TIGEROOTES(at)aol.com wrote:
> Bob, etal,
> That is not the purpose of having a condenser (or two in the case of a
> dual point unit). Although the ignition coil is charged by DIRECT CURRENT,
> it charges and discharge so rapidly, the circuit behaves similar to
> characteristics seen with ALTERNATING CURRENT...in fact it is "seen" as a
> square wave: on-off-on-off...
>
SNIP...
> Inductive reactance and capacitive reactance are INVERSELY PROPORTIONAL
> to eachother: THEY CANCEL EACHOTHER OUT...and that is the goal.
>
> Jim Leach, PTC Seattle
Jim and folks,
I have designed ignition systems using various techniques including
coil on plug, CD as well as conventional transistorized coil distrubuted
ignition systems.
I agree with your description of inductive reactance and the ignitions relation
in comparison with with AC.
BUT...
The capacitors primary function is as a reactive snubber, where the
DV/DT of the inductive voltage is limited by the capacitor.
Or simply stated to lessen the effect or points arcing.
In this way, the inductive kickback (reverse voltage induced across the coil),
is limited by the low impedance of the capacitor at that voltage slope.
This limits the voltage that is places across the points typically to less than
200vdc.
This has two effects, 1, that the limitied voltages is dependant on the coil
inductance,
2 the coil capactitor pair forms a reactive or tesonant tank circuit that "rings"
when
the field collapses on the secondary of the coil. The ringing effect results in
a longer, but weaker spark.
True, the value of the capacitor is selected with the nominal inducatance of the
coil,
but phase is really not an issue here.
Conventional electronic ignitions do not use a capacitor to protect the
transistor (equiv function to points ) as it is more efficient to use a zener
diode for the equiv function, which does not have a dependacy on the inducatnce
of the coil.
The lack of resonance results in a faster but higher enrergy spark.
Jarrid Gross
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