Low-Tension Ignition System

Bringing some understanding in a nontechnical way as to how the simple battery and coil and the low-tension rotary magneto ignition systems work.

| October/November 2020


Battery and coil: get your current up

It’s best to begin with the simple coil and battery system then progress to the rotary magneto and igniter system. The equipment used to gather all the data was not exotic: Radio Shack analog multimeter, BK digital multimeter, Harbor Freight frequency meter, variable speed electric drill, rubber hose connector, 6-volt battery, and two less common items — an LCR (inductance, capacitance, and resistance) meter and an oscilloscope. The only tools you need to analyze either system are a multimeter and a variable speed drill to spin a magneto.

Figure 1-1A, above: Typical battery, coil and igniter connection.

In Figure 1-1A, I have taken the coil, battery and igniter off an engine and placed them on the work bench to make taking data a bit easier. The coil, which I call “Fancy Coil,” is one of those good-looking coils sold at swap meets, typically wound on a 1-inch steel rod about 3 to 6 inches long with 3-inch hardwood ends and two knurled brass thumb nuts to contact the pretty copper wire.  The battery is a standard 6-volt Home Depot item that will run an engine through a two-day show without a recharge. The igniter is off a 4hp Waterloo Boy. The connection is typical of what is often used, but not the only correct way to connect the three elements. The red wire is from battery positive ( + ) to one terminal of the coil, the green wire is from the other terminal of the coil to the isolated igniter terminal. The black wire, typically called ground, goes from the igniter base/engine block to battery negative ( - ). Figure 1-1B schematically shows the circuit: a battery (“V”), a coil (“L”) with its internal resistance (“R”) and an igniter switch (“S”). Coils are labeled “L” in the electronics world, named after Emil Lentz, an early 1800s physicist.   The igniter switch is designed to snap open quickly when it’s called on to react. As an aside, in the electronics world, coils are called inductors just as condensers are called capacitors.

Figure 1-1B, left: Connection schematic

The following data was taken using a different, but very similar, coil that I will call “Fancy Coil 2.” Other coils may be shaped differently and have a different number of turns, but they will behave in a similar manner.


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