# Re: Pulse Width for Ignition Coil

```Original poster: "Ed Phillips by way of Terry Fritz <twftesla-at-uswest-dot-net>" <evp-at-pacbell-dot-net>

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)?

Last sentence is exactly correct; when the core saturates no more
energy can be stored in it.  I have found quite different maximum pulse
widths for different coils, ranging from perhaps 1 ms to 10 for coils
operated on 12 volts.  Pulse width goes down inversely as voltage.  Put
a small resistor in series with the primary (even a piece of small wire
will do), start with a narrow pulse and widen it until you see the
current at the end of the pulse increase abruptly.  You can also look at
the primary voltage and observe the pulse width at which it "bottoms
out".  You can also put your finger on the case of the transistor (turn
power off to avoid getting zapped with the HV pulse on the collector and
feel it immediately) and increase the pulse width until it begins to get
warm.  Crude method but works.  You can also watch the average primary
current with an ammeter and see where it begins to increase rapidly.
All crude things to do, but if you try you'll get a feel for what is
happening.

Every different coil also requires a different value of shunt "snubbing
capacitor".

Ed

```