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Re: Dialectric Constant
8/7/96
Snip
>> I cannot find the recent posting of modern dielectric constants and
>> their strengths. Would someone please repost it publicly or
>> privately?
>>
>> Thanks,
>>
>> RWW
>
>Richard,
>
>You will not find a suitable dielectric to do what you want. Only
>barium or strontium titanate offer a possibility and you can't obtain
>these in sheet form. There are just no real dielectric in the K=1000
>range other than these special ceramics!!! Plus, the voltage standoff
>of these are terrible!!! About 50v/mil!!
>
>Richard Hull, TCBOR
>
Modification of this variable air condenser for 20 - 40 nF is a goal.
I tested the cap with plate gaps at 1/2" in series with a 12/60 neon
and a static gap at 1/4". There was no cap arcing at 12 kV, but when
driven at 14 kV there was an occasional arc, like a Jacob's ladder
rising between the plates.
I have already increased the capacitance from 0.040 to 0.152 nF by
changing the connections of the cap. I have not physically altered the
cap, but effectively the plate gaps have been narrowed from 1/2" to
1/4". This is almost a four fold increase in capacitance. Now I need
a dielectric with a constant of 125 to 250 with stand off voltage at
250 mils.
Adding three new plates (there is room) should increase the capacitance
to approximately 0.200 nF. This would further reduce the needed
dielectric constant to 100 - 200.
Doubling the number of plates reduces the plate gaps to 1/8" (125 mils)
and the dielectric constant to 50 - 100 for a 20 - 40 nF cap. A 5 nF
cap value would require a dielectric constant of 10. How about 120
mils of polyethylene in oil? Still easy to do with the current cap
architecture.
The current radius of the plates is 3 1/2". The area is 10.25 on each
plate. If the radius is upped to 5" the plate area is 25 or about 2 1/2
times the current area. So there is another approximate dielectric
reduction by 2 1/2 times.
I hope by now the plan is clear. I know there are other factors such
as heat (loss) dissipation and all the other parameters already noted.
These estimations are only rough and I invite all criticisms and
suggestions. As this project progresses, I will keep the list posted.
I envision a safe, robust, easy to construct and cost effective
variable (tunable) Tesla Coil capacitor.
RWW