As the points open, the capacitor (or condenser) limits the voltage rise
across the points until the points are far enough away from each other to
avoid forming an arc. This is it's only purpose. Without it, the arc that
forms as the points open will both absorb energy from the collapsing
magnetic field in the coil ( reducing current at the spark plug), and
severely erode the surface of the points.
(Thought experiment : If the purpose was to 'dampen the ringing', the
condenser would be next to the coil, which is much easier than inside the
distributor.)
Although there is some ringing, it is immensely damped by the fact that
once the spark jumps the plug gap, it is basically a short circuit. And,
even if it weren't, it would be eliminated the next time the points close.
A diode wouldn't work (assuming you could find one that is fast enough, and
will handle the power, abuse, etc.) because you WANT the voltage across the
points to rise once they are far enough apart. There's also the small
problem that solid state diodes weren't invented yet when this circuit was
designed, and vacuum tube diodes don't have a low enough forward voltage
drop.
Randall
On Monday, March 29, 1999 11:16 AM, Dave Brink [SMTP:dbrink@niu.edu] wrote:
>
> 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 ri
pples 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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