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Re: new coil - basic theory

To: mgs@Autox.Team.Net
Subject: Re: new coil - basic theory
Date: Mon, 17 Aug 1998 15:21:12 EDT
Barney Gaylord wrote:

"This is the key to the whole spark trick.  Current flows into one side of
the condenser and out of the other side, but it does not actually flow
_through_ the condenser.  Internally it builds up a charge on the side
where the current is flowing in, and an opposite charge on the other side.
If you were the coil hooked up to that capacitor and only looking at one
side of it, it appears that you just crammed a big wad of electrons in
there, and then they come bouncing back out.  At the moment of the big
spark, it is that wad of electrons that provides the current through the
secondary winding of the coil and that ultimately jumps the gap at the
spark plug.  At that moment you have a circuit with current flowing out at
the spark plug, but no current flowing in on the ignition switch side of
the coil, because the charge has been stored halfway through the circuit in
the little capacitor."

I don't think so, Barney. Electrons do indeed flow into one side of the
condenser, as you stated; however, electrons also flow out of the other side
at the same rate. If you were to place a meter in the circuit, you would see a
current reading in the circuit. At the instant the points open, electron flow
is exactly as if the capacitor were a short circuit. As the electrons pile up
on one side of the capacitor, and leave the other side, current in the circuit
drops off - at an exponential rate - till the capacitor is fully charged: a
surplus of electrons on one side and a shortage of electrons on the other.
Now, if you leave everything alone, the surplus of electrons will try to get
back to the other side of the capacitor, and the meter will again see current
flow, this time in the opposite direction. This current flow will continue,
again at an exponential rate, till the capacitor is fully discharged, and a
magnetic field has again been built up around the coil. Once the capacitor is
discharged, and current is zero, the magnetic field will again collapse,
generating a current - via electron build up on the capacitor, and the whole
process repeats itself. If there were no resistance in the circuit, the
process would go on forever. With the resistance of the coil windings, the
oscillations die off very rapidly, and only a few cycles will occur. If you
look at the voltage on an O-scope, you will see only 3 -4 cycles, each cycle
significantly less than the previous.

It is not the "...wad of electrons that provides the current through the
secondary winding of the coil...," it is the collapse of the magnetic filed in
the primary. The coil is nothing more than a transformer. As the magnetic
lines of force from the collapsing magnetic field in the primary cut through
the windings of the secondary, an output is produced from the secondary. The
stronger the magnetic field and the faster its collapse, the higher the
secondary voltage.

If it were the wad of electrons, why would you need a secondary winding? That
just adds resistance to the circuit. Why not just dump the wad of electrons
straight to the plugs with a piece of plain wire? And how would that wad of
electrons get from 12 volts to 20,000 volts?

>> 6.) Approximately how much current is flowing in the wire from the
ignition switch to the coil when the spark is happening ....?
>  milliamps to 0.

Give that man an A+ and a hall pass for the rest of the period.

I don't think so. As the spark occurs, current through the primary is
transitioning from its value when the points were closed to zero. Sometime
during this transition period, the spark occurs. I take back my previous
answer to this question. I mis-understood the question, even though it was
very clearly stated.  When the spark is happening, the current is between
about 3 amps and zero.

As an interesting exercise, draw out the schematic for this circuit - just the
battery, ignition switch, coil, points, condenser, and the spark plug. I think
you will be surprised at what you see. I certainly was, and quite confused to

Barney also wrote:  

"You may have noticed that the dual spark (waste spark)
ignition coil does not need a condenser to function.  That "wad of
electrons" that fires across one spark plug to ground also comes from
ground via the other spark plug, so it does not need to be stored in the
ignition circuit."

Again, I don't think so. The primary purpose of the capacitor is to shorten
the time required for the primary field to collapse. The "points" in the waste
spark systems are actually a solid state control circuit. I don't have the
internal details available to review, but I would assume the circuits have
whatever capacitance is required built-in. The capacitors used in a points
type ignition system are not very large. Keep in mind, the Crane and the
pertronix systems don't use a discrete capacitor either, but they still use
the same coil setup as the stock points system.

Bill Schooler wrote:

How about putting an ohmmeter on the
condenser - one lead to the connector, the other to the case.  Meter
should intially peg over to zero, then gradually increase to reading
close to infinity.  (If I remember my tech school stuff, an ohmmeter
actually puts out a small voltage to obtain a reading.  Initially the
condenser appears as an "short", but as a charge builds up from the
ohmmeter's applied voltage, the condenser will change to the appearance
of an "open".)

If the capacitor is large enough, this will work, but I don't think these
capacitors are large enough. The time constant is so short that the meter will
not respond fast enough to follow the charge/discharge cycle. I just tested a
spare condensor - known to be working two weeks ago - and got no reading. If
the capacitor is large enough, as in a power supply capacitor, you can charge
it in the morning, and come back later in the evening and still get a reading.
The time constant with these capacitors is long enough that you can watch the
meter slowly return to infinity. If the capacitor is large enough, you can use
a sundial to time it.

If it's not already obvious, let me state it now - I am by no means an expert
on this subject, far from it. I just have enough understanding to ask
questions, and to make statements based on what I think to be so. With that in
mind, Barney also wrote:

a.) What is the source and nature of that "ignition noise" that bothers a
car radio?

b.) Why do high resistance spark plug wires greatly reduce that noise?

I give up, what is the answer? I think I know the answer to a.), but I don't
know b.) I'll wait for your answer.

Hey, guys, I'm retired, I'm not supposed to be thinking this hard!

Dan Masters,
Alcoa, TN

'71 TR6---------3000mile/year driver, fully restored
'71 TR6---------undergoing full restoration and Ford 5.0 V8 insertion - see:
'74 MGBGT---3000mile/year driver, original condition - slated for a V8 soon
'68 MGBGT---organ donor for the '74

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