On Wed, 18 Jan 1995, Paul McBride wrote:
> Coils are designed to operate with or without ballast
> resistors. Coils that run without them are capable of
> handling full battery voltage and the heat that this produces.
> Coils that run with ballast resistors are designed to operate
Yep.
> at approx. half of battery voltage. During low speed operation
> the points are closed longer, and the resistor heats up to
> restrict current flow to the coil.
> Less voltage is required to fire the plugs at low speed.
> As engine speed increases, the points stay closed for a shorter
> duration and the resistor cools off. This allows
> more current to flow to the coil, for increased spark output.
The resistor temperature cannot change fast enough to fluctuate at the
frequency of the point opening and closing. Instead, the temperature will
be essentially constant and proportional to the average current (OK,
electrical engineers, I suppose it is proportional to the square of the
average current), which is going to be proportional to the dwell time--the
percent of time the points are closed. Although it is true that each
point closure lasts longer at low rpm, each open period also lasts longer
at low rpm. The average current and thus the temperature of the resistor
will be regulated primarily by the fraction or percent of time the points
spend closed. Unless the rpm is so high that the points float--i.e. fail
to follow the distributor cam contour, I think the dwell time is
essentially independent of engine speed.
The current through the coil might change with rpm for some other reason,
such as the inductance in the coil. However, I think someone has fed you
misinformation about the effect of point closure on resistor temperature.
I doubt the ballast does a whole lot to regulate the voltage through the
coil; I think it is there primarily to provide a simple way of supplying
12 volts to the coil when starting, and a lower voltage when running.
> Your Lucas coil runs ok because it does not need a
> resistor. The others you tried probably require one to keep
> the coil from overheating. I`ll shut up now and give someone
> else a chance.
>
>
> paul mcbride
>
Ray Gibbons Dept. of Molecular Physiology & Biophysics
Univ. of Vermont College of Medicine, Burlington, VT
gibbons@northpole.med.uvm.edu (802) 656-8910
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