Bob:If you have the patience I'd recommend the MMC route but the second capacitor MIGHT work. Easy enough to get a quick check. Rig a spark gap which just fires with the voltage across the unloaded NST secondary and then connect the capacitor across it and let it run for a couple of minutes. If you get pretty hot sparks and the capacitor doesn't get very warm [notice both terms are subjective] it will probably due as a start until you get something better. Alternatively I could send you three of the 0.004 ufd, 20 kV HV filter capacitors I'm using with my "big" coil [3" x 15" secondary, ~27" streamers]. That's the setup I use with a 12 kV, 60 ma transformer. These things have lossy dielectric and a negative temperature coefficient but along with that have very low series resistance so they work fine until they get too warm and change the tuning [about 60 seconds at a crack with the 12 kV NST]. I've blown up a couple due to overheating but doubt if that would be a problem with your little NST. I've been intending to dig out the bigger coil and try that transformer with it but haven't had the ambition to clear enough bench space. I suspect it would give 12" + streamers at least.
Alternatively I could send you three smaller 0.004 ufd 20 kV filter capacitors of the same type I use in the little TC you saw when you visited. They have the same general attributes, lossy dielectric and high negative TC, but still have the low ESR and do indeed work OK for a while.
73, Ed W6IZJ Bob Deuel wrote:
Hello: I have questions regarding a couple capacitors that I have: 1) The first capacitor is a Sprague type 45P5 "Vitamin Q" rated at 0.012 mfd. @ 25,000 VDC. It physically measures approximately 2.5-inches in diameter and 6.5 inches long with end caps threaded for 1/4-20 thread. It is an oil filled capacitor that has a glass cylinder with metal end caps. I understand that some capacitor dielectrics do not take kindly the rigors of Tesla Coil operation and overheat. I wondering if anyone has used a capacitor of this type? This one would use for a future coil if it is acceptable. 2) I plan on building a table top coil using a 5kv @ 20 ma. NST soon. I keep looking at a capacitor that is on my bench that I believe was from an older IBM computer monitor. It is made by ERO and is 9000pF+80-20%/20KV. It actually measures 10,300 pF which is great. Physically, it is potted unit in a 1.5x1.75x2.75 plastic case with a insulated HV lead and a "ground" lead. Are capacitors like these acceptable for a small coil or should I forget it and go with a MMC? I would appreciate your input on the feasibility of these two capacitors. Thanks, Bob Deuel, K2GLO
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