Ignition Coil: Description and Operation

Typical Ignition Coil:

The ignition coil is a transformer that is used to produce the high voltage needed to generate a spark at the spark plugs. The coil consists a soft iron core wrapped by two sets of windings (primary and secondary) of fine copper wire. The primary winding contains a hundred or so wraps surrounding the secondary windings. The secondary winding contains many times more wraps than the primary. The ratio of the number of wraps in the secondary winding to the number of wraps in the primary windings determines the output voltage of the coil.

The primary windings are connected at one end to battery power (coil terminal 15) and grounded at the other (coil terminal 1) through some sort of switching device (in this case, a power transistor inside the DME control unit). When electrical current flows through the primary circuit, a magnetic field is created that permeates the secondary windings to the iron core. When the current is interrupted suddenly, the magnetic field collapses into the iron core. As the lines of magnetic flux crossing the secondary windings collapse into the core, the rapid change in the magnetic field induces a high voltage potential in the secondary windings. When this voltage is great enough to push through the secondary resistance and cross the air gap at the spark plug, the coil discharges. Although the sparking voltage is very high (as much as 50,000 volts) the amount of current that actually flows through the secondary circuit is very small (just a few milliamps).