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Re: Measuring Capacitance (fwd)
---------- Forwarded message ----------
Date: Fri, 24 Apr 1998 14:12:48 EDT
From: Esondrmn <Esondrmn-at-aol-dot-com>
To: tesla-at-pupman-dot-com
Subject: Re: Measuring Capacitance (fwd)
In a message dated 98-04-24 10:45:40 EDT, you write:
<<
Hello All:
What is the preferred method of physically finding the actual
capacitance of a capacitor? Does anybody still use impedance bridges?
I use an impedance bridge that I use in conjunction with a frequency
generator, then I can do both capacitance and inductance measurements
(comes in real handy for finding the inductance of chokes with different
cores). However, here is my situation: I made a couple of rolled
capacitors using LDPE as the dielectric. Using the k-value of 2.2 and
the physical dimensions of the capacitor, I have a calculated
capacitance to be approximately .00644 micro-farads for each.
Now when I use the impedance bridge on the capacitance setting, I
run an audible frequency through the capacitor and search for nodal
points in the amplitude of the sound wave (indicating matching
impedances for the plates), I then get a capacitance value of .021
micro-farads! This was certainly an unexpected surprise. A gain of
almost .015 micro-farads!
I have not directly measured the actual k-value of the LDPE, but I
suspect that it does not deviate far from the 2.2 mark. If in fact the
bridge is working properly, I believe that the real culprit of the added
capacitance is in the oil that I used (Caltron 60/30). I received 10
gal. from the local power company for free. They use the same oil in all
of their high voltage pole transformers. I now of course would like the
determine the value of this oil, but I am hesitant to use the bridge.
So, back to my original question, what does everyone else use to measure
capacitance?
Patrick Gustafson, Capacitance to Spare in Eau Claire.
>>
Patrick,
To calculate the capacitance of a rolled poly cap, use the formula:
C(pf) = .224 K A / d. Area is in square inches, K is the dielectric constant
and d is the dielectric thickness in inches. Then multiply the answer by two.
I have built several rolled poly caps and find this formula agrees with
measured values within a few percent if I use 2.0 for K instead of 2.2.
We have had the oil discussion here a few times recently - using different
oils, when the primary dielectric thickness is accounted for by the LDPE, will
only change the resulting capacitance by a small amount.
Ed Sonderman