"Meindert Sprang" wrote in message
...

"Vito" wrote in message
...
del cecchi wrote:

"Larry Demers" wrote
... A series cap with the diode seems like a better idea to try.


This is a DC system in the average boat. There is only one polarity
of
inductive kick. Diodes work fine. Capacitors in series with the
diode
wouldn't work at all.

That's odd considering that literally millions of DC automobile ignition
systems used a capacitor in parallel with the "points" to protect them
from the coil's inductive kick. BTW, the "kick" is damped AC.

That has a different reason. Damping a coil with a diode does the job,
since
the induction voltage is ALWAYS the opposite as the originating voltage.
But, using a diode keeps the current flowing while the magnetic field
collapses. This causes a slow collapse of the field. In a coil in the car
(is it called bobbin?) has to produce a high voltage and therefore the
field
has to collapse as fast as possible. Therefore a capacitor is used with
certain value to get the best trade-off between generated high voltage
and
RF interference surpression.

Meindert

The capacitor across the points has its voltage reset to zero when the
points close. The capacitor acts to supress arcing by slowing the rise time
of the current decrease when the points open. The resulting oscillation in
the series tuned circuit is damped by the secondary driving the spark
through the plug.

In the case of the relay coil, there is no secondary. If there were no
diode, the tuned circuit would oscillate for a long time. With the diode,
the inductive kick will be stored on the capacitor and, being unable to
discharge through the reverse biased diode, will stay there for a long time.
During that time the diode will never turn on, due to the voltage on the
capacitor.

del cecchi