Okay gang,
Before we get any more carried away, for the purpose of discussion Dan
Masters has sent me a copy of a voltage and current wave form for a typical
ignition coil. For those of you with browser capability, I have posted it
on my web site at this address:
http://www.ntsource.com/~barneymg/mgtech/ignition/it1.htm
The diagram shows voltage at the contact points, which is actually voltage
across the points and across the capacitor. It also shows the current in
the primary winding and the voltage at the secondary winding. It does not
show the current in the secondary winding, which would be the actual spark.
I will not go into any long dissertation about what it all means at this
time, but I will attempt to respond to Dan's current thoughts.
At 03:21 PM 8/17/98 EDT, DANMAS@aol.com wrote:
>Barney Gaylord wrote:
>>....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.
I think we're getting into semantics here. That "current" cannot continue
to flow for very long. Although the current flowing out is the same
magnitude as the current flowing in, the electrons coming out are not the
same ones as those going in, and there is a finite capacity for storing the
charge inside of the capacitor.
Please have a little patience while I re-quote Dan's next paragraph in its
entirety.
> 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.
By the diagram you can (could if you had a browser) see that the initial
voltage peak at the capacitor when the points first open is about 300
volts, which is the voltage that would drive substantial arcing at the
contact points if the capacitor was not there. When the capacitor is fully
charged, reverses current direction, rings for a few dying cycles, etc,
this wave worm is happening (current) is happening in the primary winding
of the coil. However, unless you have an extremely fast amp meter (a.k.a.
oscilloscope), you would not notice the current in the wire between the
ignition switch and the coil, because the net total of all this oscillating
is zero amps.
>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.
Dear Dan, a current is a flow of electrons. The voltage created by the
collapse of the magnetic field _can_ drive a current of electrons if there
is some place for them to go (a.k.a.closed circuit). In the absence of a
flow path you see a high voltage, and the energy is dissipated as heat with
no current output. In this case we are talking about the secondary circuit
being completed across the spark plug. This current that is coming out of
the secondary winding must originate from somewhere, as electrons are not
being created out of nothingness. So where are these electrons coming from?
>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?
It wouldn't. That is exactly the purpose of the secondary winding, to
boost the voltage and conduct the output current. But it's still a flow of
electrons going from somewhere and to somewhere and eventually completing a
circuit.
>>> 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 boot.
And apparently still is. As the magnetic field is collapsing the voltage
in the secondary winding is building up, but the current there has not yet
started to flow. Only when the voltage is sufficient to jump the gap of
the spark plug will the current start to flow in the secondary winding, and
by that time the current has largely stopped flowing in the primary
winding. What your wave form diagram is showing for the secondary winding
is voltage, not current. But the current is still flowing out of the
capacitor and into the secondary winding to supply the electrons that will
spark at the plug. All of this happens during the first half cycle of the
magnetic field collapse, and before all of that ringing you mentioned.
>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.
Oh Dan? I don't think so. If the capacitor was not there, and you opened
the points sufficiently fast to prevent arcing, the current would stop
instantly and the magnetic field would collapse accordingly. The presence
of the capacitor actually increases the time for the magnetic 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.
So how about another trip to the library to find us a circuit diagram for
the ignition module for a waste spark coil? Anyone want to place a bet on
whether there's a capacitor in there?
>....
>Hey, guys, I'm retired, I'm not supposed to be thinking this hard!
Aw, come on Dan. That just gives you more time to be a thinker and a
philosopher.
Cheers,
Barney Gaylord
1958 MGA with an attitude
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