high voltageinductancejoule-thieftesla-coiltransformer
This video shows a 9-volt battery connected to a wire as a primary.
The first reason I think this is fake is that he is powering a Tesla coil with 10 volts.
The second reason is that Tesla coils work on AC, or at least I thought so, and he wouldn't be able to step the voltage up too high because a transformer only works on AC and the battery provides DC.
I'm sure you figured it out by now, but you need to limit current to your power supply by attaching two reactors (modified MOT's) as inductive ballast. The arcing you experienced was most likely in the outer two MOT's as they are not build to handle more than 3000 V. Oil submersion seems to be necessary at least for the outer two transformers. Go to Steffan's Tesla Coil Page. There I have instructions on how to hook up the reactors. I have been testing without oil submersion and haven't had any problems so far.
Honestly, MadHatter's answer is the closest to being correct, although Tesla coils ARE still a type of transformer. Nobody else here seems to understand, however, that Tesla coils are resonant transformers, so they do not operate in the same way as common iron-core transformers. The most important factor required for a Tesla coil to run correctly is that the secondary coil and topload LC circuit have the same resonant frequency as the primary coil/capacitor LC circuit. This is how you get efficient energy transfer from the primary circuit to the secondary circuit. Putting too many turns on the secondary would add too much inductance (and self-capacitance) that the secondary circuit will be significantly out-of-tune with the primary circuit. You will get very little energy transfer between the two resonant circuits, causing there to be little to no output. You will also run into the issues MadHatter suggested (the waveform will be discontinuous due to current being induced in the wrong portions of the coil). Remove all but one layer of wire and just leave it as-is. Then make sure that the secondary resonates at the same frequency as the primary. You can use the following formula to calculate resonant frequency:
where 'f' is the resonant frequency, 'L' is the inductance of the coil, and 'C' is the capacitance of the system (the tank capacitor in the primary or the topload on the secondary plus the coil's self-capacitance).
Do the calculation for both the primary LC circuit and then the secondary LC circuit and make sure they match. Otherwise your Tesla coil won't work at all.
If they don't match, you can "tune" the Tesla coil using different methods:
If the primary resonant frequency is too low, do one or both of the following:
If the primary resonant frequency is too high, do the opposite.
If the secondary resonant frequency is too low, do one or both of the following:
If the secondary resonant frequency is too high, do the opposite.
You have to use math to determine which of the above to use, and how much to adjust each one.
Best Answer
It's real.
... and a transistor, and a resistor.
This is the simplest SSTC (Solid State Tesla Coil) I've seen. The transistor chops the incoming DC power supply so that it's changing at the primary. It takes feedback from the secondary, so that it oscillates at the resonant frequency of the secondary inductance and its self capacitance.
Drawing the circuit from the video (I might make one myself for the lols), we get this
simulate this circuit – Schematic created using CircuitLab
I'm not absolutely certain I have the transformer 'dotting' correct, but I think I have interpreted it correctly from the video. If it fails to work, swap the orientation of one of the coils. Eye-balling the coils, I'd guess at 32 mm x 100 mm and 200 turns = 350 µH for the secondary, and 35 mm x 3 turns = 690 nH for the primary, coupling in the 0.1 to 0.2 ballpark.
When power is switched on, the transistor is biassed as an amplifier by R1. It's quite a large value, so the base current is small, and the collector current is similarly small enough that the transistor doesn't draw too much current for the battery or let out smoke.
If you run this in a simulator, that might be all that happens until you give it a kick. In real life there's noise in the circuit, and this noise will be amplified by the transistor. This will cause a variation in the collector current, which will induce a voltage in the secondary. This voltage will drive a current into the circuit consisting of the transistor base and the coil self capacitance. If the transformer is connected the correct way round, the effect will be to reinforce the change and drive it further.
This reinforcement will continue until one of two things happen. A) If the collector current is increasing, eventually the transistor will run out of gain or B) if the collector current is decreasing, eventually the current will get to zero. At either end point, the collector current stops changing, so the reinforcement feedback stops, and the transistor switches to the other mode. This cycle now continues indefinitely.
Once started, the timing of these reversals is dominated by the resonance of the secondary with its self capacitance.
As @Hearth points out in comments, it's basically a Joule thief, or blocking oscillator circuit.
A combination of low input power and the 2N2222 being fairly tough allows this to keep working without blowing it up from overvoltage on the collector or base. More input voltage or a more fragile transistor would not work, at least not beyond the first few cycles.
I like circuits that are as simple as possible. If I'd started to design this, I'd probably ground the secondary with a pair of anti-parallel diodes, so that the discharge current is kept out of the transistor base, and then have to connect the feedback to the base. However, the capacitance of these diodes would change the phase shift and gain, and it may not start, and would use more components ... sigh! Perhaps it's worth the risk of killing a 2N2222 for the simplicity. Maybe just one little signal diode between base and emitter as shown to prevent VBE reverse bias. I'm sure that would not stop oscillation.