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Cap question???




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From:  Thomas McGahee [SMTP:tom_mcgahee-at-sigmais-dot-com]
Sent:  Monday, February 09, 1998 8:11 AM
To:  Tesla List
Cc:  mwise-at-ns.sosis-dot-com
Subject:  Re: Cap question???

 
> From:  Zuma [SMTP:mwise-at-ns.sosis-dot-com]
> Sent:  Saturday, February 07, 1998 2:24 PM
> To:  tesla-at-pupman-dot-com
> Subject:  Cap question???
> 
> I have finally decided to build some poly caps. I have come to the
> conclusion that I am going to construct .005uf caps using 60mil poly.
> They are going to be rolled caps. I have also decided to go with a very
> small size (2"x160" plates, 4"x166" dielectric). I know this may sound
> stupid to make caps this small, but I want to make my caps in intervals
> of .005 and use the smallest amount of space at the same time. I am
> planning on the plates being aluminum tape or aluminum foil which ever
> is easier. If I use the tape my question is should I leave the paper
> backing on the tape or attach the tape directly to the poly? I am hoping
> someone will help with my question and not "dog" the idea.
> 
> Chris
> 
> 

Chris,
Instead of just answering your question, I will instead do what I 
often do with my electronics students and try to get you to the
answer by asking YOU a series of questions instead:

What possible negative effects might result from the adhesive
being included between the aluminum foil, poly, and next set of
foil?  What is the loss factor of the adhesive? Will there be
any air trapped within the adhesive? How will this impact on
corona production? Will there be possible physical degradation
of the adhesive over time. If this is to be immersed in oil,
will the oil eventually dissolve the adhesive? What effect, if any,
might this have on the insulative qualities of the oil? Does the
added thickness of the adhesive have any effect on the capacitance
obtained, seeing as how it will make the plates further apart
than otherwise? Is this an increase or decrease in capacitance?

What will the backing paper do if immersed in oil for an
extended period of time? Does the backing paper also affect
the final capacitance value (as the adhesive did). Is this
effect desireable?

Another set of questions:
In a cap made using 2"x160" plates, 4"x166" dielectric, what
percentage of the poly is being used as active, useful dielectric?
How does this compare with the previous percentage obtained for
narrower sized plates/dielectric? What does this tell you
about how to get the most bang for your buck from the relatively
expensive poly? For the size sheets that you are purchasing, what
would the widths be if you cut the large sheet up into two, three,
four, or five equal width sections? Determine the percent waste
that results with each size, assuming a constant 1" border on
each side, and 3" borders at each end. What is the LARGEST width
size you would feel comfortable with building? Why is it harder
to effect bubble removal with larger caps? What practical steps
can be taken to overcome some of these problems? 

Which is cheaper, aluminum foil or aluminum tape? Does the adhesive
act more like an insulator, or more like a conductor?

As far as using the "smallest amount of space" is concerned, ignoring
for now the thickness of the aluminum foil, what would the volumetric
size of a cap be if the poly sheets were .060 inches each
and each cap used TWO such sheets 4"x166". What would it's 
capacitance be? Express the volumetric efficiency of the cap
as the total capacitance divided by the total volume.

Hint: the total volume is exactly equal to the volume of the
poly, whether the poly is flat, rolled, or melted into a glob.

Now do the same for a cap that uses TWO full-size sheets of poly.
(This is usually 48"x96")
Express the relative merit of the larger cap versus the smaller
cap by dividing the volumetric efficiency of the larger cap by
the volumetric efficiency of the smaller cap. Multiply the result
by 100 to obtain the relative efficiency expressed as a percent.

Ponder the results for a few moments. The numbers are telling you
something. Can you figure out the meaning of all this? Can you
say Bow Wow?

Hope this helps.
Fr. Tom McGahee