# Re: Pulse Width for Ignition Coil

```Original poster: "Eddie Burwell by way of Terry Fritz <twftesla-at-uswest-dot-net>" <eburwell-at-HIWAAY-dot-net>

If the core has much of a gap at all it won't saturate with any sane amount
of current. What will happen is the current will be limited by the DC
resistance of the coil. The inductance of the coil will be "charged" in a
few RL time constants. At T= one RL (resistance*inductance) time constant
the coil will be charged to 63% of the max current. 5RL would put you within
1% of max current so  2-3 sould be sufficient. Release the input current and
voila! You have a spark.

Hope this helps,
Eddie Burwell

At 10:24 PM 5/1/01 -0600, you 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)?
>
>==============================================
>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
>
>
>
>

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