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Polypropylene Protective Cover Caps




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From:  Thomas McGahee [SMTP:tom_mcgahee-at-sigmais-dot-com]
Sent:  Saturday, March 21, 1998 9:23 AM
To:  Tesla List
Cc:  bshaw-at-monmouth-dot-com
Subject:  Polypropylene Protective Cover Caps


> From:  Brandon Shaw [SMTP:bshaw-at-monmouth-dot-com]
> Sent:  Friday, March 20, 1998 12:01 PM
> To:  tesla-at-pupman-dot-com
> Subject:  Polypropylene
> 
> Would it be feasible to make a stacked plate cap out of polypropylene?
> I was thinking about making a plate cap out of those protective covers
> for paper. It would be a lot cheaper and easier to use than those 60
> mil sheets of polyethylene.
> 
> bshaw-at-monmouth-dot-com
> 

Brandon,
Polypropylene is great for making capacitors. I have several plate
polypropylene caps that I made when I couldn't get polyethelene,
and they are excellent performers.

What you want to do is figure out how much it would cost to build a
..01 mfd cap using single sheets of poly-p as the dielectric. Allow
a minimum of 1" margin around all sides. I usually allow 1" along
the top and bottom sides and 2" along the left and right sides
because I always make my caps by having the aluminum foil hang out
the left and right sides. That maximizes current carrying capacity,
but at the expense of available dielectric area.

In any case, determine the USEABLE dielectric area per sheet. Then
apply the formula

C=[.224*2.0*Area/Distance]*Nd

where
C is capacitance in ***picofarads***
..224 is the conversion factor for using inch based measurements
2.0 is the dielectric constant of polypropylene (adjusted for reality)
Area is in sq. in. and refers to the USEABLE area, not the full area
Distance is measured in inches, where 1 mil=.001 inch
and
Nd refers to the number of sandwiched Dielectric sections used.

(Call me weird and strange, oddball, unconventional and peculiar, but
I find it less confusing to talk about sandwiched dielectric sections
than aluminum plates, because some plates use one surface, others use
both surfaces, and it gets even messier when you start thinking about
number of plates and rolled caps. But for ALL types of caps it is true
that the number of sandwiched dielectric sections directly determines
the number of useful capacitance sections involved.)

Once you have determined the cost per .01 mfd, determine the breakdown
voltage for a cap using single dielectric sheets. 

With this information you can then extrapolate and find the poly-p cost
for a capacitor of any given size and voltage rating. Note that in addition
to the poly-p you also need a container, oil, and aluminum foil... 

Let us know what you come up with.

Hope this helps,
Fr. Tom McGahee