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Poly Cap Tolerances (fwd)




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From:  Esondrmn [SMTP:Esondrmn-at-aol-dot-com]
Sent:  Tuesday, May 26, 1998 12:26 PM
To:  tesla-at-pupman-dot-com
Subject:  Re: Poly Cap Tolerances (fwd)

In a message dated 98-05-23 11:23:00 EDT, you write:

<< 
 I'm curious about several issues regarding capacitor tolerance.
 
 I just completed a set of poly/oil caps for my first coil.  (I'm close to
 the test run point!)
 
 I built four caps and targeted the value at .0053uf and 7500wac.  I used 8"
 wide AL flashing and .0625 poly sheeting in the caps.  Unfortunately, I
 didn't keep accurate records of the following but I ended up with an actual
 value of .005180 with the four caps connected (2 pair in series, then
 paralleled).  This is within approx. 2.26% of the targeted value and at this
 point I figure thats pretty close.
 
 I haven't seen any info on "pulse" capacitor tolerances.  I believe that
 most "standard" commercial caps have a 20% tolerance.
 
 1.  Once you have computed the cap value for your setup, what amount of
 deviation from this value can be considered acceptable?
 
 2.  What kind of differences in computed and actual value have others
 experienced when building homemade caps? (Especially poly/oil types).
 
 3.  Having built this first set, it appears to me that in general you can
 expect any difference in the actual/computed cap values to be a lower actual
 value?  (It would seem less likely that construction techniques would "add"
 to the computed value.  It seems obvious to me that construction techniques
 could obviously cause lower values through misalignment, tightness of the
 roll, etc.)
 
 4.  If #3 is true, and one can expect say a 2-5% lower value, should we
 target a value 5% greater than the calculated need?  (Actually, I'm
 surprised my construction techniques worked as well as they did.  With some
 practice, I expect I'll get closer to my target.)
 
 Of course the whole point may be kind of mute since so much is dependant on
 the values used in the calculation.  i.e.; I used 2 for the dielectric
 constant, while most charts seem to list 2.2 for the poly and 2.3 for the
 oil.
 
 I also don't know what the tolerance on the poly thickness is but mine
 should be close to advertised since I used a single .0625 sheet in each
 layer.  Using multiple thinner sheets introduces more room for calculation
 errors I would think.
 
 BTW: I rushed my first cap and must build another.  While it is obvious to
 me now, I didn't ensure that the cap could hold a vacuum prior to filling it
 and it would seem impossible to stop the leak now.
 
 Thanks,
 
 dj
  >>

dj,

I have found the actual value of my home made rolled poly caps to be less than
the calculated value when using 2.3 for K.  If I use 2.0, the calculated and
actual values are very close.  I think what this is doing is compensating for
the fact that we never actually realize the thickness of the dielectric.  If
we use one layer of .062 poly, the actual thickness between plates is probably
a few thousandths of an inch more than that because they aren't rolled
absolutely tight, the dielectric isn't perfectly flat, etc.

Ed Sonderman