In a message dated 97-03-31, Bob Sykes wrote:
<< The coil doesn't discharge through the cap. It discharges through the
secondary
HT side. It will work pretty much the same way even if there is no
capacitor at
all, although your points won't last nearly as long. The capacitor is
primarily to
suppress arcing at the points (and RFI of course). It's not really a
resonant
LC circuit at all.
>>
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 and ran like a champ! Point
protection is a critical function, however. Suppressing the arc will, of
course, prevent RFI from the points. The primary source of RFI is from the
secondary side.
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. Without a good current path, the field collapses too slowly,
and produces a weak spark.
The secondary side of the coil is completely isolated from the primary side
of the coil, from an electrical standpoint. The only interaction is the
magnetic field. There is no path for the primary to discharge through 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.
Not too long ago, while browsing through a used book store, I found an old
college text book from the '50s and a technician's manual from the '70s, both
on the subject of automobile engines. Most of the material I have came from
the ignition sections of these two books, combined with my background as an
electrical engineer. There is still much I don't understand, but I am
continuing to research, and hope soon to have some definitive answers. ( 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.
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 appreciate your comments, and look forward to an enjoyable discussion of
this subject with the members of this list
(Looking back over this post, I realize I do go on!. My apologies for being
long winded)
Dan Masters,
Alcoa, TN
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