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i dont agree with the post that says a ballast resistor is supposed to change resistance with temperature. .




You might not agree but you would be wrong. The fact that a BALLAST changes resistance is the entire reason it's called a ballast. Otherwise it would simply be a resistor. MOST resistors used in electronics are intended NOT to change resistance, even though they do somewhat, but they are NOT "BALLAST" resistors.

In the case of Chrysler, this is exactly what they are. Somewhere in the pile of old books around here, I've got documentation to back this up. I am NOT gonna spend time looking for it



According to Wikipedia, a ballast resistor can be fixed or variable . The Mopar variety are probably variable to adjust for the different secondary current needed to fire the plugs under different loads.

http://en.wikipedia.org/wiki/Electrical_ballast



You misunderstood what they meant, there. A fixed resistor is a resistor that cannot be physically changed BY YOU. A variable resistor is like a volume control on your radio--has a knob on it or other manner to adjust the device.

A quote from the site:

"Ballasts are used where an electrical load cannot effectively regulate its current use. These are most often used when an electrical circuit or device presents a negative (differential) resistance to the supply. If such a device were connected to a constant-voltage power supply, it would draw an increasing amount of current until it was destroyed or caused the power supply to fail. To prevent this, a ballast provides a positive resistance or reactance that limits the ultimate current to an appropriate level. In this way, the ballast provides for the proper operation of the negative-resistance device by appearing to be a legitimate, stable resistance in the circuit."

What they mean by "positive resistance" is referring to temperature---As the coil / points circuit effective resistance goes down, because of changes in RPM, the coil tries to draw more, but as more current goes through the ballast it tends to heat up, and because it is a ballast instead of a "regular resistor" it has a POSITIVE temperature coefficient. That means that as it heats up it INCREASES in resistance, which now causes current through the circuit to try and drop.

All resistors have a temp coefficient, but ballasts specifically are designed to have a VERY LARGE one so that they DO change on purpous

"Ballast" sorta means "something to make something else stable" whether it's weight in the bottom of a sailboat or electrical.