Death of a Coil

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Original poster: "James Zimmerschied" <zimtesla@xxxxxxx>

Tesla List readers: this applies to new coilers but also to sages:

I recently had a 4 x 17 coil go to Tesla boot hill due to a burn 
inside the PVC form the full length of the coil from the top wind 
straight down to the bottom wind. This was a coil wound with #28 wire 
where the wire was taken into the inside and back to the outside in a 
1/2" stitch to keep the coil tight - both top and bottom. I did not 
wind it this way but obtained it from my brother who bought it on the internet.

The coil looked pretty good (except for a fairly thick coating of 
varnish on the outside). It also performed well for a number of runs 
over the course of a year.

When I first got the coil I had the inclination to pull those wires 
out so nothing was inside the PVC tube (my standard method of 
building coils). I could have put in a baffle - may or may not help. 
During the autopsy I observed that some black traces had gone along 
the inside of the 1/4" Lexan disk used to seal the top of the 
secondary. The traces lead from the outer edge where the high tension 
wire leads to the toroid across the disk to the stitch of wire that 
tied the upper end of the coil. There is a sharp bend at this point 
and ,no doubt, corona formed between the kink and the secondary wire 
leading to the toroid. The corona eventually lead to carbon traces.

When I was working with a rotary gap trying to get it in phase, there 
was enough stress in the coil to breakdown a path all the way down to 
the stitch at the bottom (ground) end. I noticed no output but 
thought this was due to tuning problems with the SRSG. I switched to 
a static gap to check things out and sure enough - smoke and the 
delightful smell of burnt PVC from inside the coil form.

I have built several other coils with the windings entirely on the 
outside and haven't had a problem. No baffles except for a top cover 
disk. Maybe some of you like to run the secondary inside the form at 
the top or bottom (or both). If you do I recommend baffles be 
installed and fairly frequent checks for soot marks. Avoid any sharp 
changes of direction with your wire since corona forms at these points.

Sincerely,
Jim Zimmerschied