Re: bridged ignition coils

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

Original poster: "Stephen Conner by way of Terry Fritz <teslalist-at-qwest-dot-net>" <steve-at-scopeboy-dot-com>

At 10:51 02/05/03 -0600, you wrote:
>Original poster: "Jolyon Vater Cox by way of Terry Fritz 
><teslalist-at-qwest-dot-net>" <jolyon-at-vatercox.freeserve.co.uk>
>
>
>----- Original Message -----
>From: "Tesla list" <tesla-at-pupman-dot-com>
>To: <tesla-at-pupman-dot-com>
>Sent: Friday, May 02, 2003 12:06 AM
>Subject: Re: bridged ignition coils
>
>
> > Original poster: "Stephen Conner by way of Terry Fritz
><teslalist-at-qwest-dot-net>" <steve-at-scopeboy-dot-com>
> >
> > At 07:34 01/05/03 -0600, you wrote:
> > >Original poster: "Jolyon Vater Cox by way of Terry Fritz
> > ><teslalist-at-qwest-dot-net>" <jolyon-at-vatercox.freeserve.co.uk>
> > >
> >
> > I don't know about the drive circuit though. You need hundreds of volts
> > going into the primary.
>
>Aren't the hundreds of volts induced in the primary as a result of the rapid
>interruption of the primary current?

Yes

>Therefore, if the rate of change in the primary current was sufficiently
>fast wouldn't the above requirement be fulfilled, and  wouldn't a bipolar
>square wave,with a fast rise and fall times, be sufficient to do this?

No, assuming that you're driving with an H-bridge like you suggested. The 
H-bridge never interrupts the current, it just changes the voltage. As you 
know, for any inductor, V=L*di/dt. With the H-bridge circuit (and the 
capacitor discharge one) you're enforcing V and the coil determines di/dt. 
The rise/fall time of the voltage waveform doesn't really come into the 
equation.

With a flyback circuit, that builds up a high DC current and then 
interrupts it by opening a switch, you're forcing di/dt, and the coil 
determines V.

The upshot is that capacitor discharge and H-bridge drivers need more 
supply volts than a flyback to generate a given di/dt, in fact about L^2 
more ;) The classic flyback circuit runs happily off 12 volts, whereas the 
others need at least 300 for the same spark.

Steve C.