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Does frequency affect inductor and capacitor value

  1. Mar 27, 2017 #1
    Hi ,

    I was measuring the inductor and capacitor value of a resistor bank with a 100Hz and 1kHz using a LCR meter by fluke. I realize that with different frequency, the L and C values were different. If the measured values were the inductance and capacitance I can understand that it changes with frequency. But the values given by the meter is in Henry (H) and Farad (F) which I do not understand why it changes with frequency.

    Thanks
     
  2. jcsd
  3. Mar 27, 2017 #2
    Do you mean you were measuring the stray inductance and capacitance of a resistor bank? What were the values you were measuring? Were the resistors wirewound?

    I'm not aware of measured variations in inductors and capacitors at those frequencies with an LCR meter unless they are very small. At frequencies of 1 GHz and higher inductors can become capacitive and capacitors inductive.
     
  4. Mar 27, 2017 #3
    yes, i am measuring the stray inductor and capacitor from the resistor bank.

    Using the LCR meter, when I set the
    freq = 1000Hz L =31μH and C = 810μF when R = 3Ω.
    When the R = 300Ω, with freq = 1000Hz L = 9.55mH C = 2.71μF

    When the frquency is at 100Hz, R= 300, L = 10.76m and C = 238μH
     
  5. Mar 27, 2017 #4

    NascentOxygen

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    Hi Sabastian. :welcome:

    If this is a figure for stray capacitance of a portable switched resistance box then it is almost certainly meaningless. I suspect the reason for coming up with an absurd figure will be found to be a limitation in the way C and L are measured by the meter.

    I don't know how it calculates values for L and C, but perhaps you can find out?
     
  6. Mar 27, 2017 #5

    Merlin3189

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    Gold Member

    What puzzles me is, how a machine applying a single frequency can think that a component has both capacitive and inductive reactance at the same time?
    Surely, if you measure a "black box" resistance, all you can know is its impedance, with a resistive and a single reactive component which is either capacitive or inductive.
    I suppose if it did a sweep of frequencies, it could obtain a resonance curve and estimate the separate inductive and capacitive elements from that?
    I think some meters apply a step voltage and measure the rise/fall time of the current to estimate reactance. If so, this is effectively applying wide band of frequencies and measuring the response curve might give enough info to get values for an LCR model?
     
  7. Mar 27, 2017 #6

    Averagesupernova

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    LCR meters are able to measure both equivalent series inductance and equivalent series resistance in capacitors. They may not even apply a frequency at all. If you apply a current source to a capacitor and plot the voltage you can learn plenty.
     
  8. Mar 27, 2017 #7

    Baluncore

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    The bridge is being used in two modes.
    In one mode, RLS, it gives resistance with inductance in series.
    In the other mode, RCP, it gives resistance with capacitance in parallel.

    Only R is a realistic value because L and C have frequency, f, dependent reactances which depending on bridge mode will partially cancel because XL is positive and XC is negative.
    XL = 2πf·L, while XC = –1 / 2πf·C.

    By measuring at different frequencies, you may be able to better estimate the values of LS and CP.
     
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