Ignition Coils and Transformers
FIGURE 36-44
Typical ignition coils.

Ignition coils and transformers both operate under the principles of electromagnetic induction Figure 36-44. They use electromagnetism to produce electricity, rather than mechanical movement. An ignition coil can be described as a step-up transformer. This is because the output can be 60 kilovolts (or more), which is higher than the input, nominally 12 volts.

Two sets of coil windings are used. One coil, referred to as a primary winding, is wound around a second, the secondary winding. The primary coil typically has 200 to 300 turns of light-gauge wire while the secondary has approximately 30,000 to 60,000 turns of very fine wire. When current is passed through the primary winding, the magnetic field builds surrounding both the windings. When the current is turned off, the magnetic field collapses with enough speed to induce high voltage in the primary winding (self-induction) and very high voltage in the secondary winding (mutual induction) by the rapidly moving (collapsing) magnetic field. Voltage is induced into each of the thousands of windings of the secondary coil. This voltage is strong enough to overcome the infinite resistance of the spark plug gap and push current across the gap, causing a spark with enough heat to ignite the air-fuel mixture in the cylinder.

The transformer action causes heat to be produced. In the past, the internal coils were immersed in cooling oil, allowing the heat to be conducted to the case. Modern ignition coils do not use oil. They are usually constructed using a heat-conducting hard resin and are cooled by their location on a heat sink or in a stream of air. The advent of computer controls has allowed the time that current flows through the primary windings to be minimized to reduce heat and electrical loads, while still providing enough spark to ignite the air-fuel mixture.

Step-down transformers operate under the same operating principles. The only difference is that the secondary coil has fewer turns than the primary, providing a lower induced output. These transformers are used on power poles to lower the voltage to your house or school and on low-voltage devices in your home that are plugged into your 110-volt outlets, such as cell phone chargers.

Technician Tip
You might expect a transformer to be a great way to boost the amount of electrical power that is transmitted, but it isn’t. The amount of power after the transformation is relatively the same as before the transformation. For example, if we raise the voltage from 12 volts to 120 volts, the amperage will decrease from, say, 10 amps to 1 amp. Thus, the wattage stays the same:

12 volts × 10 amps = 120 watts

120 volts × 1 amp = 120 watts

Also, remember that transformers are not 100% efficient, so some of the power is lost as heat.