Mike,
I have always understood the function of the Ballast Resistor to be a little
different. Its primary purpose is to protect the ignition coil. The
?percentage? of time the points are closed is the dwell, which is expressed
as an angle segment of one revolution of the distributor. Typically 30
degrees in a V8. The dwell and this ?percentage? are a constant, regardless
of engine rpm. As the coil is charged and reaches saturation, the magnetic
field creates a ?resistance? to the current flow. This happens fast enough
that at low rpm the coil is saturated in a fraction of the time the points
are closed, effectively closing off the current to a trickle for the
remainder of the dwell. It takes a certain amount of energy to charge a coil
for each spark. As the rpm increases, the number of sparks increase and thus
the energy flow through the Ballast Resistor and the coil increases. Both
parts then start to get hotter from the current flow. The Ballast Resistor
increases its resistance and slows down the current flow to protect the coil
from overheating and burning out. The limit to this system with factory
parts is around 5000 rpm. Then the restricted current flow and the amount of
time available to fully charge the coil becomes less than the energy
required for a healthy spark. With dual points, the dwell is increased to 36
to 38 degrees, allowing about 20 percent more time to charge the coil,
effectively raising the rpm limit. At start up when the Ballast Resistor is
cold, it allows for maximum current flow and maximum spark. Some cars bypass
the Ballast Resistor in Start mode for even greater voltage.
Dave Homstad
56 Dodge D500
-----Original Message-----
From: Forward Look Mopar Discussion List
[mailto:L-FORWARDLOOK@xxxxxxxxxxxxx]On Behalf Of Mike & Sharon Higgins
Sent: Monday, October 29, 2001 10:44 AM
To: L-FORWARDLOOK@xxxxxxxxxxxxx
Subject: [FWDLK] Resistors
Several questions and comments on resistors has piqued my "response
syndrome". I've got to answer! I'm COMPELLED!. It's not really my fault,
I'm just a former schoolteacher who's not asked about these old things
anymore.
The Ignition Resistor is called a "Ballast Resistor" simply because it's
encased in porcelain so it will not burn up when it gets hot. The reason
it's designed that way is so that it can get VERY hot, and, as many of you
know, when heat in a conductor goes up, the resistance increases, and
current flow goes down, and voltage is used up. In the point style ignition
system this means that less current is sent across the points, so that the
condensor can more easily controll the sparking at the points. When the
engine is running slowly, the time that the points are closed is greater,
and the points would heat up too much, and burn out quicker. But when the
engine is running faster, the time of current flow through the points is
shortened, and therefore the ballst resistor can cool down a little,
allowing more curent flow through the points, and maintaining a good
magnetic field in the primary coil.
There are other resistors in the Generator Regulator, though. The Field
strength of the generator magnetic field is controlled by passing the field
current throught magnetic switches in the regulator (those two little
clicker switches inside*). By sending the field current through these
resistors some of the time, the field current can be controlled to maintain
maximum allowed voltage and amperage output from the generator.
*Actually there are three little magnetic switches inside the regulator.
They are the Voltage sensitive regulator, the Current sensitive regulator,
and the Cut-Out switch, which is designed to disconnect the generator from
the battery when the car is not running, so the battery will not drain
through the generator..
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