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Re. HDPE found: check my capacitor, please



>Original Poster: Marco Denicolai <marco-at-vistacom.fi> 
>
>>Lampblack carbon IS a pigment commonly used in paints and it is quite
>>conductive, although the dielectric strength you listed below appears
>>quite good.
>
>I'll try to check, is it conductive or not.

I doubt there's a way to check this, short of actually using it in a Tesla
tank application.  If you have a functional "backup" capacitor that you
can run your coil on, you can do as I plan to do and build a scaled down
cap of small value but identical construction and run it in parallel with
the backup unit, just to see if it holds up.

>>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.

>I'm struggling too... Some ideas I got:
>
>- 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.
>
>This works great (tested yesterday): you don't need to cap the PP pipe (so
>you don't enclose air in it). With the cut it the PP pipe you can really
>roll thight that sheet pack: it won't slip!

I capped my pipe, not for any mechanical reasons, but to not have to fill
that volume with oil.  Given that it did burst all over my garage floor,
I'm glad to have used the minimum possible!

>- Have the rest of your family standing on the sheet pack laying on the floor
>while you roll it. Still "untested".

This won't work.  As you roll, the layers have to be allowed to slide on
the long axis relative to each other, and this is unavoidable.  This is
the reason I advocated cutting the foil plates into short sections - so
they can be taped to the poly to establish a fixed margin, AND so that
they can shift relative to the next foil segment as you roll.  Try it,
you'll see.  Cap rolling would be a breeze if things didn't shift, this
is what makes it so difficult.

>>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.
>
>23 cm is the PE width. Leaving 1.5" per side I could have a usable width
>of maybe 15 cm for the plates.
>
>BTW, cannot I leave just 2 cm (0.79") per side? That would make a 4 cm
>distance for the corona to jump, through oil. How much voltage that
>gap will stand?

With extended foil construction, you only get 1X the margin for arcover
distance, since the adjascent plate is extending over the poly edge.
For a single unit capacitor, even a 1.5" margin is "marginal" (sorry).

If you do have only a 15cm overlap, then I calculate your capacitance
will only be .0072 uF.  Is that what you're shooting for?  This is not an
efficient utilization of your two 1M x 2M sheets, you'll have enough for
two such caps.

>>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.

>But why not to leave vacuum?

No homemade enclosure short of a hermetically sealed glass-to-metal tube
will maintain a vacuum over any significant period of time.  Your vacuum
will slowly leak and you'd have air in your cap.

Gary Lau
Waltham, MA USA