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Re: BIG resonance! - thoughts...
<big snip>
> Unfortunately, determining the "best" capacitance value is not
simple
>and requires a bit of fancy equipment, but there is perhaps an "easy way".
If
>you can find some ordinary high voltage caps (~1000 volts) in the say 5nF
range
>(they do NOT have to be poly or anything fancy), you can simply try
different
>values of capacitance on the output of the transformer with your variac (or
a
>small low voltage transformer) and meter (a cheap one) until you find a
>combination that gives you the proper input to output voltage ratio (make a
>graph). Once you know the "good" capacitor value, you can deal with
>constructing yet another cap or two. If you ran this cap dry, then perhaps
the
>value will change to a better one once oil is added...
>
Alex, Terry, and All,
Terry, your above suggestions to Alex also helped me figure out my
"problem". Let me explain.
Just after I had installed a brand new 6" secondary I got a couple of
strikes to the primary while fooling with a small toroid and "poof", there
went my 15kV 60mA NST. I tried to rebuilt the 15/60 but ruined one of the
secondaries trying to remove it from the core. I couldn't find any more
15/60's so I unpotted a couple of 15/30's, removed enough shunts to make
them 15/48's and threw them in a oil bath. With the 15/48's in parallel,
(96mA), I thought that my present .011 tank cap was too small and better
performance could be had by having the "proper" sized cap. (for 15kV -at- 96mA
that calculated out to .017uF).
I spent quite a bit of time and effort constructing 4 extended foil rolled
caps that had a total series value of .006uF which when added to my
original .011uF cap gave me .017uF. Imagine my suprise when my coil's output
dropped with the .017uF combination! Couldn't understand why! Then I read
your answer to Alex' questions and the light came on!
I already had several .008uF-at-5kV caps lying around so I wired them in
various series / parallel combinations so that I could test the resonant
rise characteristics at various capacitances. It turns out that my 15/96
transformer(s) still work best with a .011 tank cap.
I fed my transformers 2Vac, got out 250Vac with no capacitor attached. I
tested various capacitances and recorded the voltages, (made a nice graph in
Excel).( I can send the graph if you want). All very, very interesting!
I then hooked my Tesla tank caps to the transformers and got the following:
C Eout
.011uF 1650Vac, (original cap)
.017uF 606Vac, (original cap + new .006uF cap)
No wonder my spark length dropped! 1650Vac is 6.6 times the 250Vac output
that I got with no cap! I hope that the 6.6 times doesn't scale up linearly.
If it does I think that my coil would have self destructed long ago.
Now on to that larger toroid and a way to increase the coupling in that
never-ending quest to achieve the longest possible spark!
Regards,
Jon