Electronic – Automobile ignition coil (spark coil)

electromagnetismmagneticstransformer

Question: How is this possible to get 1:1000 electric potential scale while primary/secondary loop scale is only about 1:100?

Best Answer

It is possible because the ignition coil is not a "normal" transformer

An ignition coil is essentially an autotransformer with a high ratio of secondary to primary windings. By "Autotransformer", I mean that the primary and secondary windings are not actually separated - they share a few of the windings. The ratio of secondary to primary turns in an ignition coil is somewhere around 100:1. The ignition coil is operated directly off a 12 volt source. However, the ignition coil does not work like an ordinary transformer. An ordinary transformer will produce output current at the same time that input current is applied. An ignition coil actually does most of its work acting as an inductor. When the ignition coil is connected to the battery, the inductor is 'charged' with current. It takes a few milliseconds for the current to build up the magnetic field - this on account of reverse voltage caused by the increase in magnetic field. During this short charging period, maybe a thousand volts are produced at the high voltage terminal, not enough to actually cause a spark.

The actual spark is generated when the breaker contacts open. For an ideal inductor, the current and voltage relate by:

V = L dI

Where V is voltage, L is inductance (in henrys) and dI is the rate of change of the current.

Thus, seeing that L is constant for the inductor, the abrupt change in current will cause a very large voltage to be produced. This produces a very short, very high voltage spike. Of course, the change in current is on the primary side, but because the primary and secondary coils have a large mutual inductance (this is where the transformer part comes in), you get a spike on the order of 100 or more volts on the primary, and 10000 volts on the secondary. Even the primary side of the coil can give you a bit of a jolt if you hold the wires while disconnecting power. Also note that any contacts you will use will get a lot of sparks, also on account of this.

From http://www.sentex.net/~mwandel/cannon/sparky.html

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