Photographic Documentation of the effects of Corona Discharge vs. Coatings of Insulating Varnish on the Secondary of a Small Tesla Coil 



The following experiment was conducted as a result of observations of a "halo skirt" of corona appearing near the top load / secondary connection of a small table top Tesla Coil flowing in a downward direction toward the primary. 



The Tesla Coil used for this experiment was originally built in 1994 under the guidance of Jeff Mullins (Dr. Mot) and has over 700 hours of operation without any breakdowns. It is of conventional spark gap design and has been used in many a classroom demonstration and was even featured on a local TV show "Brainstorm" in the late nineties. (More details of this coil at : Audiotesla Table Top Coil) I might mention that Jeff has a similar type coil and during discussions we had talked about the halo skirt forming around the secondaries of both of our coils. I don't think either one of us ever actually persued the exact cause of this pheonominon, but we did observe this effect only occuring at very low power levels. 


The Experiment

Along came the WWT2008 Arizona Western Winter Teslathon. I was having conversation with my friend and photographer Dr. Fred Starelli, who flew in from Denver for the event when the subject came up about the "skirt of corona" forming around the secondary. Fred could also see the faint glow and explained: "If you think you can see it; I can photograph it!" So we set off on a photographic adventure. The camera was placed on a tripod and set at ASA 1600 for long time exposure of 3 minutes. Here is what developed: (please click on photos to enlarge)

The skirt of corona forming at the top winding of the secondary in a downward direction is quite evident. Also evident are several "flashovers" along the secondary itself. These flashovers are also known by 'coilers as "racing sparks". It is generally assumed that the racing sparks are a result of over coupling of the primary to the secondary. The normal solution is to decouple the two windings from each other until no racing sparks are observed. This is usually accomplished by placing a spacer under the secondary to raise it away from the primary until critical coupling is achieved (usually at high power). There seems to be a fine balance between coupling and performance. Some coilers use instrumentation to achieve optimum "ring". Interestingly the coil was only running at about 20% input power during the photographic exposure and yet our eyes did not see any racing sparks. The picture obviously tells a completely different story....they were definitely there in the finished photo!


The Process

Typically once one is done winding a secondary coil, a coat of varnish (or other coating) is applied. Some folks even coat the form before winding. Over the years most everyone I've talked to agrees that the purpose of the coating is to keep the winding from unwinding and to help protect the secondary. I sprayed one or two coats of Dupont Chromaclear Epoxy Clear Coat used for automotive paint jobs. It's some pretty tough paint and looks good, too. Had a flashover during a class demonstration once...and thought the coil was toast. I cleaned the coil with denatured alcohol, borrowed some clear nail polish from one of the moms and off we went.....That was ten years ago. Close examination of the above photo might even reveal the spot!


The Solution

I shared the photo Fred took with Dr. Spark who winds some of the finest secondaries I have ever seen. He is very meticulous and spends an unbelievable amount of care and time on them. He suggested that I place the coil on my new coil turning lathe and refinish the paint job. This I did. I ruffed up the old finish and put no less than twenty coats of fresh varnish on the coil. I would put one or two coats on a day then wait a few days, then a few more coats....catch an occasional moth or bug, have to sand it down and put on a few more coats until after about three weeks or so I was satisfied with the appearance. 


The Results



Before and after photographs were both taken for 3 minutes at 1600 ASA with conditions as identical as possible for a fair evaluation of the differences....except four months later! Immediately noticable is no corona halo below the corona ring and no "racing sparks".


Now for an Increase in Coupling 

1 inch secondary spacer removed ~ closer coupling

Average spark length immediately increased 4 ~ 6 Inches with same power input with no evidence of "racing sparks" or the intriguing corona halo skirt!



We took an old coil and woke it back up

I highly recommend that one practices great care when finishing a secondary winding. The results will speak for themselves. By assuring that you have added plenty of insulating varnish you can reap the following benefits:

Less Corona Discharge

Less "Racing Sparks" ~ Flashover

Increased Coupling

Longer Sparks


"There is no substitute for good quality"


Additional Section of Interest


First Photograph Actually Bombed Out 

At first we weren't quite sure how well the varnish would work because about one minute into the first "after" exposure this happened:

Electron Leak

We thought the secondary was about to flash over and burn itself up, but it hung in there for the remainder of the shot. We sure were nervous because everything initially looked so good; at least for a while. Fred finally released the shutter on the camera and we shut the coil down, turned on the lights and discovered a bug had capacitive coupled itself to the side of the secondary coil causing a washdown of corona! Welcome to Arizona in the summer! The blue light of the coil sure attracts the bugs out here! Our second shot had a moth land on the top load:

Moth on Top Load

Moth Blasted

Moth (close-up)

Electron Leak (close-up)

We hope you found this article of interest and value


Copyright 2008 Audiotesla