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Re: Rolled Caps

In a message dated 97-02-05 03:18:36 EST, you write:

<< Hi All,
 
 Um, I think R.Quick posted a message a long time ago about 
 calculating/designing rolled capacitors.
 
 I was just curious to know if anyone has got the formula for calculating 
 the capacitance for a rolled cap?
 
 I'm looking at building a couple, but need to design for around 0.09uF, 
 so far the doc's I've read from the funet.fi site are designs for smaller 
 valued types (around .02uF), so I thought if there's a formula out there, it

 would be a fantastic guide to design and construction.
 
 I'm also needing the voltage rating to be around 40-50KVDC as the 
 pig-pole I'm using is 12700V -at- 0.787A (10KVA), and the calculated 
 capacitance required for my coil is 0.187uF. Which makes this transformer 
 fairly ideal...
 
 Thanks Guys!
 
 Catchya's Later!
 
 Rod 'Down Under
  >>
Rod,

There is a file at the ftp site that describes how to make these.  The
forumla for a capacitor is .224KA/d  where K = dielectric constant, A = area
of one plate in square inches, d = dielectric constant.  For a rolled cap,
multiply this by 2.  I just built a new one.  I used two layers of .062 poly
for the dielectric (4 total) and 14" x 51" plates of rolled aluminum roof
flashing.  Since the plates lack about 4" of overlapping on the ends (see
directions), the actual area of the plates is 14" x 47" (658 sq inches).  The
capacitor measures .00475 mfd.  The published dielectric constant for
polyethelene is 2.3.  If you solve the equation backwards for the actual K,
using  .00475 mfd, 658 sq inches for A and .124 for d, you get a K of 2.  I
would use this in your formula for K for poly as it helps to compensate for
the fact that in the actual rolled capacitor, the spacing (d) is not really
as tight as the thickness of the dielectric.  It will always be somewhat
greater due to all the materials not being perfectly flat, not rolled
absolutely tight, etc.

Using .062 thick poly is only good for about 9 kv max in a Tesla coil tank
circuit.  I would use at least 3 layers of .030 poly for your design.

Ed Sonderman