in the HVDC supply
to quit, droping the voltage to zero, thus allowing the SCR to unlatch.  But
I didn't want to bother with a switching power supply, so I am instead just
using power transistors to switch LVDC to an ignition coil.

Thanks,
--Steve