On Wed, 12 Oct 2005 22:57:47 +0100, "N Cook" put
finger to keyboard and composed:
"Franc Zabkar" wrote in message
.. .
On Wed, 12 Oct 2005 11:36:54 +0100, "N Cook" put
finger to keyboard and composed:
Estimating/determining a likely value of a small low voltage polystyrene
cap
that has no value printed on it or as in this case has melted due to
being
near a hot R and foils shorted.
Ground down the ends of a few such caps , stripped the covering off and
unwrapping the foils.
2 foils per strip length L, width W and separating film thickness d.
120pF, 22x4.5 x .04mm
2220pF, 60x4 x .05
680pF, 28x4.5 x .06
1800pF, 200x5.5 x 0.03 mm
Averaging out gives an approximate fomula of
Capacity in pF approximately = (L x W )/ (420 d^2)
Anyone aware of a more general formula,refinement or other method?
The units are wrong. I would expect something like ...
C = k * (L * W / d)
But then that doesn't fit nicely with your empirical data. shrug
The formula for a parallel plate capacitor is ...
C = k eo A / d
k = dielectric constant
eo = permittivity of free space = 8.85 x 10 -12 F/m
A = plate area
d = distance between plates
Plugging your data into the above formula gives:
120pF k=5.5
680pF k=37
1800pF k=5.5
2220pF k=52
See this URL:
"On Capacitor Dielectric Materials - A Chemist's View":
http://www.audience-av.com/on_capaci...c_material.htm
According to the author, polystyrene has a k of ~2.55 which is in
disagreement with your results, although not by much. However,
something looks very wrong with the 680pF and 2220pF data. I like the
way you've approached this problem, though.
-- Franc Zabkar
Please remove one 'i' from my address when replying by email.
Going by memory I thought the relation was inverse square law with "d".
The other factor is they are spiral wrapped with the same dielectric
material on the
other 2 surfaces, not nessarily same thickness and when wrapped these become
double
thickness - 3 dielectrics with 2 "plates" all spirally wrapped -
Ive no idea what the design formula for such construction is
I would think the capacitance would roughly double.
If the diagram on the left represents a cross section of the spiral,
where T & B are the top & bottom plates, and D is the dielectric
material, then I would think that the diagram on the right would be
its electrical equivalent.
T B T B T B B T T B
T D B D T D B T D B B D T T D B
T D B D T D B T D B---B D T---T D B
T D B D T D B T D B B D T T D B
T B T B T B B T T B
V+ V- V+ V- V+ V- V- V+ V+ V-
Let's say C is the capacitance of a single wrap of the spiral. It
follows then that the capacitance of 2 adjacent wraps is 3C. Therefore
the capacitance of n wraps would be C x (2n-1), ie roughly double,
given a sufficiently large number of wraps.
So the figure of k in two of the calculations above now approaches 2.8
which is in the ball park for polystyrene.
BTW, the value of 2220pF is strange. Shouldn't that be 220pF? If so
then k = 5.2 /2 = 2.6 which is once again looking good.
That only leaves 680pF. Was the cap marked 680? If so, how can you be
certain that this means 680 rather than 68 x 10^0? If the latter, then
k=1.9 which is again pretty close.
-- Franc Zabkar
Please remove one 'i' from my address when replying by email.