Re: Please give specs 300W 54" TC

To: tesla-at-pupman-dot-com
Subject: Re: Please give specs 300W 54" TC
From: "Malcolm Watts" <MALCOLM-at-directorate.wnp.ac.nz> (by way of Terry Fritz <terryf-at-verinet-dot-com>)
Date: Tue, 03 Nov 1998 18:56:52 -0700
Approved: terryf-at-verinet-dot-com
Hi Sulaiman,

> Original Poster: Sulaiman Abdullah <sulaiman-at-lityan-dot-com.my> 
> 
> Malcolm, could you please publish the specs for your coil achieving 4
> 1/2 ft
> with 3J / 300W? Sounds like just what I want.
> Also please impart any key features etc. that helps to achieve this
> result.

That figure is point to point measured peak performance which it hit 
several times on a day with dry atmosphere, new cap etc. More 
usually it attaches to about 3-1/2 feet because of the proximity of 
a lathe, miscellaneous tubing on the opposite wall etc.  Sshot 
attached length is about 11" at that primary energy. 

Secondary: thickwall PVC drainpipe 9.9" diameter
           980 turns 0.56mm copper, spacewound 1:1
           Ls about 45mH
           Cs about 19pF
           9" sphere mounted several inches above windings
           Ctot =  26.3pF
           Fr = 146.5kHz
           Q unloaded about 300

Primary: 3 turns 3/8" Cu tubing helical
         diameter = 26"
         height = 3"
         Lp about 11 - 12uH

Cap: dry fired 0.1uF poly.  I get about 1/2 hour of run time out of 
these if they are used in several bursts of 30 seconds or so per run 
with several hours in between to allow the poly and transformer to 
cool completely.

Vgap set to 7 - 8kV

Gap: single static, 1/2" rounded tungsten carbide tipped brass.

Transformer: modified lab demo transformer, primary 450t, sec 10,000t
effectively a c-core with windings on opposite legs. Has a lot of 
leakage inductance a bit like a neon. Vmains about 240V.
The primary brings the core close to saturation and draws 6A or so of 
magnetizing current. It was worth doing as it vastly improved output 
over the standard 500 turn coil which came with the transformer set.

I have a 1mH ferrite rod cored choke in each transformer lead. How 
much good they are doing I don't know. The transformer is right up at 
the gap. I did take the transformer home once to fire up the 17" coil 
and made the mistake of running it about 10 feet away from the gap. 
This resulted in a winding flashover and me having to rewind about 7 
layers of the secondary that had burnt. It has never given a hint of 
trouble with this coil.

This coil was cobbled together out of bits and pieces lying around 
together with the drainpipe offcut from a hardware merchant's yard.
Designing a suitable cap was absolutely critical to performance. It 
is extended foil.

     I did place a toroidal top measuring around 26" x 10" once and 
retuned by adding one more turn to the primary. This resulted in far 
hotter discharges with more stretch. It also made the gap run very 
erratically for reasons I still haven't sussed and in the interests 
of reliable running for demos I took it off again. Gap setting is 
critical for optimum performance. 
    I built this one August break in 1993 when there was little to do 
at work and since the boss insisted on paying for the parts (we used 
to use it on open days etc) I've not had any incentive to mess around 
with it further. This was the first time I really put Medhurst to the 
test. The coil was designed from the ground up to run at a particular 
frequency.

Cheers,
Malcolm