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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