Resistors 101
A ballast resistor is used in points-style ignition systems to limit
the current flowing thru the points. Without it, the coil will overheat
and the points will burn out quickly.
A resistor is not a voltage regulator. It is a current limiting device
and in so doing it drops voltage across itself, thus providing the
device in series with it a lower voltage (battery voltage minus voltage
across resistor equals voltage the other device receives). As the
current draw increases so does the amount of voltage the resistor
drops. So adding additional load to the resistor results in a) the
voltage presented to the load decreasing, and b) the work the resistor
is doing to increase, likely burning it out. That is why resistors have
wattage ratings - how much power they can dissipate without
destroying themselves.
If there is no load on the resistor (points open), there is no current
flow and therefore no voltage drop (12 volts seen at coil '+' terminal).
But as soon as the points close and current flows, the resistor drops
some voltage and the coil sees less than the full battery voltage.
So how much voltage does the coil get with a ballast resistor?
Measure the resistance of the ballast, and the resistance of the
coil. Then divide: (coil R) / (coil R + ballast R). Multiply the result
by battery voltage (give or take, about 12-14). Verify by measuring
across the coil, ignition on, CAR OUT OF GEAR, and points
closed. If you yank any wires with power on, you may get shocked!
Some points-compatible coils have a ballast resistor built in and
will be so labelled, though sometimes with some cryptic phrase.
Such applications do not use an external ballast resistor, but only
because it's already inside the coil. High energy EI-designed coils
expect and can handle the full voltage, but using an EI coil on a points
system may cause premature points failure. They _may_ also overtax
the stock Pertronix unit, but I don't know for sure (note that there is a
Pertronix II available now...)
So why not design a coil to not use the ballast resistor? Answer with
another question: Why are '66 cars sometimes harder to start than '68
cars? Because of that ballast resistor! With an external resistor, the
car can be designed to 'short out' the resistor during cranking. Doing
so gives a stronger spark and an easier start. The later cars do this;
that's why there's extra wires on the resistor/coil. Earlier cars always
had the resistor in the circuit. Weak battery on a cold day and there's
not enough voltage available for a strong consistent spark during
cranking.
There - more than you could possibly have wanted to know...
-- John
John F Sandhoff sandhoff@csus.edu Sacramento, CA
|