How does an ignition coil work?
If you’ve ever driven a car, chances are it has spark plugs. You may have wondered, how do these fantabulous little devices work, and what do they do? The spark plugs are directly responsible for igniting the fuel/air mixture in a gasoline engine (diesels ignite the fuel using compression, not spark!) and they do this by way of a high-voltage pulse generated by the ignition coil.
To most, these coils can seem like a magical little object that takes in electricity and spits it back out at a very high voltage. However, they use a very simple principle to do this: inductance!
The way that an ignition coil works is that a voltage is applied across the two terminals of the primary windings, which will cause current to flow through the primary winding. This causes a magnetic field to form inside of the ignition coil, and it encompasses both the primary and secondary windings. When you interrupt the current flowing through the primary, the magnetic field begins to collapse while forcing current through the coils (this is inductance!) Since this current has to go somewhere, the voltage in both coils will rise relative to how many windings are in each coil (think Ohm’s law, at a constant current but higher resistance, voltage rises!) Because of this, the voltage in the secondary winding will become far higher than the voltage in the primary winding. Eventually, the voltage in the secondary coil becomes so high that it reaches the breakdown voltage of the air between the electrodes of the spark plug, causing an arc to jump as the air begins to ionize and conduct current. Now that the air is conductive, the current will flow through in the form of a high-temperature plasma which is able to ignite the fuel in the engine.
While you shouldn’t try this at home (or ever, unless you really know how to handle high voltage), I was able to demonstrate the way that an ignition coil works using an STGP10H60DF IGBT that is able to withstand a high VCE (collector-emitter voltage). This is necessary because as the voltage in both coils rises, it could damage whatever is used to switch power. During my tests, I saw spikes of well over 500 volts on the primary winding using an oscilloscope. Using a TC4427 gate driver and an Arduino for simplicity, I pulsed the power to the coil for 1 ms to charge it up, then quickly cut power to it and saw a large arc jumping across an air gap that I had created across the output of the secondary winding. It really goes to show how simple electronics are and yet how powerful the fundamentals of can be, that nearly every car in the world is using at least one if not several of these for hundreds of thousands of miles!
My very safe testing setup - TC4427 gate driver, STGP10H60DF IGBT, Arduino Uno
Note the air gap between the two yellow leads, this is to facilitate a spark between them
