The base voltage rises again and Q turns on and the cycle repeats.The transistor turns off and so the magnetic field starts to reduce.Diode D prevents the base voltage to fall more than 0.7V below ground, which in return pushes the output end of the secondary high.But the tiny stray capacitance on the output resists the change, although very small, against the rise of the output end and so in return the voltage on the other end of the transformer goes down, pulling the base of the transistor low. The secondary voltage wants to grows large.The created magnetic field drives the secondary of the transformer.

The current is limited by the limited available base current. Q turns on and drives current into the primary of the transformer.Turning the circuit on, R drives the base of the transistor Q.You can add more transistors and reduce R to make the circuit drive stronger.

You might want to watch the video which inspired me here to see how Ludic Science made his own. With primary of 10 turns (could be fewer depending on where you connect the wires) and secondary of around 750 turns.

T: my hand made transformer I showed in the video.

Q: 2N2222, or 2N5550 or similar regular NPN with >50V VCE and >60 hfe.

The way the transformer needs to be connected, by showing those transformer dots.

Also in case of a Tesla coil the toroid on the top helps with creating this capacitance. You don’t add it, but the capacitor exists naturally there.

The stray or parasitic capacitance that exists on the output of the secondary winding.

This circuit is the simplest thing that can generate such high voltage I’ve ever seen, so I love it! I also added the missing information in the circuit: Well I guess it does! I was initially pretty confused about the circuit, but after finding out that there is this parasitic capacitance on the output of the secondary winding, all became clear and now I’m in love with this circuit!

Tesla radiant energy schematic