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Re: Capacitor Calculations

In a message dated 97-01-16 01:13:33 EST, you write:

<< Hi All!
 
 I've amazingly been able to get hold of a 15KVA 22KV Single Wire Earth 
 Return transformer from the local electricity board.
 It was a decommissioned xformer which lived its life on a power pole for 
 several years. The core is still in good condition, but was "too old" for 
 power distribution purposes, so when I spotted it in the scrap yard, I 
 asked and recieved!
 
 Anyway, I'm trying to work out exactly what value caps to use with this 
 xformer.
 
 With Neon's I use the formula -
 	1
 c = ----------
      2*Pi*F*R
 
 Where F = 50Hz (Aussie Freq.)
       R = V/I
       c is in Farads
 
 Since the power output of the xformer is 15KVA and the voltage is 22KV, 
 one calculates an approx. current of 0.6A.
 
 When I plug all of this into the formula, it gives me 0.09uF.
 
 Strange thing is that I've seen other coils that use this sort of power 
 level and transformer, and they use 2 0.5uF caps in series to yeild 0.25uF.
 
 Which ones do I go for, and is there a particular formula I should be 
 using for these xformers other than the above?
 
 The next question is, how does one calculate what value an inductor RF 
 choke should be for this? I don't really feel like coupling 80KHz of RF 
 back into our power grid! :-)
 
 Here's the Tesla coil specs that I'll be driving with this xformer -
 
 Secondary -
 12" diam
 625 turns of 1mm enamel coated wire spaced at 0.25mm apart
 (I know, tat tat, no spacing, but it's an old secondary from way back)
 
 Primary -
 5 turns of 3/8" copper pipe in Pancake Archimedes formation,
 spaces at 3/8" between windings
 ie
      __     __     __
  ---(  )---(  )---(  )--
      ~~     ~~     ~~
        |   |
          ^--------- 3/8"
 
 Spark Gap -
 Rotating 12 point gap.
 1440RPM (asynchronous) and does splutter occassionally.
 Wheel is 10" diam
 
 Not the best designed coil, but should work with 10KW of power, and 
 should hopefully resonate somewhere between 80-100Khz.
 
 Thanks for your help guys!
 
 Happy Sparking!
 
 Rod
 
 Rodney.Davies-at-anu.edu.au
  >>

Rodney,

It seems that the capacitor calculated to match the transformer is academic.
 You have the primary and secondary  -  assume some toroid capacitance (and
this coil will want something big like 40" to 50") and calculate the resonant
frequency of the secondary.  Now using 80 or 90% of the primary inductance
(to allow room for tuning) calculate the capacitor required for the system.

Ed Sonderman