NST PROTECTION CIRCUIT
C: .001uF/10kvdc ceramic disk capacitor with axial leads http://www.allelectronics.com catalog number HVD-102, $1.00 each.
R: 1000 Ohm/50 Watt wire wound ceramic power resistor http://www.mouser.com catalog number 588-L50J-1K, $6.54 each.
SG: Safety gap to grounded NST case or core.
Notes: For NST up to 9kvac, only two capacitors are needed. For NST above 9kvac, use four capacitors as pictured. One filter can protect two or more paralleled NSTs, if the resistors are up to it. For a big NST bank, consider using 100 Watt resistors.
NST protection circuits are one of the most common topics on the Tesla Coil mailing list. The purpose of the protection circuit is to reduce the risk of RF damage to the fragile secondary windings of the NST. Some hobbyists don't bother with the protection circuit and claim great results. I prefer to err on the side of safety and use the filter. This is a simplified NST protection filter. I've been using it for years and I've yet to kill an NST, so I guess it works. Component values are not critical, and you can get away with some pretty extreme substitutions. You can get as fancy or as crude as you want. Some guys mount their filter on perfboard, build nice brass safety gaps, etc. I have my bypass caps suspended by their leads from the HV output terminals and tied directly to the grounded steel NST case. Likewise, my safety gaps are short pieces of stiff copper wire connected to the HV terminals and bent near to the NST case. I have them set at about 1/4" (6mm) each on my 9kvac NST.