>From: "W. Ray Gibbons"
>To: Paul McBride
>Cc: british-cars@autox.team.net
>Subject: Re: Lucas coils
>
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>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.
BINGO! Current through the coil starts off high when the points first
close and tapers off as the coil "saturates". At higher RPM, the coil
spends more time in this pre-saturated state and draws more current --
but it's not linear.
>...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.
I suspect this is correct!
-Mark Joerger
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