Construction
Formerly, ignition coils were made with varnish and paper insulated high-voltage windings, inserted into a drawn-steel can and filled with oil or asphalt for insulation and moisture protection. Coils on modern automobiles are cast in filled epoxy resins which penetrate any voids within the winding.
A single-spark system has one coil per spark plug. To prevent premature sparking at the start of the primary pulse, a diode or secondary spark gap is installed in the coil to block the reverse pulse that would otherwise form.
In a coil meant for a dual-spark system, the secondary winding has two terminals isolated from the primary, and each terminal connects to a spark plug. With this system, no extra diode is needed since there would be no fuel/air mixture present at the inactive spark plug.
In a low-inductance coil, fewer primary turns are used, so primary current is higher. This is not compatible with the capacity of mechanical breaker points, so solid-state switching is used.
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