: Dear Forum,
: I'm a starting to build my 1st Tesla Coil and I've decided to use an old
: vintage 10KV spark transmiter transformer with 1/2KVA output at 110Volts.
: If my calulations are correct this transformer delivers over 4
: amps?....Should I calculate at maximum current draw or calulate at some
: smaller/safer current value. I would like the primary to use 1/4"
: copper tubing.
: Thanks for any input.
Your HV transformer will step up line voltage (usually 120 volts today instead of 110 volts) to almost 11,000 volts. Old spark gap transmitter transformers were usually designed to limit short circuit current (such as when the gap fires) to avoid drawing excessive current. If you short the HV side, you should pull about 4 amps from the LV side of the circuit. And, the short circuit current on the HV side should be about 500/10,000 = 50 milliamperes. You should always design your system so that it can safely handle the largest expected current or power draw. BTW, the insulation on old wireless gear, such as your transformer, may be degraded from age, and may fail when stressed by the demands of a Tesla Coil. You may wish to consider getting a transformer-type (non-electronic) neon sign transformer to drive your system. Older versions (without GFI protection) are preferred and can often be found when neon sign companies replace old used equipment.
If you want to add a variable voltage transformer (Variac) to control your coil, it should be rated for at least 5 - 7.5 amp to provide a bit of design margin. When you add the HV capacitor and primary circuit, it's quite possible to draw higher than faceplate current from your HV transformer (and thus higher LV side current as well), since the tank capacitance partially neutralizes the current-limiting inductance in your HV transformer. 1/4" copper tubing should be sufficient to handle the peak TC primary currents when your spark gap fires. The TC primary current can easily be hundreds of amps even in a small Tesla Coil.
You may also want to consider using a computer aided TC design tool to help you with your design, such as JavaTC or TeslaMap. These will help you optimize your design before you spend a lot of money on actual hardware. You may also wish to join the Tesla Coil Mailing List or at least read some of the archives on the Pupman site. There is a wealth of information in the archives.
Good luck and play safely!