date: 6 December 2000
Last revision: 17 October 2002
The Power Transformer is probably the most dangerous part of a Tesla Coil set up. Its input power is taken from the mains (power line) which is 240 Volts at 50Hz or 120 Volts at 60Hz depending on where you are. These voltages are lethal and must be respected at all times. The output voltages of these transformers run typically between about 8kV to 15kV and deserve even more respect for they can leap out to you. . Currents over 15mA can kill! Currents over 30mA definitely kill. Currents over 1 Ampere also roast as well as kill. It follows that the output of a neon sign or boiler ignition transformer can kill you with out roasting you, whereas transformers with good voltage regulation such as "pole pigs" will roast you in the fraction of the time it takes a microwave oven to roast a hot dog. Therefore, if you are not trained and experienced in dealing electrical power, transformers are absolutely NOT for you to play with. If you are trained and experienced, please don't get careless. Electricity makes no distinctions and gives no second chances.
This page deals only with neon and boiler ignition transformers most commonly used to power Tesla Coils. They are popular because they don't require extra components in their primary circuit for current limiting. This current limiting is designed into them to suit their original application in neon signs and boiler ignition systems.
The need for current limiting transformers in the Tesla Coil application can be explained using the circuit shown on the left. In normal Tesla Coil operation the Spark Gap arcs over and connects C across L 100 times a second resulting in 100 bursts of high voltage RF out of the Tesla Coil. The RF Chokes are there to prevent the RF from getting back into the mains (AC Supply). During these arc overs the Spark Gap appears as short circuit to the 10kV output of the Neon Sign Transformer because the RF Chokes offer negligable impedance at 50 Herz. This short circuiting at 100 times a second would reflect back to the primary and cause excessive currents to flow if current limiting didn't exist. Fuses would blow and circuit breakers open up. Neon sign transformers prevent this from happening, but "pole pigs" or any other transformer with good voltage regulation won't. As such, they require current limiting components in the primary circuit to handle such reflected short circuits.
Current limiting in Neon Sign and Boiler Ignition Transformers is due to the relatively high flux leakages deliberately built into them. This is usually achieved with magnetic shunts. These shunts bypass some of the primary coil generated magnetic flux around the secondary coil making the magnetic flux linkage between the primary and secondary coils less than unity. It is specified as the coefficient of coupling (k). Pole pigs use no such shunts, so they have k's very close to 1. Neon and boiler transformers k's of around 0.95.
The diagram at the right is an equivalent circuit for transformers just mentioned. It is often used in text books on Electrostatics and Magnetism covering transformer theory. I find it very helpful in making transformer measurements and determining their characteristics from the results. Details for making measurements and calculations on a boiler ignition transformer and using this equivalent circuit are given in the Transformer Analysis Page.