It was cold, damp and trying to rain, perfect weather for mucking about with high voltages outdoors ! So it was on with the warm clothing and out into the garden. Due to the unpredictable nature of an untested Tesla Coil, you can’t really test it indoors because of the high risk of fire etc.
The components were connected with high quality silver plated oxygen free copper loudspeaker cable ! It uses twisted multi strand cores that are then twisted into a thicker core and has a very heavy covering so it seemed a good idea at the time.
The wiring was thoroughly checked and an earthed discharge wire was pointed at the topload about 75mm away. The earthed points of the coil were connected to two 1.2m solid copper ground rods driven into the garden about 6 feet apart and connected with 8sq mm copper cable. You should never use the mains supply earth as it was not designed for the type of discharge that coils put out and can be extremely hazardous.
I retreated to the safety of my kitchen, checked that the variac(power control) was at zero and turned the power on. I very carefully increased the power and things started happening much sooner than was expected ! I had only reached about 30 volts input and there were very loud sparks jumping across from the topload to the earth probe !!
This indicates two things, one – the coil works !, and two – the primary charging circuit is resonant. This is the reason for the seemingly high efficiency but it can be very damaging because it can cause massive overvoltage damage to the tank capacitor and the supply transformer.
To counter the voltage increase caused by this resonance, I am now building in an overvoltage protection device on the capacitor board. The idea is that this large increase in circuit voltage is ok as long as it does not get higher than the design voltage of the components. This is taken care of (hopefully) by safety spark gaps in the power supply and across the capacitor. If the voltage rises too far, one or both of the gaps will fire and discharge the circuit, protecting the components.
I think the first test was very successful and very reassuring being my first coil. At 50 volts input, the spark was jumping about 250mm, not so loud but very encouraging. I am now finishing construction of the control box and base of the coil so that further testing can be done a bit easier.