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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