Poly Cap Tolerances (fwd)

---------- Forwarded message ----------
Date: Fri, 22 May 1998 23:10:42 -0700
From: djQuecke <djQuecke-at-worldnet.att-dot-net>
To: Tesla List <tesla-at-pupman-dot-com>
Subject: Poly Cap Tolerances

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

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.