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RE: CAP DESIGN

 * Original msg to: Kukkonen-at-snakemail.hut.fi


 KU> On Fri, 30 Dec 1994, Richard Quick wrote:
 
 > for the initial power up (a common practice). The coil would over
 > couple rather badly with a small discharger, but this can be
 > easily resolved by rasing the secondary coil up in relation to
 > the primary coil (for example placing some plastic blocks under
 > the secondary to lift it up, thereby loosening the coupling).

 KU> Yes - and with the large toroid the first turns of both coils
 KU> are at about the same level in your tuned coil-system?

That is generally the idea, though it may not, and does not, have to 
end up that way. I try to design a primary for a secondary where the 
first turns on both coils are at the same level. But frequently during
the course of experimenting, the coils may not even be close to the
visual "ideal" yet they system can be performimg well. 
 
Talking about bypassing capacitance and RF choking.

 KU> which does equal 700pF..

 KU> Apparently the capasitance can be somewhat lower but should not
 KU> be larger?

That is correct. I have performed several experiments in this area
to obtain my data. The larger the capcitance used, the more RF is
bypassed. But too much capacitance and parasitic oscillation will
set up in the step-up winding of HV xfmr - capacitor - ground.
 
 > then you would divide the capacitance into two parts (use less
 > that 50% of the total) and place an equal capacitance between
 > each HV line and the system (RF) ground. I have cautioned this

 KU> You are talking about the bypass-caps, right? The main capasitor
 KU> is placed parallel to the x-former - or should it be balansed as
 KU> well (equidrive circuit?) ?

Yes I am talking about bypassing capacitors used on circuits where the
HV step up transformer has a grounded center tap on the high-voltage
windings.

In the Tesla Tank circuit, which is the L - C oscillator excited by
spark discharge, the capacitor may pretty much be placed where ever
(and however) you choose. In the COLORADO SPRINGS NOTES Tesla wired
every possible combination and compared the performace. He liked the
"equidrive circuit" because it was high Q, allowed for greater break
rates, and allowed him to place his marginal capacitance in series for
greater protection during higher voltage operation. These are all
valid reasons to look at this circuit. Remember to discharge the
capacitors, they do remain live in the equidrive circuit after shut
down.
 
 KU> About the ferrites : if memory serves, one should indeed use
 KU> ferrites - not iron cores - to get rid of rfi, as iron-core chokes
 KU> pass the rf through them like the ferrites pass 50Hz through..

Yes ferrites are definately the way to go.

 KU> About the caps:
 KU> I really can't find 60mil (or thicker) ld-PE here in Finland 
 KU> - it is definitely not manufactured in Finland and not used 
 KU> for anything :( - The companies I contacted even stated that 
 KU> it is impossible to make ld-pe  foil that thick.. Well - I do 
 KU> could get .2 mm thick foil and use 8 layers of it to get 1.6 
 KU> mm which equals about 60mil=1.52mm - would this do?

Yes this will work. But you are better off building a flat plate 
capacitor construction with this thinner material. Use maybe two
thicknesses of plastic between sheets of AL foil. Build up stacks
of very large capacitance (like .15 uF) and wire a bunch of the 
stacks in series to get around .015 at 15kVAC pulse.

 KU> I could get some 2 mm thick hdpe but it would not be 
 KU> transparent and thus have "adulterants".. Do you think
 KU> this would do for a stacked cap? 

Nope. I would not use anything but virgin poly, no additives.

 KU> btw: I just got a nice idea for the secondary - how about 
 KU> making the tube by laminating polyester-resin on some mould 
 KU> and then winding the coil on the polyester-tube and finally 
 KU> casting a few layers of polyester-resin on the coil? This way 
 KU> one could make the walls as thick (thin) as one would like to..

It would work, but would the results be worth the work (and the
mess?) Probably not. You would be better off perhaps buying an 
acrylic or HDPE tube and machining the plastic down to make a thinner 
wall tube.

 KU> ps. I put your file about the experiments with electrical
 KU> counterspoises etc. to the ftp site last night.

Well at least I know it got out and that I did not waste my time
writting it. Thanks!

Richard Quick

p.s. I will try the thing with the mail site this weekend!
... If all else fails... Throw another megavolt across it!
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