Daniel:
Per your request, following is the text of the response I sent to Bob. Unless
we get a lot of complaints, I will continue to post to the list. I am pleased
that someone finds our rambling enjoyable! It is getting hard to follow,
because we are responding to a response, to a response, etc. I will try to
clean it up in future posts.
Bob:
I agree, we should take this off the list before we get flamed. I am glad you
picked up this thread, I am enjoying the exchange!
In a message dated 97-04-01 21:10:42 EST, you write:
<< > Without a good current path, the field collapses too slowly, and
produces a weak
>spark.
>I'm not so sure about this. With a good current path (points closed) the
field
>does not collapse at all. If we suddenly interrupt the primary current
path
>(points open, no cap) the field collapses at the maximum rate, producing
the
>highest voltage. The capacitor slows the rate of change in current and
consequently
>the rate of magnetic field collapse, reducing the voltage.
I am not so sure, either. The books I read implied this, and to some extent
it makes sense. When the field collapses, it tries to maintain the original
current flow, one of the characteristics of a coil that makes it a good
filter. With no path, where does the induced current go? But, as you said,
the whole point of opening the contacts is to stop the current flow. At this
point, I am not sure. Maybe your original statement is correct, except it is
the secondary current that returns through the cap?
>[we are in agreement about the primary and secondary sides being
>linked only by mutual inductance, no DC path]
I am beginning to have second thoughts about this,also. After our e-mail
exchange, I went back and did a little more research. Looking at a TR4 manual
(about 15 minutes ago, in fact), I noted the ignition schematic did not show
a ground path for the secondary winding, except through the
breaker/capacitor! Thinking this was an error on the part of Triumph, I went
back to the '50s book, and looked again. The schematic clearly showed a
separate ground for each winding, but the physical cut-away drawing showed
the same as the TR4 manual, and the text described it as so. Thinking about
the coils on our cars, the only way for a separate secondary ground would be
through the case, and I don't think that is the situation. Now, I am really
wondering how the heck the spark current gets back to the secondary.
>I agree that resonance is not of concern here (at least I don't think it
is),
> but any time you have a capacitor and an inductor wired together, either
series or parallel,
> there is a resonant circuit. In this case, I think the resonant frequency
is
> outside the range of concern.
>Agreed, but I'm beginning to wonder if resonance doesn't play an active
part here.
>Initially I didn't regard this as a resonant circuit because the the
capacitor is effectively
>shorted out by the points ~90% of the time.
(in a 6 cylnder car, with a dwell angle of 45 deg, it is closed 75% of the
time)
May be. At this time, I am not sure on the primary side. The resonance I
noted on the output side occurs AFTER the plug has fired, and the points
close. Could this be the results of distributed capacitance in the wiring,
etc? If so, is this the return path for the secondary current?
>[interesting information about books snipped for brevity & bw]
Unfortunately, there seems to be discrepancies within the books, not only
between each other, but with-in themselves as well. I am going to go to the
University of Tennesse bookstore sometime in the next week or two, and see if
I can find an up-to-date text. Alcoa is only 15 miles from UT. (by the way,
when reading the '50s book, I nearly went crazy trying to figure out how they
got their numerical values - they were always either 1/2 or twice what I
thought they should be. Then it dawned on me, in 1950, cars had 6 volt
batteries!
...> ( have a reasonal grasp of electrical engineering principles, but the
peculiar aspects
> of automotive ignition systems is new to me). Now that I am retired, I
have time to
> persue such stuff as this.
I spent my entire career designing Instrumentation and Control systems, so
much of my education on this type stuff is long since gone. I graduated 29
years ago! Very little of this was required for I&C work.
>The more I learn about automotive ignition systems, the more peculiar they
appear to me.
>My understanding of inductive circuit principles comes mainly from the
'laws' of Kirchhoff &
>Lenz but these guys obviously weren't exposed to automotive electrical
systems ;-)
Amen to that! Thats one thing a lot of people don't understand. Princibles
only get you started. There has been so much design work done on any aspect
of engineering, that the steps taken to get where we are at now are taken for
granted by the engineers versed in that aspect, and you really have to dig to
understand.
>As an example of my lack of understanding, according to the books, there is
some
> resonance in the output of the secondary, which has no discrete capacitor.
> This puzzles me. Can anyone explain that?
>I'm afraid I can't, but would love to hear someone's theory!
See above (?)
>I appreciate your comments, and look forward to an enjoyable discussion of
this
> subject with the members of this list
>I have enjoyed this and learned something that I never would have thought
>possible. I guess I'll have to add a condenser to my 'road trip kit' ;-)
>I fear that this subject may be getting a little esoteric for the "Triumphs
List"
>though. Especially in light of some recent posts.. I would be glad
>to pursue this further 'off line' ( I can get a little long-winded).
Posting
>schematics and equations describing series resonant circuits might elicit
>some complaints<g>.
>>
Yes, I have enjoyed it also. <<snip>>
As I learn more, I will keep you informed, and please do the same for me.
Dan Masters,
Alcoa, TN
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