Will a Tesla coil work with a square wave inverter output, used as input?

Question

I wanted to try an unusual thing, I couldn't find enough resources explaining this approach.

Few days back, I built a fairly simple CFL inverter using a TL494 IC and a couple of power transistors. It's fed with 12V DC supply from a battery and it generated ~ 220v high frequency square wave AC output which can drive a CFL Light or similar load of approx 35 watt. It uses a 12-0-12 4 amp transformer.

I also created a Slayer exciter using my hand made 1000 turn secondary coil which is working ok. The problem is - the slayer exciter runs on 12V input and thus I have not been getting a super high output voltage on secondary and naturally the sparks are short in length. I wanted to amplify this.

I was wondering, if I can feed the output of my inverter into the primary side of the Tesla coil and get a larger output voltage at secondary or not. Theoretically I do not find a problem in this, except the power limits. 5-6 turns of primary coil will try to draw a lot more current than 40 watts is my assumption. But I can work on the current limit somehow and not overwhelm the power transistors or the inverter transformer. I am also not worrying about the lack of an interrupter right now, I can work on perfection if it works.

I want to know if the Tesla coil would work if powered this way. Where should the non-open terminal of the secondary winding of my Tesla coil be connected to, in order to achieve this? I am just asking this in reference to the Slayer exciter design. It won't have to contribute to the oscillation of the input signal unlike Slayer exciter, because the input signal, i.e. 220v AC is already a high frequency square wave. I am assuming it has to be grounded to make it work. Is that correct?

Sorry if my question sounds silly, I am not an electronics expert. I am just a newbie hobbyist.

Answer 1

A "high frequency" if it is high enough will actually limit the current in the primary coil for 2 reasons.

1. The skin effect will increase the resistance of the coil as current will only be conducting in the surface of the coil. This will limit current at very high frequencies, and cause more losses as well. The skin effect will not be significant for a thin conductor at low frequencies say in the kHz region
2. The coil will have some inductance which causes an increase in reactance (impedance) at high frequencies - hence a reduction in current. Although, with only a few turns on the coil and no ferrite core the inductance will be very low - so you will need high frequencies for this to be significant.

Depending on the frequency of your square wave supply, the current levels through the primary coil might be acceptable. - but you need to make sure there is no DC voltage component to your AC supply. DC will draw lots of current through the primary.

As JRE has mentioned, the tesla coil needs to operate at its resonant frequency, which a slayer excitor does for you. Your AC supply won't be effective on its own unless it is tuned to the resonant frequency of your tesla coil - are you able to adjust the frequency to match your tesla coil?

Alternatively, if you add an interrupter with your AC supply, you will get inter-modulation which will shift the frequency of excitation into 2 frequencies, one above and one below the actual interruption frequency and this will make it hard for the interrupter to excite the coil at resonant frequency. Although you could still use this method by adjusting the interruption frequency up or down so that one of the modulated frequencies matches the resonance frequency, you will then suffer loss of efficiency as a result with half the power being lost to a non-resonant frequency.

From your question about grounding the secondary I assumed you were thinking of using it as an ordinary transformer. But I realise now this is not the case.

On a Tesla coil, the connection to ground is through the air, which is possible because the voltages exceed the breakdown voltage of air - and also due to parasitic capacitance with surrounding surfaces. If you add a grounding point your wiring might add too much parasitic capacitance which will suck all the energy out of a very high frequency, high-voltage, but very low current transformer. I don't think you need to change anything on the secondary coil, just get the drive for the primary right.

You could rectify the voltage from your AC source into DC and then use an interrupter circuit. Or possibly, adjust the frequency of the AC source to resonance - the disadvantage of this is that would probably be no feedback loop to automatically tune in to the resonant frequency, but it could work if you can tune close enough to resonance manually.