If you wish to move this discussion to a public forum, then please note
that I'm arguing that the capacitor's main function is to save the
points.
rtriplett@bjservices.com wrote:
>
> You're right, that's an important part of the capacitor's practical function
>in
> the circuit, but not the main function.
So having the points last more than an hour isn't the primary concern?
> The reason a car will not run (or run
> very well) without the capacitor is because the capacitor serves to "dampen"
> the ringinig of the ignition coil,
Uh, I don't think you want to "dampen" the "ringing". The more energy
flowing through the plug, the mo' better. If there were no capacitor,
you would have an open circuit when the points opened, and excellent way
to dampen any ringing if ever there was one.
> this "ringing" being is a natural effect of
> high voltage step-up transformers. On an Oscilloscope, it is seen as a big
>wave
> followed by a bunch of similar patterns, each one a little smaller till it
> finially rings itself out.
Most people would call that a tank circuit.
> Now, if the point closes again during the decay of
> this ringing, there is much less energy available, ie lower spark.
Hmm, points close, capacitor gets shorted out and discharges, voltage
across primary gets clamped at 12 volts by the battery (read: GIANT
capacitor) in short order. I don't see what you are talking about.
> Think of
> this as a rock thrown in a big pond (ie, no capacitor): the ripples dissipate
> slowly across the pond, with the energy dissipated likewise. Using a
>capacitor
> is like the same rock thrown in a small bucket; it dissipates the ripples
>faster
> and energy is more concentrated where it is needed (at the spark plug). Maybe
> not the best analogy, but you get the idea...
I do get the bucket idea, I don't see how you got there. But you
ignored my point entirely. The capacitor has to charge when the points
open. It's voltage will increase and it's opposition to the applied DC
current will increase. If you just opened the points (quickly enough,
of course) then the rate of change of EMF in the coil would be faster if
there was no capacitor. Now, the capacitor does allow for recursive,
albeit dimininished firings of the plugs, and if the plug gap is still
ionized then it might just bridge the gap a couple times, but not a
primary concern, IMHO, especially if a ballast resistor is used. Does
anyone have typical L, R & C values handy?
> If protecting the points were the
> only consideration, a simple inductive bucking-diode would suffice.
Ok, I'll bite. How is that simpler than a capacitor?
> Richard
Dave
P.S. Please respond via the list OR e-mail, not both, thanks.
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