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
 Patrick Gustafson, Capacitance to Spare in Eau Claire.


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