Nolan Penney wrote:
>
> This discussion of points ignition capaciters has fascinated me. I too
> believed the purpose of the capaciter was to prevent the points from
> arcing. Now I know that's not the case.
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ ???
Okay, Nolan... let's skip down to the latter half of the paragraph:
Therefore, by placing in the coil a condenser which is
> connected across the interrupter points (P, Fig 3), we can absorb or
> suppress the spark across the interupter points of the primary circuit
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Think you were more correct than you think, Nolan.... <g>
> and stop the flow of self induced current which is still trying to build
> up lines-of-force in the core, and thus aid the primary current to fall
> to zero value more quicly. Hence the reason for placing the condenser
> across interrupter points of the primary circuit.
And, this phrase essentially says the same thing--the path to the
capacitor has less resistance than that of the air gap between the
opening points, so the cap gets charged up, and rapidly comes up to 12
volts, so there's 12 volts on either side of the primary winding,
causing current flow to fall to zero very rapidly. The only thing I
wonder about the phrase "self-induced current" in the language about the
primary is the date of the publication... does this section talk about
magnetos??? (!) In a magneto, there would be magnetically-induced
current in the primary. In a coil, it seems there would not be--after
all, if self-induction occurred in both the primary and the secondary of
the coil, it would be a perpetual motion machine, and would require no
battery current. (?) <g>
The only other thing this may refer to is the tendency of one coil to
induce current in the other. And that effect seems neglible, if one
keeps in mind typical high-voltage behavior as seen on an oscilloscope.
A normal trace shows an extremely sharp vertical rise in voltage in the
spark lead, followed by a 1-2 kv drop in voltage as breakdown occurs
across the air gap of the spark plug. The secondary discharges virtually
all of its energy to ground through the arc, and there is a steep drop
in voltage to near base line. At baseline, there are a series of
diminishing oscillations in voltage with a peak of perhaps 2-300 volts
in the secondary. Given the coil windings, that would translate to
perhaps an induced oscillation of 0.2-0.3 volts in the primary, which
seems almost negligible. Depending upon the leakage rate of the
capacitor, the capacitor would probably dampen these oscillations fairly
quickly--but what stops the oscillations dead is not entirely the
effects of the capacitor in a resonant circuit, but the closure of the
points at the initiation of dwell.
In short, the capacitor does two things--supresses the arc between the
points as they open (which has two benefits--stops current leakage to
ground through the arc, which quickly raises the voltage on the points
side of primary, and keeps the points cleaner by preventing metal
transfer from contact to contact through the arc), and creates a
resonant circuit for all the previously mentioned reasons.
Cheers.
--
My other Triumph doesn't run, either....
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