The principle of ignition coils

A ignition coil usually has two sets of coils, a primary coil and a secondary coil. The primary coil is wrapped with thicker enameled wire, usually around 0.5-1mm of enameled wire for 200-500 turns, while the secondary coil is wrapped with thinner enameled wire, usually around 0.1mm enameled wire for 15000-25000 turns. One end of the primary coil is connected to the low-voltage power supply (+) on the vehicle, and the other end is connected to the switching device (breaker). For secondary coil, one end is connected to the primary coil, and the other end is connected to the high-voltage line end to output high-voltage electricity. 

The reason why the ignition coil can turn low voltage electricity into high voltage in the car is because it has the same form as a regular transformer, with the primary coil having a larger turn ratio than the secondary coil. But the ignition coil works different from a regular transformer. The working frequency of a regular transformer is fixed 50Hz, also known as a power frequency transformer, while the ignition coil works in the form of pulses, which can be seen as a pulse-transformer. The ignition coil repeatedly stores and discharges energy at different frequencies according to the different engine speeds. 

When the primary coil is powered on, a strong magnetic field is generated around it due to increasing current, and the iron core stores magnetic energy.  When the switching device disconnects the primary coil circuit, the magnetic field of the primary coil rapidly decays, and the secondary coil induces a high voltage. The faster the magnetic field of the primary coil disappears, the greater the current at the moment of current disconnection, and the larger the turn ratio of the two coils, the higher the voltage induced by the secondary coil.