Re: [TCML] sealing a cardboard form

[jimlux <jimlux@xxxxxxxxxxxxx>]

quarkster@xxxxxxx wrote:
> Dave -
>  
> I would venture an educated guess that very, VERY few Tesla coils have actually been built using the expensive Dolf''s varnish as a secondary overcoat. Based on closely monitoring this list for nearly ten years, attending a fair number of Teslathons, and coresponding with scores of TC builders over the years, I estimate that for operating amateur-built TC's of all sizes, the "real numbers" for secondary overcoatings may look something like this:
> 1. No secondary overcoating at all;
> 2. Clear varnishes of all types, including oil and water base types (most commonly polyurethane, but also good old fashoned marine spar varnish, etc);
> 3. Two-part epoxy materials;
> 4. Dolph's AC-43, "Glyptal", and other types of  commercially-sold "corona dope" insulating coatings.
>  
> I think that a very interesting experiment would be to wind at least 5 identical secondary coils, coat 4 of them with the most commonly used insulating material, and then subject them to ever-increasing electrical stress in a large bang-size coil, gradually increasing coupling until insulation failure (racing sparks, turn-turn arcing, etc.) occurs.
>  
> I'm strong proponent of evaluating new processes and materials in an attempt to improve the technology. Successful evaluation means pushing the stress levels to the threshold of failure, and beyond. If someone is willing to take the time to evaluate new "stuff", and understands that this unavoidably includes the risk of failure, then let's not dissuade them from trying.
>  
> If a small-to-medium size secondary fails this testing, what's the cost? At max, $20 or so of magnet wire, $10 worth of PVC pipe, and a few hours of labor, it's not terribly significant.
>  
> If DC Cox (or anyone else) has spent lots of time and money functionally evaluating multiple kinds of other secondary insulating materials before finally selecting Dolph's AC-43 as the very best, then I invite him to share this interesting and useful data with the TCML. 
>  
> Personally, I suspect that DC read some Dolph's advertising claiming that it was "good for high voltage applications", found that he could make a profit by re-packaging and re-selling this product at a markup, and thereby claims that it is "the best material" for insulating  TC secondary coils. 
>  
> If there really IS comparative test data, developed under controlled conditions, that shows the clear superiority of Dolph's for this specific application, then let's see it.
>  



Products like AC-43 were developed for things like motor windings, for 
which some amount of HV insulation, as well as tolerance to some amount 
of mechanical abuse is desirable. I doubt that it is an order of 
magnitude better than anything else, but it does have useful properties: 
it's flexible when cured, it's a convenient viscosity for application, 
it's formulated to not corrode commonly used metals in electronic equipment.

Something like polyurethane shares many of the properties, although one 
might want to check about the corrosion issue. (e.g. there are lots of 
kinds of silicone sealer, some good for electroncs, others not). 
Polyurethane may not handle high temperatures (100C) that occur in 
motors, but I doubt that's a problem for tesla coils.

Hey, you could grind up beetles and dissolve it in something distilled 
from fermented apple juice, and it would work too, and I doubt it would 
make a HUGE difference in performance.

It's the mechanical properties of the material (both during application 
and after drying/curing) that are probably of the most concern. 
Toxicity, viscosity, and flexibility after cure, would be my list.




> I happen to prefer two-part epoxy for coating secondary coils, and I have a number of very specific reasons for this preference (that I have shared with the TCML over the years through several detailed posts). However, I do not have side-by-side test data comparing epoxy with other materials, so I cannot prove it's superiority on all possible evaluation criteria. And no, I don't sell any epoxy coatings of any kind.
>  
> Regards,
> Herr Zapp
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