Re: rolled cap formulae (fwd)
---------- Forwarded message ----------
Date: Tue, 21 Apr 1998 12:40:07 -0400
From: Thomas McGahee <tom_mcgahee-at-sigmais-dot-com>
To: Tesla List <tesla-at-pupman-dot-com>
Subject: Re: rolled cap formulae
> From: D.C. Cox [SMTP:DR.RESONANCE-at-next-wave-dot-net]
> Sent: Monday, April 20, 1998 9:04 AM
> To: Tesla List
> Subject: Re: rolled caps
> to: Kevin
> The two equations are equivalent -- the units conversion factor of 10^7
> just makes them look different.
> Regarding your rolled up caps -- when you roll the caps very tightly you
> compress the dielectric thus reducing it to 1/2 its normal thickness. A
> 1/2 appears in the demoninator which goes "upstairs" to appear as a 2 x
> the final value. If you use 1/2 the unstressed thickness as the "d" in
> your equation you will get the right values on the first calculation
> without multiplication being necessary.
You must have really humongous muscles to be able to compress
polyethelene dielectric to half its normal thickness....
remind me never to arm wrestle with you! :)
Seriously, though, what happens when you roll the cap up really
tight is that you reduce the thickness of the air/oil segment,
and in this sense you are correct in stating that you have reduced
the thickness of the total dielectric, as in homemade caps
this air/oil interface can be substantial. The dielectric
constant of the air is 1.0, whereas the dielectric constant
of the oil is closer to about 2.0 - 2.2, and it closely
matches the dielectric constant of the poly.
However, I wanted to make sure that anyone reading your post
doesn't get the idea that by tightening up their rolled cap they
are actually compressing the POLY to 1/2 its thickness.
Different coilers will get slightly different values for the
"same" capacitor because some will be more aggressive with
bubble removal and compression. Also note that as time goes
on the value of a homemade cap may change due to bubble
removal and expansion of the poly as it absorbs oil.
As regards the last part of the cap formula for homemade
caps, some coilers experience some difficulty with understanding
why rolled caps are then multiplied by two. It has to do with
the fact that both sides of each plate are used. With a
flat plate design the final *outer* plates only use one of the
surfaces from each plate. All *other* plates use BOTH surfaces.
Thus the "N-1" in the formula. Personally I change
the formula slightly so that N instead of N-1 is used, and
the N now refers to the number of sandwiched poly sheets.
Note that the formula then applies equally to flat plate
and rolled type designs. I prefer this formula because the
charge is, after all, stored in the intervening dielectric,
not on the plates. A capacitor section is always two opposing
plate surfaces and the intervening dielectric.
Hope this helps.
Fr. Tom McGahee