At 10:20 PM 8/18/98 EDT, DANMAS@aol.com wrote:
>
>In a message dated 98-08-18 16:06:49 EDT, barneymg@ntsource.com writes:
>> As there is a current flowing from the secondary winding across the
spark plug to ground, what path do you ascribe to that current for it to
return to the other side of the secondary winding to complete the circuit?
This was the basic question from the beginning.
>
>I don't know. .... let me describe the circuit, and current flow, as I see
it, and tell me what you think. ....
>
>Prior to the opening of the points, electrons are flowing from the
negative post of the battery, through ground, through the points, through
the primary, and then back to the battery positive post. When the points
open, the coil tries to maintain electron flow in the same direction as
before. Now, however, instead of flowing through the points, electrons pile
up on the ground side of the capacitor, leave from the other side, and flow
through the primary to the battery just as before. Current will decay as
the capacitor charges. Until the primary current flow drops to zero, we
will have current flow through the battery.
I hesitate to even mention which way the electrons are flowing, but
otherwise, you have the right idea.
>Sometime during the decay, but before the primary current drops to zero,
the plug will fire. When it does, electrons will leave the high tension
post of the coil secondary - this post being negative with respect to
ground - flow through the tip of the plug, jump to the electrode, to
ground, and.......... ?
>
>Somewhere between the ground plane and the other side of the coil
secondary is a return path for the electrons.
Still stuck on the direction of flow, but still okay.
>Is it the capacitor? If it is, the capacitor is being charged by the
secondary current, not discharged.
Yes, it's the capacitor. And no, the capacitor is not being charged and
discharged at the same time, it's just acting as an accumulator. Bear with
me if some of this sounds like a re-iteration (because it is).
When the points open the primary current continues momentarily and the
capacitor charges up. (And yes, I know there's a complimentary charge and
current on the other side of the capacitor.) While the capacitor is
charging, the current is decaying rapidly, resulting in a rise of voltage
in the secondary winding. When the voltage in the secondary is
sufficiently high, the current suddenly starts to flow through the
secondary and across the spark plug. At the exact instant that the
secondary current starts to flow, the voltage at the capacitor has reached
about 300 volts (peaked), and the capacitor is as fully charged as it gets.
Current then flows from the capacitor through the secondary winding to the
spark plug.
And now I'm about to make a small concession (extremely small in
magnitude). At that precise moment when the spark starts, there is still a
small flow of current in the primary, before it stops dead and starts to
"ring". Both the primary and secondary windings are connected to the
capacitor, and the flow into or out of the capacitor is the sum of the two
currents. When the current starts to flow out of the capacitor into the
secondary winding, the small amount of current still flowing in the primary
winding then flows also into the secondary winding, but it is quite small
in magnitude compared to the current coming from the capacitor. Now the
spark is extremely fast, and the current in the secondary stops again
before the current in the primary is fully decayed (just barely before),
and as was stated earlier, it's all over but the ringing.
The fact that the current from the capacitor into the secondary is much
larger than the current from the primary into the secondary is the reason
that the ignition will not function well without the capacitor. If the
engine runs at all without the capacitor, it would run very poorly because
of an especially weak spark.
And as to how much current is flowing in the wire from the ignition switch
to the coil while the spark is happening, I believe the answer was "0 to
milliamperes". And that answer is correct. When the spark starts the
current in the primary (decaying exponentially) is already less than 1 amp
(milliamperes), and by the time the spark is finished the current in the
primary is WAY much closer to 0. After the spark has ended, the ringing in
the primary is reflected in the wire from the ignition switch, but is
ringing at very high frequency and at less than 24 volts rms (within +12 to
-12 volts) on the switch side, and the net sum of that current is 0.
>Is it the battery? If so, the current would also have to flow through the
primary, and I think we would see that on the waveform, which we don't.
The ringing seen on the waveform is at the capacitor, on the points end of
the primary winding, and at a relatively high voltage (but less than the
300 volt peak). Because of the inductance [is that a new word here?,
sorry] of the coil providing a flywheel effect to the current, there is the
high voltage ringing in the coil, but the voltage on the switch side of the
primary is much lower because it is damped by the battery. That is, the
connection to the battery allows the current to flow and oscillate mostly
unrestricted so the voltage does not build up on that end of the primary
winding. The ringing is in fact present on the switch end of the primary
winding, but the voltage is especially small there such as to be negligible.
>Distributed capacitance? Maybe, but I don't see that accounting for all of
it.
(Chuckle, chuckle.) I would truly love to hear someone explain that one.
<VBG>
>I'm missing something here, and probably something obvious. When the light
finally goes on, I will most likely be embarrassed that I didn't see it
sooner. What am I missing?
>
>Dan Masters
Well, you're probably not missing anything. Very few people in the world
actually have a need to know any of this, including me. But I've always
thought that challenging thoughts were interesting. In the end it's an
extremely simple function with a truly bizarre (counter-intuitive)
explanation. And when the light does finally go on (if it ever does), you
should feel downright proud of yourself for grasping such a strange concept
and conquering your intuition. And furthermore, you should be absolutely
entitled to a large portion of the beverage of your choice and at least a
one hour chat with someone close to you who most likely will not give a
damn about the explanation.
May we all one day be graduate philosophers,
Barney Gaylord
1958 MGA with an attitude
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