Original submission by T Mar 20th 2007:
In Holdens that had breaker point ignition as standard, from EH up to HZ, UC and VB, the ballast resistance was a length of pink wire with "Resistance - Do Not Cut" printed on it, that attached to the coil positive at one end and tapped into the ignition feed either at the bulkhead connector or at the ignition switch.
These cars did not use a separate Ballast Resistor. One advantage of incorporating the resistance into the wire is that the heat generated in the resistance is dissipated over a wide area. Ballast Resistors burn out more frequently than the wire because of the high heat concentration around them.
Ballast Resistors and Duty Cycle:
Submitted by T on Tue, 20/03/2007 - 09:46.
The Ignition Coil is Not A Pure Resistance:
===The Direct Current Behaviour of an Inductor:=== As you know, when an inductor (coil) is presented to a DC voltage source it initially behaves as an open circuit. As time passes it eventually becomes a short circuit.
===The Direct Current Behaviour of a Capacitor:=== This behaves as a short circuit initially and in time becomes an open circuit.
===The Effect on the Ballast Resistor with the Increase in RPM:=== As the Engine RPM increases, the amount of time available for the coil primary to reach a maximum is reduced. After about 3,000 RPM a points coil starts to lose output because its buildup time is reduced.
===Dual Point Ignitions and Dwell Extension:=== Dual point ignitions and electronic dwell extension sustain the coil's output by making the coil "on" time as long as possible and the "off" time as brief as possible. The increase in output is achieved just by changing the duty cycle.
===Ballast Resistor Heat:=== The power reduction effect with the increase in RPM is exactly the same as if the on time were getting shorter the faster the engine revs because the coil primary is given progressively less time to react.
On another note, if the coil had 0 resistance and the points were closed a typical ballast resistor might be expected to dissipate
( 3 amps x 12 volts = 36 watts ). You could fry more than one egg on that lot. A 2 watt resistor can give you 1st degree burn.