# Is this video fake? I thought Tesla coils only worked on AC

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.

• The transistor + feedback winding can work as an oscillator. Commented Jul 27, 2021 at 3:38
• Who has a schematic? tinyurl.com/yjzmeypv. tinyurl.com/yj3up6pm Commented Jul 27, 2021 at 4:59
• Regarding the other thing going on here - a fluorescent bulb being powered wirelessly, I can attest to that definitely being a thing. I grew up half a mile away from some overhead power lines, and you could take a fluorescent bulb (in those days just the straight tube kind) and bring it near the power lines (and by "near", I mean, just on the ground 100 ft. below them), and they'd glow very nicely. Commented Jul 27, 2021 at 13:32
• Not only that this is not fake, you can get one for less than 2 dollars online from China. Commented Jul 28, 2021 at 6:43
• @MeenieLeis Electricity is flowing in a sense, but it is not completing a circuit like you would imagine. Look HERE Commented Jul 31, 2021 at 15:05

It's real.

this video shows a 9-volt battery is connected to a wire as a primary

... 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.

• It reminds me of a joule thief. Commented Jul 27, 2021 at 4:32
• Coincidentally looks like mine and it oscillates in either polarity probably weak magnetic coupling Commented Jul 27, 2021 at 5:03
• Good simulators start with a residual voltage that is a tiny %age of the supply voltage on caps , so that the step voltage has the similar bandwidth of “white noise” or the same occurs with a step current from bias. So noise is less important an assumed requirement for oscillation, self start. Same applies to XO’s. Commented Jul 27, 2021 at 13:24
• A 2222 is a big fat gold-doped transistor that can tolerate a lot of base-emitter breakdown current. It makes a nice 7V zener. So the extra diode probably isn't needed. Commented Jul 27, 2021 at 14:44
• If it hasn't already been said, I believe this circuit is also known as the "slayer exciter" Commented Jul 27, 2021 at 15:51

So its real. Could you explain it a bit simpler please?

Figure 1. Schematic drawn by Neil_UK.

• When SW1 is closed current flows through R1 into the base of Q1. This turns Q1 on.
• Current can now flow through XFMR1 primary through Q1's collector and emitter.
• The current in the primary induces a voltage in the secondary. As drawn the dots indicate that the secondary voltages on the top of the coils will be positive.
• The output voltage is fed back to the base turning the transistor on even more.
• Eventually the current through the primary maxes out and can increase no more. Since transformers only transform when the primary voltage is changing and don't pass DC through the output voltage starts to collapse. This reduces the base current and Q1 starts to turn off.
• The decreasing primary current causes the secondary voltage to go negative and this turns off Q1 even more until it is turned fully off.
• The cycle then starts again.

A similar type of circuit is used in gasoline engines so that the 12V alternator can produce a high voltage spark to ignite the fuel/oil mixture. See https://en.wikipedia.org/wiki/Ignition_coil for more information.

A transformer does not need alternating current (positive-zero-negative-zero repeated indefinitely) but only requires that the voltage change with time. If you run DC power through the primary coil, it will store energy in the transformer, but no power will come from the secondary. If you suddenly cut the power to the primary, you have a very high rate of change of voltage (difference of voltage divided by unit of time) in the primary, resulting in a high voltage spike at the secondary. If you want more details, the Wikipedia article appears to be a good source.

• The alternator charges the battery and supplies 12 volts DC nominal to operate the vehicle. People have gotten several hundreds of volts from a car alternator but it is designed to output something in the 13 - 18 Volt range, not kV. The engine will start without a battery look at the drawing in your reference. There is a "transformer" normally called an ignition coil. When DC is supplied to the primary it stores a charge as a magnet flux, When the supply is removed the polarity reverses and the primary discharges causing energy to transfer to the secondary creating a multi kV pulse.
– Gil
Commented Jul 30, 2021 at 2:53
• I may have been unclear in my statement. I understand that the alternator only produces 12 volts and I indicated that it was a twelve volt supply. t Commented Jul 31, 2021 at 21:52