# Re: Pulse Width for Ignition Coil

Original poster: "Malcolm Watts by way of Terry Fritz <twftesla-at-uswest-dot-net>" <m.j.watts-at-massey.ac.nz>

Hi Steve,

On 1 May 01, at 22:24, Tesla list wrote:

> 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)?

The diminishing returns come when the current ramping up in the
primary coil reaches a maximum imposed by the DC resistance of the
coil. At that point, more voltage is needed to increase it and the
core energy further. So the answer depends on the DC resistance of

> ==============================================
> 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?

Yes. They are known as GTO thyristors (SCRs). I have not personally
used them. Another method used in flashguns (a largely non or slow
repetitive process) is to fire a small SCR with a capacitor inseries
with its anode such that it connects across the main SCR and robs it
of current for long enough to allow conduction in it to cease.
Obviously the cap then has to be discharged to allow the trick to be
re-applied. If I was really interested I could probably design a
circuit to make use of the effects of the now shut off inductor to
also allow a rapid discharge of this little capacitor.

> 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.

Probably far simpler and just the way I've done it in the past.

Regards,
malcolm