I'm aware that neon sign transformers (NSTs?) produce high voltage – up to 15kv. But I cannot picture what's inside of one. Wikipedia seems to indicate that they may be a form of resonant transformer but provides no diagram. Googling "resonant transformer" shows images that may be laminated core or toroidal based, close proximity series transformers, but it's hard to tell.
What would a typical NST look like if it was cut open, and would it be possible (not that I want to try) for someone to home-build a HV transformer like a NST?
Note: I'd never trust myself to build one, so that may answer the second part of the question – if I wanted a tesla coil, I'd buy a working NST
Best Answer
Ordinary power transformers are designed to have good voltage regulation (the ratio of input to output voltage is fairly constant regardless of load)- the flux is mostly linked between primary and secondary and the leakage inductance is minimized.
Neon sign transformers are made to have high leakage inductance- ferromagnetic "shunts" are inserted (magnetically) in parallel with the secondary, which is similar electrically to having a large inductor in series with the output. That makes the transformer more of an AC constant current source than a constant voltage source. It allows the voltage across the neon tube to rise in order to start the ionization, and prevents excessive current from being drawn once the discharge starts. Below is a photo of a disassembled Neon Sign Transformer from this website (arrows and caption added).
It's possible to fool with the shunts in order to change this behavior (removing them increases the short-circuit current).
Modern neon sign "transformers" are typically switching power supplies designed to have similar characteristics to the older style of mains-frequency transformer. The old style ones are practically indestructible, and the new ones are not going to last nearly as long.