Motorcycle Ignition Current

[Hugh Janus]

Even the human body can act as a condenser, storing an electric charge. We’ve all experienced this after shuffling across the carpet and touching a doorknob. (Viktoriya Kuzmenkova/123rf.com/)

Condenser Trouble

One gentleman wrote in to ask why a condenser (electronics people call it a capacitor) is always wired across the mechanical contact breaker points in a distributor or magneto. I too wondered about this until a hi-fi enthusiast friend looked at me as if I had two heads.

“It’s there to give the flowing primary current someplace to go besides trying to jump across the gap as the points are opened by the breaker cam.”

That explanation also tells us why ignition operation is poor if the condenser is missing or defective: Instead of sharply cutting off primary current in the coil to generate a spark, the current decays more slowly as it continues to arc across the gap. The whole idea of sharply cutting off the primary coil current is to rapidly collapse the magnetic field it has created, thereby inducing a much higher-voltage and lower-current pulse in the coil’s very numerous secondary windings, which are connected to the spark plug. But with no condenser, the primary current decreases more slowly, doing a less-good job of inducing high voltage in the secondary.

What Is Flashing Across The Points?

And the “flashing of the points” that occurs if the condenser is defective? That is the visible arcing as the points open, whose intense heat (like that of an arc welder) rapidly erodes the high-melting-point tungsten of which they are made, causing them to conduct less well and finally not at all. Then you are walking.

Without a properly functioning condenser the points will be eroded by arcing and will eventually fail. ( Jeff Allen/)

What Is A Condenser?

A condenser or capacitor conceptually consists of a thin sheet of glass with a square of aluminum foil on either surface, to each of which foils a wire lead is attached. Depending upon the area of the foils and the thinness of the glass (or other insulating material), quite a lot of electric charge can be stored on such a device, but no current can flow through it. With a working condenser across the contact-breaker points, as they open, the current readily flows to the condenser rather than jumping the much higher resistance path across the increasing points gap.

Sign up here to receive our newsletters. Get the latest in motorcycle reviews, tests, and industry news, subscribe here for our YouTube channel.

For packaging, foils and insulators may be rolled up into cylindrical or other forms. Back in the days of cathode ray tube TV sets, service people took care to discharge the large high-voltage capacitors on the electronics chassis so they would not inadvertently touch the wrong thing and get a possibly fatal whack of current from one of them. When charged, one foil becomes positive, the other negative.

Condensers, like these from Lucas units for Norton Commandos, give the primary current somewhere to go when the points are opened. (Ray Nierlich/)

You yourself have capacitance, as you can discover when you scuff across the carpet on a dry winter day and reach for the light switch. Zap! A spark startles you by jumping between your finger and a screw in the switch plate.

On an early Orient motorcycle (1899–1907) I once had the privilege of examining, its ignition capacitor actually was a thin glass sheet between foils!

Source