Ballast Resistor

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Original submission by T Mar 20th 2007:

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Ballast Resistor to extreme left of centre, has green wire attached. Click to Enlargen. Photo by Griff_one_tonner.

Ballast Resistors:

Early type Ballast Resistor wire as found in the wiring harness of Holdens. Click to Enlargen. Photo by HQ_SS.

Ballast Resistance:

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:

Ballast Resistor wire as found in the wiring harness of Holdens. Click to Enlargen. Photo by HQ_SS.
If the coil were purely resistive, any increase or decrease in RPM would make little difference because the duty cycle would remain at largely 50% (equal on and off times).

Operation of Ballast Resistor.

Early type Ballast Resistor wire showing pink and yellow wires. Click to Enlargen. Photo by juicematic64.
Aftermarket Ballast Resistor on a 253 HJ. Click to Enlargen. Photo by Zed-Tonner.
===Because the Ignition Coil is an Inductor:=== Because the coil is an inductor it takes time for the primary coil current to reach its peak.
Aftermarket Ballast Resistor (upper left). Low Energy Coil (upper right), HEI Distributor (lower left). Latest version Bosch HEC-716 Coil (lower right). Photo by 1963EJ. Click to Enlargen.
===The Coil and Ballast Resistor at Idle:=== At idle the coil has time to reach a peak and sustain the peak meaning that more current flows into the coil than needs to since the coil only needs to saturate the coil former, then have the primary current path broken. The resistor minimises heat buildup in the coil by soaking up a percentage of the power.
===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.
A firewall mounted Ballast Resistor had to be removed from the HEI circuit. Photo by KyleWA. Click to Enlargen.
HQ Ignition Switch Connector. Brown - Accessories Feed to Fuse Panel, Pink - Ignition Feed to Fuse Panel, Yellow - Coil +ve, Braided - Ballast Resistor to Coil +ve via Bulkhead Connector, Purple - Starter Solenoid, Red - Primary Feed from Starter Tap. Photo by Old73qe. Click to Enlargen.
Diode used for providing full Voltage under Cranking plus isolation after Starting on a Ballasted Ignition Circuit. Photo by Zed-Tonner. Click to Enlargen.
The Connector to the right of the Wiper Motor is known to burn and cause intermittent problems where the pink Ballast Resistor Wire meets the yellow Ignition Wire. Photo by  HX_HZ_Monaros. Click to Enlargen.




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