Measure capacitance in a coil of wire

Discussion in 'General Electronics Chat' started by theegb, Jan 4, 2008.

  1. theegb

    Thread Starter New Member

    Jan 4, 2008
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    I am trying to measure the capacitance in a coil I just made. I have a middle of the road handheld LCR meter (about $300). When I connect the possitive lead of my LCR to one end of the coil and the other lead to the other end, my meter goes to infinity no matter what capacitance setting I use. It's a small coil so the capacitance should not be anywhere near the mF range of my meter. Can anyone offer information on what I am doing wrong?

    Thank you
     
  2. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    The coil is shorting out the capacitance. A short circuit looks like infinite capacitance to a capacitance meter (yours, anyway). I would first measure the inductance, then try to measure the self-resonant frequency of the coil. With that information, you can calculate the capacitance.
     
  3. theegb

    Thread Starter New Member

    Jan 4, 2008
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    Thank you very much for your reply. I am trying to find the resonance by measuring the capacitance. Is there an easier way to find the resonance?
     
  4. Ron H

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    That depends on the value of your inductor. Have you measured it?
     
  5. theegb

    Thread Starter New Member

    Jan 4, 2008
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    Yes. My inductance is 1.4mH
     
  6. Ron H

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    If you have a VNA (vector network analyzer), it's simple. Otherwise...

    I would just put a resistor (a few kohms) in series, connect to a variable frequency signal source, put a scope probe across the inductor, and adjust the frequency until you reach resonance (Fr), as indicated on an oscilloscope. Resonance is when the amplitude reaches a peak. You can measure the frequency of the signal source with a counter, or estimate it with the oscilloscope.

    Then, C=1(L*(2*pi*Fr)^2). Subtract the capacitance of your scope probe, and the rest is self-capacitance of the inductor.
    *********************************************************
    I had already composed this next part, so I'll post it. It should work with your 1.4mH inductor. You won't need it, because you already have an LC meter. I don't.
    (Note: the following method won't work well for small inductors, like a few microhenries)
    I have measured inductance and capacitance of a coil by putting a known capacitor value C1 in parallel with the inductor, forming a resonant tank. Add a resistor (a few kohms) in series with the resonant tank, connect to a variable frequency signal source, and adjust the frequency until you reach resonance (F1), as indicated by an oscilloscope. You can measure the frequency with a counter, or estimate it with the oscilloscope.
    Now, remove the capacitor and replace it with another value, C2. Find F2 the same way you found F1.

    We know that, for a resonant circuit, L*C=1/(2*pi*F)^2
    Let Cs=parasitic capacitance of coil
    (1) L*(C1+Cs)=1/(2*pi*F1)^2
    (2) L*(C2+Cs)=1/(2*pi*F2)^2

    (L*(C1+Cs))/(L*(C2+Cs))=((2*pi*F2)^2)/(1/(2*pi*F1)^2)

    The L's cancel, and the 2*pi's cancel:

    (C1+Cs)/(C2+Cs)=F2^2/F1^2
    Solve this for Cs.

    Now that you have Cs, you can plug it back into (1) and (2) to solve for L. You will almost certainly get two values for L, due to measurement uncertainties, actual changes in L with frequency, series resistance, and phase of the moon. I would probably take the average of the two, or do several more tests with different cap values, and then average all of the values of L.
    Don't forget to take out the probe capacitance from the value of Cs, as well as any estimated fixture stray capacitance.

    I didn't "invent" this method, but I have no idea where I got it.
     
  7. theegb

    Thread Starter New Member

    Jan 4, 2008
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    Thank you once again for your quick reply. I had tried to find my Fr the way you described a while back and had a little luck, but though it would be easier and more accurate to measure C and L and calculate my Fr. I will put more effort into it with my function generator and ossciliscope this evening.
     
  8. Ron H

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    Apr 14, 2005
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    The only way I know of to measure C of a coil directly is to use a VNA. Some of them have an impedance test jig that allows you you select the topology of the circuit (inductor with parasitic R's and C, quartz or piezo resonator, etc.).
    I haven't used any of the newer LCR meters. Some of them may be able to do it also.
     
  9. alvarocanivell

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    Dec 12, 2008
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    I am currently modelling a UTP cable made of two single wires and have measured the RLC values for this pair of cables.

    For a single wire, capacitance is neglectable compared to inductance or resistance (see http://en.wikipedia.org/wiki/Transmission_line) on the same wire.

    RLC meter would only measure capacitance between wire pairs. For a single wire, it will only tell you about resistance and inductance.

    My problem now is checking my RLC measurements with theory so as to verify them.

    So which formulae would you use for calculating the capacitance between two wires of a UTP (unshielded twisted pair :) ) cable?
     
  10. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
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    This can be a bit tricky. As mentioned above, the wire itself shorts out its own capacitance. If you have a lot of turns of thin wire, you are likely to find SEVERAL resonant frequencies, some parallel, some series.....since the per/length capacitance is distributed with per-length inductance....in other words, you have a big transmission line.

    If you do use the resonance method, be sure you use the LOWEST frequency of resonance you can find.

    Eric
     
  11. PasadenaDave

    New Member

    Dec 12, 2008
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    Can also measure it, approximately, by placing a much larger capacitor, like a 0.1ufd, in series with the coil, and measure the coil and capacitor. The readout value will be a little bit off, but the larger the capacitor, and a 0.1ufd should be fine, the more accurate will be the reading. The series capacitor allows you to read the smaller capacitance of the coil.
     
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