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Re: [TCML] Primary selfC



Hi Chris,

Granted for a rolled cap situation, N = 3. But in a primary, use 2. This is what I was trying to state in my correction to the list. A rolled cap has 2 sheets rolled up. But a primary is a single sheet. So, say 5nF which is somewhat close to my calcs. But I agree with Phil, it is extremely high for what anyone would expect. However, let us say we had a 2 plate capacitor of 600 square inches separated by 1mm (or .03937 inches), then what? Would we assume then the basic plate cap formula is wrong? It's worked just fine for other plate caps, why would it be wrong here?

I'm not saying it's correct, I'm asking the question to anyone who might have insight. BTW, you mentioned a typo. Can you specify?

Take care,
Bart

Chris Swinson wrote:
Phil,


C = 0.2248*k*A*(N-1)/(d*x)  (in pF)
where
k = Dielectric constant (1.0006 for  air)
A = Effective plate area in square inches (length  x width)
N = Number of conductive plates (3 for a  rolled cap)
d = individual dielectric film thickness  (.03937" in this case)
x = number of stacked sheets of  dielectric between plates (1 in
this case)


So if the effective capacitance is as 1/4 of that  of a single
turn-to-single-turn coil, using the above I calculate around 200pF. Still I bet in real
life it's much less than that!

I think Chris is looking at about 6.4nF for the ribbon primary.



If we could get that much capacitance out of reasonable ribbon primaries, we
wouldn't bother with primary tank caps!



I think Bart had a typo in the last line, it should be 1mm not 1inch for the dielectric... if it was 1" apart then it probably would be in the order of 200pF.... ( I got 268pF ) Though my primary is 1mm spaced (only running low voltage as its a solid state design) and there is 25foot of primary length 2" wide.. equates to 600 square inches in total, but slightly less will be needed to obtain 4 turns at about 22" dia.. pretty much I get tot he value of 10nF for 4 turns.



Chris




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