Original poster: "D.C. Cox" <resonance@xxxxxxxxxx>Dozens of experimenters have reported similar results, ie, racing sparks, when using nonconformal coil coatings such as polyurethane, boat epoxies, etc. These coatings are much less tolerant than a proper coating designed and thoroughly tested for HV applications. I use Dolph's AC-43. It's an excellent product that is specifically designed for HV coil winding and coating applications. It works and holds the wires solidly in place with no tracking or racing sparks. Also, using a proper size topload of at least 2 x coil dia. helps reduce unwanted side frequencies that can produce standing waves thru constructive and destructive interference modes. 2 to 3 x sec dia. seems to work well and keep the sec coil properly elevated with regard to your specific sec coilform dia.
Dr. Resonance
The first medium-large coil I built used a 4.2kva 19,000 volt transformer. The secondary coil was an 8" x 40" pvc pipe wound with 21 gauge magnet wire. A large home made toroid of at least 30 inches outer diameter sat on top. Because of the angle of my inverted cone primary, I didn't know that the coil was horribly overcoupled. To add to my problems, I had observed on one of my old TCBOR video tapes that Richard Hull stated that "All coil coatings are Q killers" Thinking that I should try for as high a Q as possible, I simply taped the wire to the secondary at both ends, and left the whole coil un-coated. At first power-up, sparks were fantastic. We managed to hit the right spot on the primary tuning, and it was pushing out 4 to 5' streamers at 40 to 50% power. Satisfied that I had put a killer coil together, I started to throttle it up and see what it would do at 80% power. Immediately, I had a very nasty racing arc, more like a power arc, creating havoc on the secondary. It killed the coil and made it unusable. I was amazed when I took off the lower third of the windings how deep the channel was where the arc had occurred.I became convinced at that moment that it was better to loose a little q by applying a coating then to completely kill a secondary if you are a little off in your coupling. Since that coil, I have only used low q components and they work fine. I don't like PVC pipe, so I use kraft phenolic tube. Strong and light weight. Once wound, I coat the secondary with water based Minwax Polycrylic. I have been told that this would be marginally better than winding my coil on a soaked rain barrel. Again, my coils perform fine. On the occasions that I have had racing arcs, they usually glide along the outer coating in a light manner, and have done no harm. In my more recent coils, I have used only flat primary coils, and started the first turn of the secondary an inch or so above the plane of the primary. I don't know that I have the coil putting out as much as possible, but I do get very good performance. My last coil with this arrangement used a 9/60nst, a 4" x 20" phenolic secondary coil form wound with 30 gauge magnet wire, a 4" x 13" toroid on top, and 12.5 turns on the flat primary. The coil at 120 volts pushed out 36" sparks with a nice steady tone. Spark gap set at 13/64"The bottom line: If you cross the line of max coupling, a good coil coating may save your coil from non-repairable damage.Dave Goodfellow ----- Original Message ----- From: "Tesla list" <tesla@xxxxxxxxxx> To: <tesla@xxxxxxxxxx> Sent: Friday, April 21, 2006 4:31 PM Subject: RE: Primary coil configurationOriginal poster: "Lau, Gary" <Gary.Lau@xxxxxx> Increasing the coupling until racing sparks occur, then backing off, is the only way I have ever heard of to set the coupling. Since the exact mechanism responsible for racing sparks is still unknown, we are unable to predict the coupling threshold at which they'll occur. As setting the coupling much lower than that threshold will degrade performance, we strive to keep it as high as possible. Some may consider it "horrible engineering", but it's the best that can be done. How do you propose to determine that threshold without ever crossing it? I agree that running a coil with racing sparks with reckless abandon will permanently damage the secondary, but I think most of us have briefly coupled too high, seen the sparks, backed off, and have had no further trouble. Or are you suggesting that a brief racing spark will permanently lower the critical coupling threshold? An interesting and reasonable suggestion, but very difficult to prove. I don't know that it's ever been conclusively demonstrated that any secondary sealing technique will render it invulnerable to racing sparks. Regards, Gary Lau MA, USA > Original poster: "D.C. Cox" <resonance@xxxxxxxxxx> > > Someone advocated adjustment until racing sparks occur and then > backing off. This is horrible engineering. Once a spark punctures a > solid insulation it does not "cure" or repair itself. Once the > damage is done it is permanent unless the coil is rewound. You can > back off the coupling to reduce sparkover, but then you always have a > weakest link in your coil system. Properly sealing the sec coil and > then not puncturing it will produce a long lasting coil. > > Dr. Resonance