<VV> Secondary voltage at the coil

[Jim Simpson]

The output voltage of the coil -- when it's connected to a spark plug -- is
only indirectly related to the input voltage to the coil.  The spark plug
acts essentially as a surge suppressor.  Once the voltage at the output of
the coil reaches the point that the spark plug fires, the spark plug keeps
the voltage from getting any higher.

If the secondary of the spark coil was an open circuit, that is not
connected to anything that would provide a current path to ground, it would
act according to the standard transformer and inductor laws.  The current
flowing through the coil, when interrupted by the opening of the points or
by the Pertronix controller, would induce a voltage into the secondary
proportionate to the turns ratio and inductance of the primary.  That would
be a pretty high voltage -- in the 40 - 50 kv range.

But since there's a load -- the spark plug -- connected, once the secondary
voltage gets high enough to start the arc, the arc acts as a short on the
secondary keeping it at about the voltage required to maintain that arc.

As Matt Nall pointed out, the voltage required to create that spark changes
with the pressure at the spark plug.  Low RPM tends to have the lowest
pressure hence lowest voltage requirement.  Higher RPM tends to have higher
gas pressures in the cylinder, hence higher voltage requirements.

And of course the voltage required is affected by the actual spark plug gap
(the smaller the gap, the lower the voltage) and the condition of the
plug.  Sharp edges on the electrodes requires lower voltages to create the
spark.

Someday I need to find a dead Pertronix unit and take it apart to see what
makes it tick.  I'm using a Pertronix I on my turbo Corsa to trigger a
"Safeguard" knock sensor.  There's very little current flowing through my
Pertronix; the Safeguard is doing all the heavy duty switching for the coil.

Jim Simpson
Group Corvair