IGNITION COIL CONSTRUCTION:

An ignition coil is composed of a core, two winding, a metal case and mounting bracket.

The core of the coil usually consists of thin soft iron strings or lamination. Its purpose is to increase the efficiency and output of the coil by promoting faster and more complete coil magnetic saturation. The soft iron core readily conducts magnetic lines of force, so less energy is used then if the magnetic lines of force had to travel through an air core.

The two winding are identified as a primary winding and a secondary winding. The primary winding consists of approximately 250 turns of relatively heavy wire, which is laminated with a special varnish. The secondary winding is wound inside the primary winding and consists of approximately 20,000 turns of very fine varnished wire.  The many layers of secondary winding are insulated from each other by high dielectric paper. One end of the secondary winding is connected to the high tension tower, while the other end is connected to one of the primary terminals inside the coil.

Ignition coil are often filled with oil or special compound to provide additional insulation and to help dissipate the heat that is created by the transformation of battery voltage.

The dissipation of heat is very important in an ignition coil as heat tends to weaken insulation. An insulation results in partial or total coil failure.

IGNITION COIL CONSTRUCTION

IGNITION COIL ACTION:

When the ignition switch is turned on and the breaker points are closed, current flow in the primary circuit. As current flows through the primary winding of the ignition coil, a strong magnetic field is produced, with the aid of the core. When breaker points open, current ceases to flow through the primary winding of the coil and causes the magnetic field to collapse across the man thousands of turns of wire in the secondary winding. This action induces a very high voltage in the secondary circuit which forces current to jump the rotor and spark plug gap.

When a piece of wire is connected across a source of voltage, current will immediately reach a maximum value determined by the resistance of the wire itself. But this is not true when the same wire is wound into a coil as in the primary winding of the ignition coil. This characteristic of a coil conductor is called reactance or counter-electromotive force, and is due to the self-induced voltage in the coil.

When the breaker points close, current starts to flow in the primary winding. As the magnetic field begins to build up, the lines of force not through the primary winding and induces a voltage that opposes battery voltage. Therefore, it takes a definite period of time for the primary current to reach a maximum rate of flow after the breaker points close. This period of time is called a “build-up” time. When maximum current is flowing in the coil winding, the maximum magnetic field is present and the coil is said to be fully “saturated”.

If the breaker points remains closed for too short a period of time, maximum current flow will not be reached in the primary circuit and the maximum magnetic strength will not be attained. As a result, when the breaker points open, there will be less lines of force to cut through the secondary winding and coil output voltage will be reduced. This can cause the engine to misfire under certain operating conditions.

Reactance or counter-electromotive force not only opposes the build-up of current through the primary circuit, but also opposes any attempt to stop the flow of current. As the breaker points open, the magnetic field starts to collapse. The lines of force cut through the primary winding, but in the opposite direction from the build-up. This causes an induced voltage in the primary winding which is in the same direction as battery current and tends to keep current flowing. If current continues to flow when the breaker points open, there will be an arc between the breaker points. This arc has two very detrimental effects. First, it causes a transfer of metal from one point to the other, resulting in point pitting. Second, unless the flow of primary current is stopped quickly, the magnetic field will collapse gradually and the secondary winding output voltage will be considerably reduced.

To control the arc that takes place between the points as they separate and to quickly stop the flow of primary current to develop maximum coil voltage, condenser is connected across the points.

IGNITION COIL REPLACEMENT:

As previously stated, the ratio of coil secondary turns to primary turns is approximately 100 to 1. Therefore, a typical coil for a standard ignition system would have 200 turns of primary winding and 20,000 to 26,000 turns of secondary winding.

The coils used in transistorized ignition systems have a turns ratio of either 275 to 1 or 400 to 1. Because of the high current- carrying capacity of the heavier gauge transistor coil primary winding, approximately 95 turns of primary winding will be used in a coil having 26,000 turns of secondary winding.

It is very important when coil replacement is required that the proper replacement coil be installed on the engine. Mixing standard and transistor ignition coils or coils of the wrong polarity will be quickly reflected in poor engine performance or total ignition failure.