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Many dry caps in series
Dear List:
Several weeks ago I proposed doing away with the oil immersion of poly
caps by constructing it as MANY caps in series. Since no one appeared to
have first-hand experience doing this, I took it upon myself to conduct
an experiment to test this. I built a scaled down version, using two
layers of 4 mil Home Depot Poly between each foil plate-pair, in a flat
plate construction, times 20 sections. The total capacitance measured
only 47pF, but my goal was only to see if any signs of corona were present.
I ran this test cap in parallel with my .01uF cap on my TC for about 2
minutes, intermittently, and looked for signs of corona, but saw none.
The cap was constructed with each plate wrapping around a corrogated
cardboard spacer, so the edge of each foil plate was plainly visable.
I also inspected one of the sections for discoloration of the poly at the
foil edge and saw none.
It's clear that using only a single layer of thin poly between plates is
asking for trouble as surface defects in the poly are very common.
Hopefully the use of two sheets between plates will guarantee a
puncture-proof dielectric.
My original plan was to use 20 extended foil rolled caps in series, but
after attempting rolling this with two thin poly sheets between plates,
gave up in utter frustration. Instead I'll use flat plate construction.
Has anyone had experience trying to spot-weld standard .75 mil kitchen
aluminum foil? Each of my proposed 20 flat plate segments will have 34
sheets of foil, 12" wide, extending from the poly that needs to be
connected to the 34 sheets from the next segment. Spot-welding sounds
neat if it can be done.
Gary Lau
Waltham, MA USA