# Re: Pulse Width for Ignition Coil

```Original poster: "Jim Lux by way of Terry Fritz <twftesla-at-uswest-dot-net>" <jimlux-at-earthlink-dot-net>

A useful source of information on this sort of thing is a data sheet for one
of the ignition IC's, like the Motorola MC3334 ("high energy ignition
circuit") data sheet
(online data sheet at
http://www.mot-sps-dot-com/books/dl128/pdf/mc3334rev0f.pdf).
The data sheet describes the chip as being used in aftermarket Delco (GM)
five
terminal ignition applications.

Basically, you want to limit the current to around 5A. Knowing the nominal
inductance (around 8mH), you can figure out how long it will take to get
there (Ldi/dt = V... 12V = 8E-3 * di/dt.. 1500Amp/second, 5A is 1/300 second
(3.33 msec))

You can kind of bound your work by noting that the coil has to "charge" fast
enough (i.e. the current has to get to saturation) to allow a V8 to run at
6000 RPM (i.e. 100 Rev/second * 4 sparks/rev => 2.5 msec/spark).

----- Original Message -----
From: "Tesla list" <tesla-at-pupman-dot-com>
To: <tesla-at-pupman-dot-com>
Sent: Tuesday, May 01, 2001 9:24 PM
Subject: Pulse Width for Ignition Coil

> 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
>
>
>

```