Barney,
You're right, this could be it. Not because we've solved the problem, but
because we've run out of things to say. I've said everything I know about it,
and your posts are, as you yourself stated, just a re-hash of what has been
said before. You still haven't addressed the concerns I brought up last time.
After this post, I will stop my participation until I know enough to make a
meaningful contribution. When I get the answers, and I will get them, I will
post back to the group with what I find.
> >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>
Good grief, Barney, you don't understand the concept of distributed
capacitance? That's simply amazing! Distributed capacitance is real, it's not
just a figment of my imagination. Distributed capacitance is so real in fact,
that often, especially in high frequency circuits, designers use it in place
of real capacitors. Most often, though, designers have to figure out some way
to negate the effects of distributed capacitance. As an example of using
distributed capacitance, the output stage in the radar systems I used to
maintain for the Air Force had neither descrete capacitors nor inductors. The
circuit relied entirely on distributed capacitance and distributed inductance.
Not randomly distributed of course. They machined a cavity into a solid chunk
of steel (or perhaps aluminum, I don't remember now) - the walls of the cavity
formed the capacitor, and the surface of the cavity formed the inductors. We
tracked and guided missiles for up to 200 miles using this system.
I wouldn't even attempt to explain this to you, but for the benefit of others
on the list that may still have an interest, I'll give it a try. Between the
HT post of the coil and ground, there is a capacitor. Between the plus and
minus terminals of the coil, there is a capacitor. Between the case of the
coil and the windings, between the secondary and the primary windings, between
each loop of each coil winding, in fact, anywhere there are two pieces of
conductive material separated by a non conductive material, there is a
capacitor. You won't find a part number for them, in fact, you won't even see
them, but they are there never-the-less. These capacitors are refered to as
"distributed" capacitance, simply because they are not contained in tidy
little containers, but are "distributed" all over the place. The capacitor
from the HT post to ground, for example, is "distributed" along the entire
length of the high tension leads, both the coil to distributer lead, and the
individual plug leads.
As an exercise, refer to the diagram on Barney's page and start drawing in
capacitors. Just put them any where, between any two components you choose.
Wherever you put them, you are not wrong. They are there. After you've drawn
them in, imagine what this does to a circuit analysis problem! Particularly as
the values are not known. Fortunately, in most cases, the capacitance values
are so small that they may as well not exist.
Not always, though. Suppose you have a capacitor between the secondary winding
and the case of the coil (you do, as a matter of fact - I just don't know the
value of it). The case is grounded, so you have another capacitor, in addition
to the one we normally think of, directly between ground and the secondary. If
this capacitance is large enough, wouldn't it (chuckle, chuckle) make a nice
return path from the plugs to the secondary?
As I said before, I don't know if this capacitance is large enough to make a
difference or not. I have read, though, in at least one text book, references
to this capacitance as a return path. Unfortunately, I no longer have access
to this book, so I can't confirm this. And, as I remember, the reference was
not definitive.
I said earlier that I had exhausted my knowledge and had nothing more to
offer, but that's not entirely true. While looking for a reference for
distributed capacitance, I ran across another interesting article. According
to Herbert E. Ellinger, auther of the book "Automotive Electrical Systems,"
the spark consists of two components. According to him, the spark is initiated
by capacitor discharge, and maintained by the magnetic field decay. What
capacitance was he referring to? The points capacitor? Not at all, he very
clearly and precisely identified this capacitance as the (chuckle, chuckle)
distributed capacitance, mostly between the windings of the secondary. In
fact, he made no reference whatever to the descrete capacitor across the
points. Whether that omission is significant or not, I don't know.
Maybe I'm wrong (or maybe Ellinger is wrong), but it seems to me that if this
capacitance is large enough to generate a spark, the coil-to-ground
capacitance just may be large enough to provide a return path for the
secondary.
One other observation: According to P. M. Heldt, author of the book "High-
Speed Combustion Engines," some of the earlier ignition coils had the
secondary winding connected directly to ground, rather than to the points as
in modern systems, ie the coil had four terminals rather than three. If
capacitor discharge is vital to producing a spark, then the capacitance would
have to be (chuckle, chuckle) distributed capacitance, as there would be no
other capacitance in the circuit.
> 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.
Trust me, Barney, the light will go on. I will understand this circuit. Will
you?
> May we all one day be graduate philosophers,
Some of us are content to go through life just knowing that things do work;
some of us have a burning desire to know how they work. For good or bad, I
belong to the latter group.
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
|