Secondary Construction

 * Carbons Sent to: usa-tesla-at-usa-dot-net

Quoting  Steve_Crawshaw_at_erith6-at-smtpgwy.supertension-dot-com: 

> I have asked this before,  but I can't remember if I got an 
> answer. When constructing the secondary coil, I seem to recall
> that RQ recommends sealing end caps on the former. Why is this? 

In a high performance design, properly driven, the secondary coil
is easily capable of producing sparks that greatly exceed the
physical height of the secondary coil. In other words, your coil
may be 20 inches tall producing sparks that are 30 inches long. 

Under these conditions an "open" or uncapped coil will almost
certainly fail. All of the electrical failures that I have seen
occurred inside of the coil form. The failures resulted in
sidewall perforations, arc scoring, and carbonization of the coil
form. I have never been able to successfully repair a coil once
it has failed in this manner.

To prevent electrical breakdowns in the secondary coil the
following points are important:

1) Never drill holes into the sidewall of the coil form.
2) Do not allow wire (or any conductor) inside of the coil form.
3) Both the top & bottom of the coil must be capped air tight.
4) Solvent based adhesives must not be used for capping the coil.

If these simple guidelines are met, the coil will have an
enormous electrical strength and failure is highly unlikely.
Coils wound on all plastic coil forms that are constructed
following the simple guidelines above can produce sparks that
exceed the physical winding height by a factor of 5 to 1 without
failure or breakdown in the construction.

Every set of printed coil plans that I have seen shows two holes
drilled into the coil form sidewall. The wire used to wind the
coil is fed through the hole near the intended base. The wire is
then routed inside of the coil form to a ground terminal. The
coil is wound, and the other end of the wire is fed through the 
hole near the intended top. The wire is again routed inside of
the coil form through an insulator mounted on the top of the
coil. If end caps are even used, wood is frequently recommended. 

While these instructions will work fine on an open wooden or
cardboard coil form that is space wound with d.c.c. (double
cotton covered) wire a-la 50 years ago, if followed today using
close wound magnet wire on a high-Q plastic form it is lethal.

Modern high-Q construction materials demand a change in con-
struction practices and techniques. A high inductance design
(close wound magnet wire on a plastic coil form) will produce a
very potent secondary winding which will promptly smoke when
coupled to an efficient tank circuit unless care is taken to
prevent internal arcing and sidewall failure of the coil form. 
If the wire is never allowed to enter inside of the coil form, 
no holes are drilled into the sidewalls, and both ends of the
coil form are properly capped, there is no way the spark can get
inside of the coil. The result is that the coil will not fail.

BTW, the reason I have recommended that solvent based adhesives
NOT be used when capping the coil form is because the explosive
vapors are trapped inside of the coil. If points #1-3 are
followed then point #4 should not be too critical, but if an
internal failure of the secondary coil does occur, you can count
on any trapped vapors igniting and an explosion will result. This
is not just an idle warning either. I have had it happen and it
was nothing but dumb luck that a serious accident did not result.

Richard Quick

... If all else fails... Throw another megavolt across it!
___ Blue Wave/QWK v2.12