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Cap Experiments and now less ??



 Original Poster: "Barton B. Anderson" <mopar-at-uswest-dot-net> 
 
 Hello Bart, Mark, All,

Well I didnīt give up on the cap problem. I hate trying something and getting
more questions than answers. I have good news and a little bad news.

Bart:
If you want to try the experiment on your cap I would be interested in the
results. If you have some thinner PE sheets, try it, too. Until then have a
look at my results.

Okay here goes:

First of all I redid my last cap experiment with the thin poly. I used a new
materials and remeasured the PE thickness (still 1.75mils). Both flat and
rolled cap had approx. the same values I posted earlier. I made double, triple
and quadruple sure of having a minimum amount air, dirt, etc trapped inside.
At least as good as you can without vacuum, etc. I even tried using different
materials for connections (AL foil, copper wire, copper pipe, etc) It made NO
difference. The results were NOT satisfactory, to say the least.

So.....

I built another cap with the following materials:

2 sheets of thick PE (6.8 mils) I measured it at several different places. No
difference
2 pieces of AL foil 6.888" wide x 11.690" long. Same type of kitchen foil.
Just new.

I assembled the cap like the previous one making absolutely sure that the two
AL foils where exactly on top of each other. I put the cap on a hard surface
and put two heavy books on top. I also sat on these books. My cap meter read
3.82 nF (inductance was approx. the same as with the thin PE material).
Further pressure on this flat (very flat!!) cap did not change the cap more
than 0.01nF. This is the max accuracy of the DMM, anyway. I also tried
different material for the electrical connections. This didnīt change the
value (as I was expecting)

Now I rolled the sucker up, really tight. The suspense rose. I connected the
DMM and first measured the inductance. It had gone up (as I expected). Now I
switched to the cap measurements and.... It measured 9.87nF. Squeezing or
putting pressure on the cap did not change the value much. Around 0.08-0.10nF.
Not too bad. The factor (rolled to flat) was 2.589:1. This seems to be
resonable. 

So I ran to my computer and fired up my HB TC prog. Plugged in the
values......... and......YAHOOOOOO!!!! Gimmme a beer, please. My compi told me
it should have 10.582nF. This difference is okay I think.

Last experiment I did was to take a piece of the thin and of the thick PE. The
thin stuff is really clear, where as the thick stuff is slightly milky. I
ignited both pieces and observed. Both smell almost the same while burning.
Also the way they burn (yellow, slightly sooty flame) is the same. The thick
stuff feels "waxy". The clear stuff feels a little different. It doesnīt quite
have the same "waxy" feel. It feels more like Saran wrap, but it doesnīt stick
to itself like Saran wrap does.

Now I know:
a.) my new experiment did give me the correct values.
b.) my HB TC programing was okay, too.
c.) I CAN build my caps with the thick PE sheets.
d.) I wonīt use the thin stuff for my 8" coil cap.

I donīt know:

a.) is the thin stuff really PE?
b.) if so why do the results still differ ?
c.) why is the "effective" dielectric K of the thin stuff less than 1 (K of
air) not really careing what the stuff is made of.

There is something else I noticed earlier. All of my test caps are built and
run dry. None of the caps exhibit any signs of corona, when run in a darkend
room on a tc (main gap was shielded in a box) There is (always) one difference
between the thick and thin caps:

All caps built with the thin PE get really, really hot. In fact I have had two
(one was even tested under oil !!) that have gotten so hot, that they have
melted!! They got so hot that I burned my hand (after 5 min of cooling time)
while unwrapping them, to see why they had died. I was using a 10kv-at-20mA OBIT
as a xformer. Run times around 45 sec to 1 minute.

On the other hand all caps built with the thick poly run and stay almost cold.
Even on longer runs. MOT input power (full open at 9.6kV and 700mA) didnīt let
them get too warm either. My power cord runing the 4 MOTs got warmer than the
caps. Run times were clocked at about 5 minutes.

My conclusions are you canīt use the pretty thin poly (if that is what my
stuff is) in a homemade cap. I have unrolled a couple of commericial 10kV DC
filter caps. (Plastic Cap Chicago) that had died in tc usage. They seem to use
a sort of paper (which has about the same total thickness as my thin PE stuff)
These caps got so hot in TC use (like my thin PE caps.....?!) that the glass
cracked.


Bart: 
" I just couldn't stop myself (I tried, but no success). "

RWB: No problem. I like getting comments.

"see how that works out. If it was me, I might go as far as getting a couple
 alluminum sheets of some given size and try the test (who knows, maybe it's
 the foil? everything should be considered). As a matter of fact, I have an
old
 flat plate cap which was built out of alluminum sheets about 8" x 12" now
 disassembled. I might try this test myself tomorrow and see what happens with
a couple of the alluminum sheets."

RWB: Okay I tried to rule out as many factors as I could. Iīll be waiting for
your results.

"More plates (capacitance in parallel) = C1 + C2 + C3 etc.. = higher
capacitance.
 Means capacitance goes up, just like Cself and Ctop would equal Ctotal on a
 resonator. Same math, same paralleling of capacitance."

RWB: Yep, I goofed on this. I now accept the fact that my rolled caps go up in
capacitance. As a matter of fact I am glad that it does. It lets me get my
needed cap sooner (less stuff to roll)
 

Coiler greets from germany,
(were it gets bitter cold in the winter with lots of yucky snow. I miss
California....)

Reinhard