NSTs & MOTs for Tesla Coils

For a tesla coil to get a sufficient output voltage (in excess of 500,000v) we need a starting voltage that is somewhat higher than the normal household mains. The first step therefore is to boost the mains voltage by using a conventional HV (high voltage) transformer.
A popular choice is the NST, or Neon Sign Transformer. These typically give output voltages of between 7,500 volts to 10,000 volts in Europe, or in the USA and other parts of the world, a considerably healthier 15,000 volts, with an average current of 60 m/amps.
I have used the word healthy from a performance point of view, rather than what happens if you come into contact with it.
An accidental brush against a live NST terminal with that level of current (60m/a) and 15KV will be extremely painful causing burns, or may, dependant on the path the current takes, even prove fatal.

From a safety point of view I use a switchable power feed to my coil. This is simply a box housing the 30amp variac and output ammeter and a key operated double pole switch. This is wired via a 32 amp plug for use on my big tesla coil.
The key can only be withdrawn when the power is switched off, so the key is on a lanyard around my neck, meaning it has to be off before I can approach the coil. Simple measures could save your life.

Given the right circumstances a NST power source may kill, but more likely though, is that it will give you a long lasting memory of how painful a high voltage burn can be. The situation with MOTs from a Microwave oven and OBITs (both lower voltage than an NST but much higher current) is even worse, as these stand a real chance of killing rather than just hurting you.
While things like Pole Distribution Transformers (PDTs) or Homemade Transformers definitely will kill.

NST Transformer for Tesla Coil
A Single 10Kv 48 m/amp unit

NSTs are not the most robust things as they were never designed to run at a high output voltage continually. Normally they would only be open circuit, and therefore at max voltage, for half a second or so until the neon tube fires. The tube then presents a fairly low impedance short to the NST and its output voltage drops to a much lower level at its max current rating.
For this reason it can be a good idea to use alternative more robust sources for a high voltage power supply, such as a oil burning furnace's ignition transformer (OBIT) and Microwave Oven transformers (MOT).

High Voltage Pole Distribution Transformers (often called PDTs or Pigs) can be used if wired in reverse. By that I mean the output terminals are used for the input, and vice-versa. Although we strive for a high voltage, old X-Ray machine transformers are too high a voltage for tesla coil use, but once the windings are removed the cores can be used to make either a ballast or to wind your own transformer.(See link below)

Despite their fragility the easy availability of NSTs still make them a popular choice though. Because of this a safety circuit to protect the NST is needed. See Safety Gap  and Terry Filter     

Another option is to build your own homemade transformer like I did here: (click photo)

Homebuilt Transformer.

Tesla coil MOT
A Microwave Transformer (MOT)

With most transformers you will need to have a Ballast added to stop excessive current being drawn through the transformer's windings when the secondary is short-circuited. This is not needed with NSTs (all NSTs except some very old ones). Because NSTs are actually designed to operate with virtually a short circuit across their output at times, they are wound onto a core that is designed to self limit the maximum amount of current that can flow (so are most OBITs).
If you use other types of transformers you will need to add some ballast (usually in the form of an inductor) in series with the primary (or secondary) winding of the transformer to limit the current that can flow.
MOTs do have some in built ballast in the form of shunts like the NST does, but sometimes they are not effective, and you may need some additional external ballast with these as well.