Just a comment:
This stray or distributed capacitance in the ignition circuit would only be
a small capitanence; many orders of magnitude less then the capacitence in
the capacitor that is supposed to be parallel to the points.
Bob Reisse
76 MGB
NAMGBR 8-3559
At 05:14 PM 8/19/98 EDT, DANMAS@aol.com wrote:
>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
>
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