Scions,
An interesting discussion (on caps & coils) in today's digest.
FWIW an additional 2 pence:
It was written:
[R.T.]
> > ...A capacitor stores an electrical charge, a coil doesn't.
The energy delivered to the spark plug can be stored in either
an inductor or a capacitor. Although the inductive storage
method is the more common approach, both are used.
[R.T. (about arc suppression)]
> > You're right, that's an important part of the capacitor's
> > practical function in
> > the circuit, but not the main function.
[D.B. replies]
> So having the points last more than an hour isn't the primary concern?
My points will last for a very long time with the capacitor
disconnected. The car won't start. FWIW it's a ballasted Spitfire.
Point is (groan)- The capacitor performs a couple of very important
functions. It limits coil secondary voltage rise-time, it third-
harmonic tunes the coil, providing for rapid primary current decay
and increasing secondary output voltage. Oh yea, and it prevents
points meltdown due to arcing ;-)
[D.B.]
> 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.
The "ringing" occurs after the spark event is over.
The (secondary) voltage starts to rise when the points open and
increases until it's high enough to "jump the gap". It then
remains constant at the sustaining voltage for ionization across
the spark plug. When there is insufficient energy left to maintain
the discharge, current flow ceases and the remaining energy is
dissipated by ringing.
[D.B.]
> Does anyone have typical L, R & C values handy?
Typical Ignition-Coil Parameters
Turns Ratio 100:1
Secondary 25,000 turns #41
Primary 250 turns #22
Primary Inductance 6 to 10 mH
Primary Resistance about 1.5 ohms
Secondary Inductance 40 H
Secondary Resistance 10 kilohms
For a much more comprehensive description of how points, plugs
and coils play together in LBCs, take a look at:
http://www.vtr.org/maintain/ignition.html
I have some graphs of ignition system dwell waveforms (voltage
& current) that were the basis for the above page. If you're
really really interested, contact me off-list and I can send
them.
--
-Bob
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