In a message dated 97-03-31, Bob Sykes wrote:
[...]
> The capacitor is primarily to
> suppress arcing at the points (and RFI of course). It's not really a
resonant
> LC circuit at all.
In a message dated 97-03-31, Dan Masters wrote:
>Actually, no. I had been told all my life that the capacitor was for the
purpose you
>suggested, but not too long ago, I tried to run my TR6 without one, and it
flat would not
>run. Would not even start! Never fired once. Replaced the capacitor, and it
kicked right
>off [...] -snip-
I tried this today and my car would fire intermittenly but would not start
without
the capacitor connected either. I stand (sit) corrected and a little wiser,
but this
still puzzles me.
>When the points open, the magnetic field in the primary collapses to produce
the HV
>output from the secondary. This collapsing field produces a current in the
primary, in
>the same direction as when the points were closed. Without the capacitor,
there would
>be no place for this current to go. An arc is an ineffective current path.
The instant the
>points open, the capacitor, which has been discharged, looks to the coil
like a short
>circuit, as it charges to the 12 volts.
Agreed so far.
> 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.
[we are in agreement about the primary and secondary sides being
linked only by mutual inductance, no DC path]
>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.
[interesting information about books snipped for brevity & bw]
...> ( 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.
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 ;-)
>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!
>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>.
Bob Sykes - - - __,@_\____ s1500@worldnet.att.net
'78 Spitfires - -- }-0-----0-> s1500@aol.com
"Every great and commanding movement in the annals of the world is the
-triumph- of enthusiasm. Nothing great was ever achieved without it."
--Ralph Waldo Emerson
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