RE: Primary/NST case coupling effects

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: Thomas <tom-at-pwrcom-dot-com.au> 

 > Original poster: Esondrmn-at-aol-dot-com
 >

 > >
 > >Thanks Ed.
 > >My NST is less than 6" below the inside of my conical
 > primary, I will move
 > >it and see if it makes an appreciable difference.
 > >This is going to be a pain as the primary winding is part of
 > the lid that
 > >encloses the NST in a wooden box.
 > >See: http://www.users.bigpond-dot-net.au/broken.trout/DSCF0015.jpg
 >
 >
 > Nice looking coil project.  Because you are using a conical
 > primary, I
 > doubt you are loosing much power by having the nst below the
 > primary.  Try
 > taking it out of the box and using a tempory lash up to test
 > it.  Let us
 > know if performance is changed.  Boy, it sure looks like your
 > coil would be
 > overcoupled.  Do you get racing sparks?  How much power are
 > you running and
 > what size cap?
 >
 > Ed Sonderman
 >

Ed,

I will try the temporary hook-up you suggest, probably not for a few weeks
though (flat out with work at the moment) and let the group know the
results.

The coil uses an NST with the following name plate rating: 11kV 80mA (odd
values, perhaps it was an overwound 10kV, or under wound 12kV
transformer??).

I use a nearly resonant 22nF MMC (18x18 of 22nF 1500v DC), it actually
measures 20nF but that's within the caps tolerance range (and it has not
changed after a few hours operation). It's over-engineered slightly, for
reliability and the possibility of using a larger NST if I ever find one at
the right price.

The secondary coil is wound on 6" dia. resin impregnated paper tubing
(professional coil winding stuff), with 3 coats of "Electrolube" MR8008
insulating varnish.

I've had no problems at all with racing sparks. The line of speckles down
the centre of the secondary visible in the photo are reflections of the
camera flash.

However a couple of weeks ago (in very humid hot weather) I noticed a large
amount of corona between the earth tap on the strike rail and the secondary
winding. Removing the pointy tip from the earthed alligator clip solved this
problem.

In fact if you look really closely at the picture in the link above you can
just see the corona (just to the right of the secondary, white alligator
clip poking up on the strike rail). This was really weather dependant.


I think the main reason for lack of racking sparks is the reduced coupling
from using only 6.75 turns on the primary, as the top load is quite small
and there are only ~755 turns of 20AWG on the secondary. The low input power
is probably also a factor.

I have yet to check the quenching capability of my single blower gap. I have
an Ion Physics CT, just perfect for the job, but again very little time.

I plan to add a larger top load at some point and with the extra turns on
the primary required for this, racing sparks could become a problem.

Regards,

Tom L.