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Re: Build own capacators
Original poster: Terry Fritz <vardin@xxxxxxxxxxxxxxxxxxxxxxx>
Hi Dave,
At 09:10 PM 12/12/2005, you wrote:
Hello,
Newbie here--
I have a source for accurate thickness sheets of mica
up to 12 inches square. Didn't Tesla build his own
capacators? Something to do with current storage and temperature?
Tesla's Colorado springs coil used salt water and large bottles for
the primary cap. They often failed but they were easy to fix. Salt
water caps are still often used because the price is right ;-)
There are mica caps around that have been used for Tesla coils. But
they are expensive and fail hard if the voltage goes too high.
I am not sure what kind of mica sheets you have. If they are
composite, they might be more for heat insulation than electrical
use. Capacitor mica is pretty pure stuff and I would think it would
not be easy or cheap to get.
The capacitance is:
C = E x A / D
Where:
E = 8.854x10e-12 x ~ 6 (mica Permittivity)
A = Surface area in square meters
D = Distance between plates (thickness) in meters
More info here:
http://www.allaboutcircuits.com/vol_1/chpt_13/3.html
Typically the "plates" are vacuum deposited on the mica to get good
airless contact. You need to have the mica thick enough to stand off
say 30000V at say 300kHz. You need to leave enough space at the
edges to prevent to thigh voltage from tracking around and shorting
(like an inch). So you would have to figure out how much you would
need to make say 20nF...
It is a pretty tough thing to build. Unless you want to make it
basically for fun, challenge, or some other special reason, I would
just say spend like $50 on an modern MMC primary cap and be done with it.
How hard are they to build today?
Today, synthetic sapphire is used instead of mica since it's supply
is limitless (but not cheap) and it's properties can be super well
controlled. I suppose it could be done at home but maybe a big
factory could make it cheaper...
Would the mica allow for very large capactors to be built?
You can make it as large as you want until you run out of mica
;-)) You should figure out how much you need and everything first to
see if it would be reasonable though.
The vast majority of people just use MMCs these days....
Cheers,
Terry
Thank you,
Dave Luke