Reply to: RE>Re: Dwell and points gap
Bob Spidell writes:
Now I'M cornfused. I always thought the purpose of the ballast resistor
was to limit current at normal running speeds[...]
***** OK. Time for a ballast resistor tirade. Pat has oversimplified things a
bit, doubtless wishing to save his fingers from typing a few more pages.
Here they are:
Once upon a time, cars mostly had six-volt electrical systems. This seemed
about ideal to run an ignition coil at the time. It still is. Modern coils
are designed to run off six volts. We run them off 12 volts by putting a
resistor in series with the coil that drops the voltage down to six. Why do we
do this?
It turns out that the energy stored in a coil is proportional to the current
going through it. When we hook up to a dead coil, it looks somewhat like an
open circuit. An ideal coil would look indeed like an open circuit, and we
would be totally unable to put any current into it at all. Fortunately, there
are no ideal coils, and we can indeed "charge up" the coil by putting a voltage
across it, thereby forcing current through it. The more energy we put into the
coil, the lower it's apparent resistance goes, until, fully charged, it looks
like a short circuit. Well not quite. Remember, it's a real coil, not an
ideal one, and it's made of real wires that have resistance. So it looks like
a small resistor, and the current rises to some large, constant, value.
In the case of automotive ignitions, we shortly afterward interrupt the
current going through the coil. The magnetic field quickly collapses,
releasing the energy into the secondary winding, which then produces the
20-30kV we need to fire our plugs.
The trouble with the above scenario, is that we want to charge the coil up
quickly. If the motor is turning, say, 5000 RPM ( the Clankster's redline ),
the crankshaft is turning round some 83.3 times a second. For a four-cylinder
4-stroke engine, that translates to 167 sparks per second, which gives the
coil a little less than 6 milliseconds per spark. If the dwell is 50%, then
the coil has 3 milliseconds to charge up after we close the points.
Now, what makes it difficult to charge a coil quickly, is that it looks
like an open circuit when we first start charging it. So we are only able to
force a small current through it. Wouldn't it be nice, if we could raise the
voltage ( push harder ) when we first connect up the coil? With a
ballast resistor, we can do just that. Instead of connecting the coil directly
to a six-volt source, we connect it through the ballast resistor to a 12 volt
one.
At the moment that the coil first connects ( points close ), the current
through the coil is small. Ohms law tells us that the voltage drop across
the ballast resistor is also small. As the coil accepts energy, it starts to
act less like an open circuit, and the current starts to rise. As the current
rises, the voltage across the ballast resistor also rises, until the coil is
fully charged. Then the voltage across the ballast resistor is usually about
six volts, leaving six volts across the coil, or enough to keep it charged.
In electrical engineering terminology, the 12V source and the ballast
resistor form a "current source", that is, a source of electricity which will
try to maintain a certain current flowing from its output. It turns out that a
current source is ideal for charging a coil quickly. In fact, you
could charge the coil even quicker with a 24V battery and a bigger resistor.
Or a 48V battery and an even bigger resistor. Or.....
So, saying that the ballast resistor limits current through the coil is an
oversimplification. It's like saying that the condensor is there only to keep
the points from pitting. ( Don't believe me? Try an experiment: take
the condensor off your distributor. Take your car for a ride. Ran lousy,
didn't it? )
Running a coil directly off 6V worked well in its time. ( What's the
redline of a Model A, anyway? ) But nowadays we like to run our engines
faster.
We bypass the ballast resistor during starting because the battery voltage
drops to 9V or so, and the revs aren't very high anyway.
- Jerry
p.s. I like to think of a coil as a heavy flywheel that turns in response to
current going past it. It's hard to get it started, more or less ignores
steady-state current going past it, and hard to get it stopped....
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