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Re: Cap Question

In a message dated 10/2/98 8:05:00 PM Mountain Daylight Time, tesla-at-pupman-dot-com
writes:

> 
>  1.) I had calcīd this cap to be aprox 21 nF. It turned out to having only
10.
> 5
>  nF. Thatīs only one half. I scratched my head, unwound the whole thing and
>  tried to rewind it tighter. It didnīt help at all. Why? Is there a 
> possibility
>  that the two AL sheets are acting capacitivly on each other, because I 
> didnīt
>  cover the top AL foil with more PE sheets effectivly reducing the expected 
> 21
>  nF to the measured 10.5nF ?!?. I donīt think so.
>  
Reinhard,  
    The most likely cause of your difference in calculated and measured
capacitance is the distance between the plates.  You assume this thickness is
the sum of the individual sheets of poly, but there is likely something
causing this to be in error.  Maybe the sheets are not perfectly flat against
each other causing an increase in the total thickness.  Another possibility is
that the sheets are slightly thicker than advertised.    These are obviously
just guesses, but seem to be the only thing that I could think of that would
cause the measured capacitance to differ from what you calculated.
   I am working on a plate cap using LDPE sheets that are 1/16 inch thick.  I
assembled it temporarily(still need to clean everything before final assembly)
and the measured capacitance is nearly identical to the calculated
capacitance.  
      Have you checked your meter against some known caps to insure it is
working properly?   There is also the possibility that you are not getting
good contact somehow with the plates.  I brought some commercial caps to a
gathering last weekend.  I had them connected in parallel with some copper bus
bar.  I measured each one separately and the total of their capacitances was
20nF. However,  when Chip put his meter to the assembly, his meter only read
14nF???  When I got home I remeasured them and it read 19.6nF.  I wonder if
some surface oxidation of the copper caused a mismeasurement or maybe a bad
contact with one of the caps that somehow became better from them sitting in
the hot car.  I took the assembly apart and redid the bank using copper sheets
instead of the bus bar, but made sure to clean the copper with a bit of 400
grit sand paper before  assembling them to make sure of good contact.  It
still measures 19.6nF :)
Good luck on figuring this one out.
Mike H.