Fellow listers,
I'm sorry it has degenerated into this. If I said anything in any of my
previous posts that in any way could be construed as a put down of Barney, or
anyone else for that matter, it was completely unintentional, and I apologize
for it. It is unfortunate that Barney now feels the need to talk down to me.
As I said before, I don't fully inderstand the operation of the ignition
system, but I do understand basic electrical engineering principles.
I would let this drop, but Barney has made some incorrect statements that I
believe need to be corrected. Many of you have, understandably, lost interest
in this subject a long time ago, and you may wish to hit the delete key now.
In the interest of brevity, I have made massive snips, so you may want to
refer to the previous posts.
Barney wrote:
> 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.
Yes, the current cannot continue to flow for very long. In my previous posts,
I stated when and how it would stop, and what would happen next when it did.
As for the electrons coming out not being the same as the electrons going in,
that is completely immaterial. What ever electrons are flowing through the
meter I mentioned will register as current just the same. The meter does not
do an identity check as they flow past. Do you believe the electrons that flow
through a switch are the same electrons that flow through the light bulb
controlled by that switch? Contrary to what you may believe, electricity is
not a "flow" of electrons through a wire in the same manner as a flow of water
through a pipe, although it is helpful to think of it that way when trying to
understand a circuit.
Where do these "alien" electrons go when they leave the capacitor? There are
only two options - either through the battery back through the primary and
then to the other side of the capacitor, or backwards through the spark plug
and through the secondary to the other side of the capacitor. There is no
other option that I can see. They (or other electrons just like them) WILL get
back to the other side of the capacitor. Which path do you think they will
take? Although there is no "flow" of electrons through the capacitor itself,
there is in every other part of the circuit.
> Dear Dan, a current is a flow of electrons.
Where did I imply otherwise - at least for the purposes of this discussion, as
stated above? I managed to make it through the required course work to get
both a certification as a radar technician in the Air Force and a BSEE from
the University of Tennessee, so I have a basic concept of electricity. This is
not the place to get into a discussion of the "current flow" vs "electron
flow" schools of thought.
> 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?
I will answer this question with two questions, and then with an answer. Where
do you think the electrons come from in your alternator? Is there a capacitor
stuffing "wads" of electrons into the windings? The electrons are always
there, in the wires which make up the windings, either the coil secondary or
the output windings of the alternator. As you learn in any freshman electrical
engineering or physics course, the movement of a magnetic field with respect
to the wire simply "motivates," if you will, the existing electrons to move,
provided there is a path for them to move. There is no "electron pump"
injecting electrons into the circuit. Electrons leaving one end of the winding
must/will return to the other side.
> 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.
Explain to me please, if you will, how a capacitor pushing electrons through
the secondary of a coil can boost the voltage. I will admit, this is a concept
I am not familiar with. If true, though, I can see enormous applications for
such a process. I'm sorry, but the high voltage (and the spark current) coming
from the secondary is the result of the collapse of the magnetic filed in the
primary, and not the discharge of electrons from the capacitor.
> 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.
If primary current is only "largely" stopped flowing, then some current must
still be flowing. If the primary is still "stuffing" electrons onto the
capacitor, how then can the capacitor now be ejecting electrons through the
secondary? Are the electrons ricocheting off the capacitor like bullets off an
armored tank?
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.
Again, you have electrons going into and out of the same side of the capacitor
at the same time. A pretty neat trick if you can get it to happen. Do you have
any idea of the energy capacity of the capacitor? Do you think it can store
enough energy/electrons to support an arc across the plug gap?
Up to this point, I have no trouble understanding the circuit. It is only
after the plug fires that I am confused. As we have both said, the secondary
fires during the first half cycle of the primary discharge. Now, we have
current flowing from the primary into the capacitor, and current flowing from
the secondary to the plug, across the plug gap, to ground, and to....where?
There are only three possible paths for this current - a) through the ground
connection to the battery, through the battery and the wiring to the primary,
through the primary, and then back to the secondary, b) back to the secondary
through the capacitor, which we have already said is still flowing current
(aka electron flow) from the primary in the opposite direction, or c) back to
the secondary through distributed capacitance in all the large amounts of
metal surrounding the engine/ignition system. Or perhaps a combination of all
three. If some one could clarify that for me, someone who actually understands
the theory involved, I would be most appreciative.
> >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.
YOU may not think so, but EVERY engineering text book I have read says so. And
they give the equations to prove it. Remember, we are not just talking about
stopping current flow - our primary interest is in reducing the time for the
magnetic field to decay. The next time you are at your local library, check it
out, don't take my word for it. Want to make a small wager?
As a matter of fact, no matter how slowly or how rapidly the points open,
current flow will be stopped instantly. Maybe not at the time you would like
it to, if there is arcing, but instantly never-the-less. Current will flow,
almost un-impeded, as long as there is an arc. When the points are open far
enough to interupt the arc, current flow is stopped.
> 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?
Not I! I don't know what's in the Crane/Allison or the pertronix unit that
lets then get by without an external capacitor. What do you think is in there
- a super fast switch? One that opens fast enough to prevent arcing? A super
fast, non arcing, transister? Technology marches on. At one time, a capacitor
was required to get fast enough primary decay, who knows what they have now.
But you can bet I'll find out.
Perhaps a switching transister and a control circuit that regulates the
conduction of the transister to mimic the characteristics of a capacitor? Such
circuits exist. Perhaps the capacitor is only required to compensate for the
switching chracteristics of a mechanical on-off switch (points)? Perhaps the
circuit generates the output voltage by turning ON the current through the
primary, rather than OFF? Some ignition circuits do just that.
One thing I do know, though; If there is no capacitor in the Crane unit I'm
using on my old fashioned coil ignition, then it certainly isn't stuffing
electrons through the secondary. Maybe I'm wrong and something else is
stuffing electrons, but it ain't the capacitor if there ain't no capacitor.
Dan Masters,
Alcoa, TN
'71 TR6---------3000mile/year driver, fully restored
'71 TR6---------undergoing full restoration and Ford 5.0 V8 insertion - see:
http://www.sky.net/~boballen/mg/Masters/
'74 MGBGT---3000mile/year driver, original condition - slated for a V8 soon
'68 MGBGT---organ donor for the '74
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