[Healeys] coilt induction ingition

[Austin Healeys List]

A capacitor is effectively a dead-short the very instant power is
applied to it. When the points open, flyback voltage from the coil
appears across the capacitor but since it is effectively a dead short
for a few nano seconds, the voltage across the points gap is very
close to zero and no arcing can occur because all the current is
rushing into the capacitor instead of trying to form an arc. A few
more microseconds later the points are far enough apart to prevent any
arc jumping the gap and all is well.

When the points close again, the fully charged capacitor gets
dead-shorted and if it were significantly bigger, this would cause a
big enough spark to erode the points in the opposite direction. You
can look at a well used set of points and determine whether the
capacitor is the correct size by whether metal has migrated from one
contact to the other, or vise versa. Unfortunately I can't remember
which direction corresponds to too big or small. The size of the
capacitor is a compromise between creating the smallest possible arc
when the points open vs when they close. At least that's what I was
taught at tech school.

Andy.


On 12/29/16, Ed O'Neal <Ed at wadsworthoneal.com> wrote:
> Gents,
>
> By way of recap:  When the points close current flows through the primary
> (12 volt) side of the coil.  When they open the magnetic field around the
> coil collapses across the secondary coil windings (high voltage) driving a
> high voltage current across the plug gap and firing the cylinder.  At the
> same time the magnetic field also across the primary windings attempting to
> drive a current across the open points and creating a relatively high
> voltage (perhaps 400 volts) which will want to arc across the open points
> and wearing them out quickly.  To mitigate this a condenser (capacitor) is
> places between the points coil connection and ground which absorbs the
> “shock” of the current being driven towards the open points and reducing the
> potential for arcing across the points allowing them to last longer.
>
> Question:  Why would a larger condenser (capacitor) not reduce the potential
> for arcing across the points even more?  What might be the down side of such
> an attempt???
>
> Thanks for your expertise in advance.
>
> Ed