Re: volts and amps

[brian whatcott <inet@INTELLISYS.NET>]

At 14:10 7/14/00 -0400, >John D mentioned this:

> > The coil on some older cars (Fords in particular) would also burn out
> > if their dropper resistor were somehow shorted for long
> > (for imagined spark boosting?).
>
> That's an example of a circuit designed for a special purpose.  It is (to a
> salable but not very good approximation) trying to be a constant-current
> source.

Coil current limiting was a very useful trait if the ignition could be
live on an older spark ignition engine that stalled. The Russian roulette
played by the distributor shaft meant that sooner or later a closed
ignition point applied 12V DC to a coil that was much happier with a
duty cycle of 1:4 or less.
   But there was another feature that makers like Ford strived to provide
 - not always successfully: easy starting.

When the starter relay or solenoid/breaker applied the battery output to the
starter motor, it was helpful to edge the system towards maximal
energy transfer - where load and source impedances match.

In fact, a battery terminal voltage of 8 or 9 volts was often the result
 while cranking, just when a glittering high energy spark would have been
preferred - so a relay activated by the starter switch that shorted the
ballast resistor to the coil was just the ticket for boosting the spark
when most needed.

Survivors of those forgotten times can be identified by their propensity for
starting the engine *before* turning the lights and heater on, not after.

brian whatcott <inet@intellisys.net>  Altus OK
                Eureka!