Ballast vs. Non-Ballast Ignition Coils
by Dan Masters, firstname.lastname@example.org
Many Triumph owners opt to replace their standard coil with the Lucas Sport coil, to get a hotter spark. Often, though, they neglect to bypass the ballast resistor, used on some of the later models. Failure to do this will negate the benefit of using the hotter coil, as the Sport coil is designed to be used without a ballast resistor. What is the difference between the two types of coils?
Basically, a non-ballast coil is designed to produce full spark output with 12 volts on the input (+ terminal). A ballast coil is designed to produce the same spark output, but with only 6 to 9 volts on the input.
With a non-ballast coil, the input to the coil is the same, 12 volts, whether the engine is running, or being cranked by the starter motor. With a ballast coil, the starter relay 1 by-passes the ballast resister when the starter motor is spinning the engine, and applies the full 12 volts to the coil. Since the coil is designed to provide full spark with reduced voltage, the application of the full 12 volts produces a much hotter spark, which is an aid in starting. After the engine starts, and the starter motor is off, the coil voltage is dropped to the lower voltage, and the coil output is the same as for a non-ballast coil. The reason the ballast type coil is not run at the full 12 volts, for a hotter spark, is to prevent damage to both the coil and the points.
With a non-ballast coil, power is applied to the coil directly from the ignition switch, via a white wire. Power to the ballast coil comes from the ignition switch to the resistance wire, and then to the coil. When the starter relay operates*, power from the battery, via a brown wire, is routed through the contacts of the relay, via a white/yellow wire, to the coil. This shorts out the resistor wire, by placing 12 volts on both ends of the wire. With the same voltage on both ends, no current flows, so no heat is generated. The current flow is shunted around the resistor wire. This bypassing of the resistor wire places the full 12 volts on the coil.
As for the modifications required to switch from a ballast coil to a non-ballast coil, it couldn’t be simpler – just run a wire from the most convenient white wire you can find (probably at the fuse box), directly to the + terminal of the coil (of course, all the rules of good wiring practice should be used). No need to remove the resistance wire, because it will now be constantly bypassed,