# Where do spark plugs get their electricity?

Where is the electric charge coming from and does it have a large amount of electric charge to it?

From a device called a coil. The coil is basically a transformer which takes in the 12vdc voltage from the battery and upconverts it to around 40k vdc (depending on the ignition system). Here is a breakdown of what a basic coil looks like from the inside:

The battery input feeds the primary coil. This is a thicker wire with a number of windings. The secondary coil is where the magic happens. There are a larger quantity of windings within it. Basic electrical theory shows us as we transfer power from the fewer windings of the primary coil and the secondary coil picks it up with the huge number of thinner windings, this will increase the voltage of the electricity. While this increases the voltage, the amperage is decreased at the same time. You can read more about how it works in this How Stuff Works article.

• Its not the 40K volts that can kill you its the amps....modern ignition systems can stop your heart if you get a good hit.
– Moab
Commented May 27, 2016 at 23:02
• I've had the "pleasure" in auto shop once or twice to get a jolt, and it was not pleasant. If I had my hand on bare metal of the engine, I'd likely not be typing this now! Commented May 28, 2016 at 4:57
• The piece that's really important is that transformers only work when the magnetic field is changing. So an electronic device cuts the voltage to the coil which makes the field collapse. Its the collapsing field that causes the large voltage spike on the secondary coil. Commented May 28, 2016 at 5:18
• @cdunn - Very good point! Remember, too, it's electronic today, but in the days of yore, it was the points/breaker system which caused the field collapse and voltage surge. Great fun! Commented May 28, 2016 at 12:34
• For some vehicles things have advanced to multi-coil, multi-spark ignition systems under MPU control. I miss the older giant single coils. They were fun to play with.
– user16540
Commented May 28, 2016 at 20:30