Winding An Ignition Coil.

[Vintage Engine Repairs]

When winding an ignition coil, one end of the primary winding goes to the condenser and points, the other end connects to both the coil core and the secondary windings.

The other end of the secondary windings goes to the spark plug terminal.

When the magnet passes the iron core there is voltage created in the primary windings and when the points open and the voltage drops, electromagnetic induction induces a voltage in the secondary windings like a step up transformer. However, what I don’t understand is, why doesn’t the induced voltage in the primary when the points are closed automatically also induce voltage into the secondary windings?! They’re both wrapped around the iron core, they’re both physically joined and the flywheel magnets pass the iron core that both are wrapped around?

There should be current flow on both windings, but there is not.

 

[Old2stroke]

Due to the nature of an inductive circuit, when the points open and break the flow of current, the current wants to keep flowing and this causes a LARGE (400v-ish) inductive spike that is discharged into the capacitor. It is this large discharge current that is stepped up by the secondary to produce the spark. Normal current through the primary does induce a current in the secondary but it is too small to do anything.

 

[ChillyB]

There is a voltage induced to the secondary but because there is a spark plug gap it has no path for current to flow. And the voltage while points are closed isn't very high. So while points are closed it is in fact a step-up transformer with an open secondary operating at relatively low voltages.

Once the point opens a decently high primary voltage spark then induces an pretty darned high voltage to the secondary.

 

[ChillyB]

The purpose of the capacitor is to delay spark development across the points until they can open fully. Without a capacitor to buffer the voltage growth the points would begin sparking almost instantly as they open, before the field can develop a very high voltage.

 

[echomeister]

sometimes they put in a diode on the secondary to keep the on close voltage from reaching the spark plug.

 

[SteveSr]

There should be current flow on both windings, but there is not.

No, Until there is enough voltage to jump the spark plug there is zero current in the secondary, essentially an open circuit.

The main purpose of the capacitor is to create a resonant circuit with the primary. When the points open this excites the resonant circuit to produce a high voltage "ring" wave which will be stepped up to fire the plug. This is why a bad capacitor will reduce the plug voltage and quickly burn up the points.

You can see these waveforms on an oscilloscope. You can also diagnose shorted secondary turns as a shorted turn will "suck" energy out of the system greatly reducing the ring wave. This is how the now "old fashioned" ignition/coil analyzers worked.

 

[Old2stroke]

No, Until there is enough voltage to jump the spark plug there is zero current in the secondary, essentially an open circuit.

Good correction to my post and a better explanation.