What Battery and Capacitor Are Best for a DIY Spark Plug Ignition Circuit?

[nukengr10]

My friend and I are trying to build a very basic to get a spark plug to fire. Right now we have a plug, and Ignition coil that outputs 20 kV. The Ignition coil says on it 12 V and was bought at an auto parts store so I am assuming its an ignition coil for a car. What kind of battery should we test it with? Do we need as much power as a car battery or can we live with a much smaller size? Also i heard about using a capacitor to absorb the back emf in the primary coil that will run across the switch normally. i was thinking maybe a high voltage capacitor of like 10-100 microfarads?

 

[Marty]

I've done this circuit a couple of times. The first time I did it I tried to theoretically calculate the value of the capacitor. It was a pretty interesting problem in a lot of ways. I can tell you my calculated answer was around 0.010 microfarads, but I won't bore you with the details. The voltage won't be more than about 400V because the ignition coil has a turns ration of around 100:1.

There's a lot of physics in the circuit, but the most fun is making sparks. I got the best fireworks by dragging a screwdriver across a long piece of threaded rod.

Marty