Neon Sign Transformer (NST)
Neon sign transformers can be purchased used from cooperative sign companies. The transformer is a step up from the 120 or 240 volt line to 5000-15,000 volts. Current ranges can be from 30 milliamperes to 120 milliamperes.
For beginers it is probably good to stay in the 30 milliamp range for safety sake. For bigger sparks you definetly need the higher current ratings. The high current transformers are made for cold cathode lighting. Two stars if you know what that is.
There are altertatives to NST's but they are a good place to start. Some coilers remove transormers from microwave ovens and some use transformers from oil burners. I believe these tend to be lower voltage and current than most NST's.
I have seen some coilers who have successfully put multiple transformers in parrallel to boost the current. It would be important that the transformers be identical in every way or problems will arise.
This is a photo of the name plate on my main NST. You can see the following info on the plate.
Primary Volts 120
Sec Volts 12,000, the number is not clear.
Sec Amps 0.100 (same as 100 milliamps)
KVA 1.25
Cycles 60
NST Filter
It's a very good idea to include a filter to protect your NST from the high frequencies developed in the coil. High frequencies even above the tuned frequency of the coil can damage the NST by causing a breakdown in the insulation. This is bad.
I settled on a RC Resister Capacitor filter using wire wound resistors and high voltage capacitors I found from a surplus dealer on the internnet.
The filter I used is a basic low pass filter. The most basic RC filter is illustrated schematically here:
The formula for calculating the cutoff for an RC filter is stated :
In my coil the Resistor is 1000 ohms and the capasitor is 920 pf.
The cut-off for this combination is 173 kHz. At this frequency, the output of the filter is about 70% of the input signal. In the Tesla coil this means that the transients above 173 kHz are prevented from entering the NST.
Here you can see The RC filter made from C1,C2 and R1,R2. In this case we are trying to block the high frequencies from the primary tank circuit so the resistor is to the right rather than on the left as shown above. The safety gap is another preventive measure to disapate transients that make it through the filter. The two filters are mirrored around the ground connection since the center of the NST secondary is grounded so each side of the transformer is filtered separately.
Here is how my filter and safety gap look on the coil:
My NST is mounted behind the filter panel. There is an extra sheet of polycarbonate to insulate the panel from the case of the NST. The brass angle brackets are from the home center. My wife had the copper sheet. The red caps...I don't remember where I ordered them from. Sorry.
Here is a side view of the NST (Black box), Spark gap (above) and the NST Filter (On the right) The two silver boxes at the bottom are line filters. The brass ribbon on the left is the ground connection.