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| Jon hit most of it. It really depends on the coil and the battery voltage. Coils can also be wound for 12 volts without a resistor.
It not important on a gas tractor with 4 or 6 cylinders running no faster than 2500 RPM, but in some automotive applications the resistor is critical. Any inductance has the fundamental characteristic of resisting a change in current. That's what makes an ignition coil work. Opening the current is a rapid change and that makes a very high voltage compared to the voltage it took to get the current through the coil. On closing the available voltage is small, like 6 or 12 volts depending on the vintage and that means the current rise to be limited by the DC resistance is slow. In a V-8 or V-12 running over 6000 RPM, the time between firings is a lot shorter than on a tractor and so it takes a lower inductance coil to get a rapid enough current rise. Then when the series combination of a resistor and a coil has an applied voltage, the current rise is faster when the resistor is bigger and the coil smaller, so for distributor ignition racing cars, even at 6 volts a low resistance and minimum primary turns coil was used to get the current to rise within the available dwell time. So in that high speed engine application a lower voltage coil with a resistor runs better.
Gerald J. | |
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Resister ahead of coil??
