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Re: Measuring Capacitance (fwd)
---------- Forwarded message ----------
Date: Fri, 24 Apr 1998 21:17:09 -0700 (PDT)
From: bkm-at-jps-dot-net
To: Tesla List <tesla-at-pupman-dot-com>
Subject: Re: Measuring Capacitance (fwd)
At home I use a BK Precision #878 LCR Meter. It's a nice instrument but
will set
you back about $275 new. GenRad made(makes?) some nice LCR bridges, but
they are about $5k. Hewlett-Packard (of course...) has the finest one I've
seen, but
(no surprise) it is even more. You can find the Capacitance from the phase
shift in a
RC network with an oscilloscope, but scopes aren't cheap either.
One possible approach would be to measure voltage drop across the cap when it is
in
series with a resistor and the series pair is being driven by an AC source
such as a
12V, 60Hz transformer (must be a low impedance source). Line frequency is very
accurate, so you don't need a frequency meter.
Your 0.0064uF cap would represent a reactance of about 412k ohms at that
frequency, so a 100k resistor would be a good pick, given that most DMMs are
10Mohms (you don't want to introduce errors due to meter resistance).
Measure the voltage across the resistor with a DMM (again, use a DMM that has at
least 10Mohms impedance to minimize loading error). Note here that you
can't get the capacitor voltage by subtracting the resistor voltage from the
source voltage because of the phase shifted current in the cap..
Divide the resistor voltage, VR, by the resistance, R, to get the current in
the circuit
(I = V/R). Now measure the voltage across the cap, and calculate the reactance
from Vcap/I = Xcap.
You can now find the capacitance from C = 1/( 2 * Pi * f * Xcap), where
C = capacitance in farads
Pi = 3.1416
f = the test frequency in hertz
Xcap = the calculated reactance
PE caps don't change value much for the range of frequencies found in
tesla coils, so the results should give a good ball park figure (within 5 to
10%).
This "trick" works for inductors also- just substitute the C equation with
L = 2 * Pi * f * Xl
Note that most DMMs and VOMs are only accurate to around 5kHz, and if the
impedances you are measuring are more than 1/20 of the meter's impedance,
the error introduced by loading will likely exceed 10%.
Hope this helps!
Bruce Martin
bkm-at-jps-dot-net
At 08:36 AM 4/24/1998 -0600, you wrote:
>
>
>---------- Forwarded message ----------
>Date: Fri, 24 Apr 1998 00:23:53 +0000
>From: "Patrick J. Gustafson" <gustafpj-at-uwec.edu>
>To: Tesla List <tesla-at-pupman-dot-com>
>Subject: Measuring Capacitance
>
>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.
>
>
>