Patric wrote ...

And what ballast? And then you have only 5000V without the ballast.

Download wrote ...

I'd be worried about using three MOTs as it requires running them past their designed voltage.

loneoceans wrote ...

Hello Patric, it looks like you have a reasonable setup already but with several tweaks and making some new parts you should be able to get your coil going well. What I believe is the main problem now is that your spark gap is not quenching due to the huge current and low voltage you are getting from your paralleled MOT stack.

As mentioned by a few people, the first thing you should do is to fix your power supply and decide what sort of power you want to run the coil at. With 4 MOTs, I’d wire all the primaries and parallel and all the secondaries in series, being sure to make the phasing correct. This results in higher voltage than the MOTs were designed to handle, so you need to put them under oil for reliability. I used this in my Tesla Coil 2 which I built a decade ago and it’s still going strong. See the schematic on how I wired it up here.

You will find that this will draw a lot of power (as you have noted), so you can use MOT caps as a ballast, or simple, a fifth MOT with the secondary shorted out and the primary placed in series with the paralleled primary, acting as a rudimentary ballast to about 1000 to 2000W depending on your ballast MOT. For my setup, I eventually ran it from a 30A 240V line, so I wouldn’t trip the breaker. It draws about 4.2kVA when running.

You can also do a dual MOT setup for 5kV, but this will be very tricky to get a static spark gap to fire reliably. This brings me to the next thing you need to fix – your spark gap.
Due to the huge current the MOT stack can deliver, quenching the spark gap is very difficult. The way you have it set up currently will not work at all since the moment the spark gap shorts, the high current of the MOT stack will simply sustain that arc. If you make a static gap, you will need a very strong blower or vac to quench the gap.

What I prefer instead is to make a rotary spark gap. There are lots of information online on how you can make one, as I have done for my Tesla Coil 2.

I would also try to re-do your primary coil, to make it a bit more tidy. As it is, it looks like your coupling is very low as well since your secondary coil begins much higher up than where the primary is. I would spend a bit of effort to tidy up the primary coil, perhaps mounting on a flat board in a spiral shape, and position it so it is level with the first turn of your secondary coil. You should also improve your primary cables to some beefier ones – it looks like you used some very thin wires for your primary circuit, which will result in huge losses and very poor performance.

As for MKP caps, they are one of the best capacitors you can use for Tesla Coils and there is nothing wrong with using them. The foil-type ones are better, but the MKP caps do just fine. Likewise, my TC2 uses MKP as well (instead of metal foil). However as people have pointed out, it would be great if you can double your primary capacitance to something closer to 50nF or more.



Here’s a photo of my TC 2 running with the 4 MOT stack. It has generally similar parameters to what you have, so hopefully it will be a good basis for building. Construction information here: