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Tank Capacitance Riddle
Subject: Tank Capacitance Riddle
Date: Tue, 27 May 1997 23:46:32 -0400 (EDT)
From: Noetic1-at-aol-dot-com
To: tesla-at-pupman-dot-com
After much head scratching and correspondence with Ed
Sonderman, I've got to put this to the list:
When is more capacitance less?
For the last few weeks I've been noodling with a new
vertically stacked plate capacitor, to replace our lossy
(lousy) barium titanate hockey pucks. The old caps
were 4000 pf, two by three (series and in parallel) giving
us somewhere in the vicinity of 4500 to 7000 pf by my
estimation.
Our discharges have been averaging about 24", and not
wanting to lay out $250 for a nice .010 Plastic Cap, I built
a plexi box about 14" wide by 12" tall by 4" deep, and
stacked about 20 plates total (10 on each side) with .090
Poly dielectric. Aluminum flashing plates were approx 9 by
12", with tabs at the sides for connection to 2" brass bolts and
wired to outside lugs with 10 ga. insulated wire. Refrigerator
compressor oil to 1\4" over the top. Should work great, right?
Hooked it up, and using Ed's Excel spreadsheet, calculated it
to tune at 10 turns.
It tuned at 10. But with a lousy 8" discharge, and that was the
maximum. Wasn't sure if that was way out (since the old
caps tuned at about 17 turns) so checked both way inside
and out, only to get nothing different. Disappointing. 2 weeks
of driving, cutting and messing up my wifes kitchen table with
oil. I had it measured on a meter and it registered .009 (less
than the .010 design) so I took it apart TWICE and rebuilt
it with aluminum foil, to make sure the clearance between
the dielectric was right. (I double checked for pin holes or
flaws in the Poly.) Same results before tearing the foil trying
to hook it up for the second and third time.
Bottom line, our old SMALLER capacitance was better. Ed
gave us some suggestions about our tank circuit, which
improved it somewhat, but overall a big disappointment. I
thought this was by the book...
We're driving a 15/60 transformer with a 6" secondary and
big primary winding.
.010 should work great...
???????
any help appreciated
Chris Dunagan