Re: Capacitor Applications

[Leigh Palmer <palmer@SFU.CA>]

At 11:34 PM +0100 1/15/01, Mark Sylvester wrote:
> At 14:17 15/01/01 -0800, Leigh Palmer wrote:
> >  >   --) The ignition-spark system for a car has an important
> > capacitor in it.
> >
> > Oh? It had one in the old days which was a spark suppressor, used
> > solely to extend the life of the breaker points by preventing a
> > high current spark when the current through an inductor (the spark
>  >coil primary) was interrupted.
>
> Was that its sole function, or did it also increase the rate at which the
> current dropped to zero, thereby boosting the ignition voltage? I have a
> model ignition coil in the lab, with a removable "condenser". Removing it
> makes the induced spark across the high voltage coil more or less disappear.

Since I failed to address my last remark to Bernard in private, perhaps
I shouldn't say any more. I can't even make email work right!

If the breaker points are allowed to carry a current as a spark, the
resulting drop in current is *faster*, but it overshoots zero primary
voltage and continues as an oscillator given the irreducible capacitance
of the circuit. There is also a primary resistance and some reflected
secondary impedance to complicate and complete the picture. The
condenser slows the decay of the primary current, and the oscillation
is damped completely. The resulting peak voltage produced is lower than
it would be without the condenser.

>  >  That capacitor was usually called a
> > "condenser", by the way. In a modern* electronic ignition system
> > the breaker points operate at low voltage and the switching is
> > done by a solid state device. The lifetime of the points exceeds
> > that of most automobiles, and a condenser is not needed.
> >
> > Leigh
> >
> > * Few if any of our students are old enough to remember any other
> > kind of ignition system, and a fraction now approaching 0% work on
>  >their engines.
>
> Well, it's not so surprising. Under the bonnet of my car it looks more like
> inside a TV set than a car!

That is to be lamented, I'm afraid. Old fashioned ignitions were a
wonderful real world example of the application of Faraday's Law,
and the boys, at least, had their interest piqued.

I may have figured out ignition systems all wrong; I've never read
anything on the theory; I just worked out an explanation that
satisfied me about why the condenser was there. I never gave that
explanation to a class because the question never came up. The
condenser was always supplied with the kit, so I replaced it as a
matter of course. I couldn't get the kit cheaper without it. I no
longer work on my own car (a 1989 Dodge Caravan) except simple
things (I have a shop manual). So few things are simple any more,
and I have so much more money and less time now than I had in my
youth.

Leigh (in Canada, where we look for a lovely lass under a bonnet!)