Creating sparks with a mosfet and ignition coil

Oops, correction: I tried manually touching the ignition coil primary to my 12V battery...

yeah, that's the normal ignition system capacitor size.

energy in a capacitor is 0.5V^2/C

So reducing the capacitance by a factor of 10 has the potential to boost the voltage by a factor of sqrt(10) which is about 3

if you measured the peak voltage seen across the MOSFET that may give some clue as to how much lower you can go in capacitance.

I bought a 1uf cap from Radio Shack and at first I got beautiful sparks. Then after I felt my MOSFET to see if it was hot, it didn't work anymore. I disconnected the coil and replaced it with a 12v auto light bulb which worked fine. Kind of puzzled.

Oh and my DMM said I get voltages of hundreds, sometimes 1000+ volts across the cap. (Maybe my cap is dead?)

Oddly, the sparks are nice without any cap at all. My MOSFET doesn't even get hot, at least during my 5-second trials.

sounds about right.

you used a polarized Electro didn't you?

Jamie

its not a polarized cap. It is a metalized film cap.

Since I am having so much trouble with a cap, would a Schottky in parallel with the coil primary be enough to protect my irf530 mosfet?

That'll make a single spark. In ignition systems ringing is desirable. ...Jim Thompson

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Oh I won't mind a single spark too much, since I'm generating them at 100 Hz anyway. :p

Michael