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Re: Coilforms, sealants/glyptal (fwd)
---------- Forwarded message ----------
Date: Sat, 11 Jul 1998 08:04:06 -0700
From: Jim Lux <jimlux-at-earthlink-dot-net>
To: Tesla List <tesla-at-pupman-dot-com>
Subject: Re: Coilforms, sealants/glyptal (fwd)
> Richard, and other glyptal advocates:
>
> I'm a little cunfused about glyptal - only heard of it a couple of posts
> ago. What is the nature and design purpose of the stuff??
Glyptal is normally used as a generic name (it is actually a brand name for
a varnish popular a few decades back) for electrical insulating varnish or
paint. It has good electrical properties (i.e. it doesn't absorb moisture)
and is slightly flexible, so that thermal expansion and contraction and
vibration don't cause it to crack. It is popular for sealing vacuum system
leaks as well.
>
> Do you contend that all coilforms should be sealed inside and out with
> it, or just cardboard and other forms with 'poorer' electrical qualities?
Depends on the moisture absorbtion characteristics. A moist coilform will
breakdown under the HV stress, so you'd want to seal a porous coilform.
There has been much discussion about the loss characteristics of various
materials, but there are enough other losses in a typical TC (like the
series resistance of the secondary, unless you've wound it with silver litz
wire) that the dielectric loss from the form is going to be negligible.
>
> I ask this partially because I'm having trouble finding a coilform as
> large as I want, and I've been considering just doing a 12 inch one out
of
> one of those concrete post pouring forms made of spiral-wound cardboard.
I
> had been planning, if I used that, to seal the form inside and out with
Bondo
> fiberglass resin (a two-component product with the 'fiberglass' (?!)
resin and
> a hardener which is methyl ethyl ketone peroxide.) I believe this is
widely
> used for auto body repair.
Bondo in thick layers isn't particularly flexible. It will crack if the
form gets bumped. It is normally used to make a very smooth finished
surface for painting (not for filling in huge dents, as it is sometimes
used for). It is also quite porous (so the primer or paint will adhere).
The cardboard concrete forms (e.g. Sonotube) are usually quite rigid (the
walls are 1/2 to an inch thick) and covered with a sealant (after all, wet
concrete is, as you might imagine, wet, and you don't want the form to fall
apart). However, the sealant isn't very durable, so you would want to
finish the cardboard with something, after drying it well.
If you want to make it stronger and more rigid, I would suggest a wrap of
fiberglass cloth and resin. Talk to your fiberglass material supplier about
a suitable resin.
>
> I don't know anything about the electrical qualities of the material.
> I chose it because it will make the cardboard tube rigid enough. Any
experience
> with this stuff? Would you go ahead and use 'glyptal' after sealing the
tube as
> I described??
The other reason for using glyptal is to hold the secondary winding in
place and to insulate it after you have wound it, a purpose for which it is
well suited.