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Re. HDPE found: check my capacitor, please

Comments interspersed...

>Original Poster: Marco Denicolai <marco-at-vistacom.fi> 
>
>After calling nearly 30 companies here in Finland, I eventually found and
>bought 2 sheets of black HPDE, 0.8 mm thick, 1 x 2 meters. The company
>ensured me that black color is just pigment, not coal or other conductive
>stuff.

Lampblack carbon IS a pigment commonly used in paints and it is quite
conductive, although the dielectric strength you listed below appears
quite good.

>Declared properties are:
>Dielectric constant:                    2.4
>Dissipation factor (-at- ? Hz):            0.0005
>Dielectric strength (? mm thickness):   80 kV/mm  (i.e. >2036 kV/mil)
>Water absorption:                       <0.01%
>
>
>The material is stiff but I have already tried and I can bend it. I have
>decided to go with it and to build two capacitors as follows:
>
>- rolled caps, sheets 23 cm wide, 2 m long
>- extended foil design with alumininum food packing foil
>- I roll the foils around a 25 mm PP pipe were I have cut a long hole to
>  insert the foil's edge and keep it there. Same idea that cash machines
>  use to roll their printed paper bills
>- Aluminium extended edges are bended about the PP pipe and fixed with
>  two hose clamps (Gary Lau design). Before to do this I cut away some
>  parts of the aluminium to allow for oil to reach the capacitor inside

I recommend cutting the foil AFTER tightening the hose clamps.  The foil
is fragile and it's best to clamp it intact.

>- One of the clamps is taken through a copper strip to the upper
>  side of the capacitor, so I have both capacitor terminales on the same
>  side of the capacitor
>- Then two bolt terminals and two small valves.
>
>Foils are arranged as:
>
>- paper    INNER LAYER
>- Al
>- paper
>- PE
>- paper
>- PE
>- paper
>- Al
>- paper
>- PE
>- paper
>- PE      OUTER LAYER
>
>Paper is 0.05 mm (40g/m2) kraft paper vaxed only on one side.

"Vaxed" = Waxed?  The paper should be unwaxed, to be as absorbant as
possible.

I don't know if you've thought this far ahead, but I found that when
using extended foil construction, if there's more that one layer of poly
and/or paper between plates, they're going to shift on the short axis as
you roll, and since the foil extends on both sides of the poly, you can't
see it, get at it, or do anything about it.  I've found no way to assemble
this with more than one layer, and even then, I had to tape the foil to
the plates to keep it aligned.  Do let me know if you have a way, as I'm
struggling with this now.

If your "sheets 23 cm wide" refers to the foil plate dimention that
overlaps with the opposite plate, I calculate the capacitance per roll
to be .012uF.  I assume you have about 5cm poly margin on each side of
the foil beyond the 23cm width.

C(pf)= .224(k)(2)A/T, k=dielectric constant, A=overlapping plate area
(sq-in), T=dielectric thickness (in).
k=2.4, A=9.05"x78.75"=712sq-in, T= 2*.0315"poly + 3*.002"paper = .063"
C = .224 * 2.4 * 2 * 712 / .063 = 12,151 pF = .012uF

>Questions:
>
>- is the sheet design OK?

Assuming you were targetting .012 uF, yes.

>- is the overall design OK?
>- were do you get those small (2-3 mm) valves?

I looked in the phone book under hydraulic and neumatic supplies.  I
think what you want coming out of the end caps is threaded-to-barbed hose
fittings, with the valves in-line in the tubing.  You could probably just
clamp down on the tubing with locking vice grip pliers instead of valves.

>- how much voltage do you believe my capacitor will stand?

You're in uncharted waters using black poly.  The real answer of course
is that no one has ever created a definative formula relating dielectric
material, thickness, number of series sections, vacuum(y/n), edge margin,
dielectric perfectness, etc.  Sounds like we need a database containing
detailed descriptions and applications of home made caps that worked and
of those that failed, and how.  Any takers?

>- after vacuum and oil fill, should I:
>
>  A - fill the whole cap with oil
>  B - leave some vacuum empty space
>  C - let some air get in on the top of the oil (then I wouldn't need valves,
>      just hose fits)?

A).  Having learned my lesson about sealing the caps, if I were to repeat
the design, I would have a tube containing oil exit the cap enclosures
into a small bottle.  If the temperature or pressure of the cap rises,
the oil will expand like fluid in a thermometer and visibly vent slowly
into the overflow bottle as an early warning sign.  Unless you can
guarantee that the position of the cap will never change, I'd not leave
any air in the enclosure.

Good luck,
Gary Lau
Waltham, MA USA