Reply to: WHY DO WE USE BALLAST RESISTORS?
Okay, this is going to be hard. I hope I'm not going to lose the
non-engineering types on the list....
Jim Muller asks why we use a 6V coil and a ballast resistor, instead of a
12V coil. The reason, Jim, is that such a setup will energize the coil
quicker. How can it do that, I hear you ask....
What we are trying to do here, is to dump a lot of energy into an
inductor. The energy is stored in a magnetic field, and when that field
collapses, it is transferred into the secondary winding and creates the spark
voltage.
It is characteristic of inductors that they want to maintain a steady
current flowing through them. When there is zero current flowing, the inductor
would like to keep it that way. Therefor, at the instant when we connect the
coil up to the battery ( by the points closing ), it looks like an open
circuit. To pump current into that open circuit, we would like to have a
"current source" to pump it in with.
In electrical engineering, a "current source" is a source of electricity
that "wants to maintain a certain constant current" at its output. An ideal
current source would not care what the voltage at its output was, as long as
the current was what it wanted. It would do whatever was necessary to get that
output current. One common form of current source would be a high voltage with
a resistor in series with it. If the voltage is high enough and the resistor
big enough, then that source will try to maintain its output current regardless
of whatever small load we put on its output.
With the ignition coil, this "current source" action is important because
1) The energy we dump into the coil is equal to the *current* we manage to
squeeze through it
2) The coil offers a changing load. In particular, it's nearly an open
circuit when we first connect it.
At the moment when the points first close, the coil looks like an open
circuit, and the ballast resistor has zero volts across it, since there is zero
current going through it. Therefore, the full 12V is across the coil. The
current in the coil immediately begins to ramp up. This happens much quicker
than if you just hooked a 12V coil across 12V. When the coil reaches
saturation, its current is such that there will be six volts across the
resistor.
I used to work with Teletype machines. These had an electromagnet that
works off the incoming digital data. It was a requirement that current be
dumped into the magnet with all possible speed, so that the magnet's flapper
would thrash back & forth in a faithful reproduction of the bits. It was
common practice to use a 100-150 Volt supply and a big resistor to run these
"selector magnets" even though the magnets themselves ran off of 10V or so.
It used to be common practice in big American cars, to have a secondary
contact on the starter solenoid, to short out the ballast resistor. This was
done because the battery voltage can get quite low, down to 9 Volts or so,
during starting, especially when trying to turn a big V8. And the
current-source action is not needed during starting, because revs are so low.
And there's no danger of burning the coil or points because starting doesn't
last too long.
- Jerry
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