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Ignition coils can handle quite high power levels.

Bipolar ignition coils. 70V input voltage with 20000uF smoothing cap and MOSFET (IRF740) switching. Significant current draw - primary peak 20A. What might not be apparent from the video is that the arc is very loud. I'm letting the voltage fly after switching. The output voltage is approximately 80kV.

I've collected info to this page:

http://www.niksula.cs.hut.fi/~plahteen/misc/old_projects/ignition/ignition.html

In order to get long sparks you need to feed the coil enough power for each pulse (meaning long enough on-time to build the current). You also need to switch the current off fast at it's peak. That will result in inductive kick which will rise to infinity regardless of input voltage as long as enough current is flowing at the switch off time. However it will be limited by the switching device maximum voltage (and other practical reasons such as capacitance). We want the voltage to fly as high as possible within safe limits. Secondary voltage is usually multiplied over 100x the primary so 400V kick should be around 40kV at the secondary.

These coils are anti-parallel so the spark is between -40kV and +40kV terminals. That's why 80kV.

The circuit I used is here:

http://users.tkk.fi/~plahteen/circuit2.gif

Be warned however that this is only a quick and dirty way. High voltage spikes may destroy the MOSFET in the long run. The cap should be rated well over 400V and 100n is pretty critical (compromise between high current and high voltage spikes).

The MOSFET avalanching at around 400V offers a natural clipping limit. As long as that pulse energy is (s)low enough it will survive. The parallel capacitor (s)lows down the kick and helps to protect the MOSFET. Too large capacitor will however result in too large short at the moment of switch-on.

I however didn't use any protection with this specific circuit. The component values may be quite critical. I used it with two different coils (primary inductances 5mH and 9mH).

Better circuit here:

http://users.tkk.fi/~plahteen/circuit1.gif

C should be determined so that the primary voltage swing after the switch-off remains below the safe limit. R should be determined so that discharging of C is slow enough while getting maximum current to flow through the inductor. These values depend on your coil and switching device and can be determined in practice with an oscilloscope. The gate diode ensures that switch-off is as fast a possible. They are not too critical though. R may be in the range of some 100kohm.

Here is one possibility too:

http://users.tkk.fi/~plahteen/circuit6.gif

R and C determined so that the voltage is low enough to be safe but as high as possible to give long sparks. R will become hot.

I've tested the basic circuit up to 200V and it will work up to a point but voltages much more than 24V aren't too safe in the long run without some of the suggested solutions. If you are out of luck you may destroy the MOSFET even with 12V. The parallel capacitor may get destroyed too if it's of low quality. It sees high voltages and get's discharged very rapidly.

Low voltages such as 12V will limit the primary current due to internal resistance too much and aren't very impressive. So at least 24V is typically required for good results.

Best solution would be a full bridge like the ones on a solid state tesla coils. However that would require higher voltage to operate and/or a resonant capacitor since it's not allowed to fly. More complex in any case.

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