Resistance And Frequency

Discussion in 'General Electronics Chat' started by Teslafan, Nov 17, 2014.

  1. Teslafan

    Thread Starter New Member

    Mar 8, 2011
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    Hi everyone,
    I am trying to simulate a circuit containing resistors, inductors and capacitors. When i calculate the Impedance of the circuit i find it to be for example 150+100j for a frequency of 5Hz. When i run the same simulation for a frequency of 15 Hz i find it to be 162+134j. I understand that inductive impedance increases with frequency but what about resistance? It cannot be independent since it changes too! Is there any possible relationship formula connecting these two parameters?
    Thank you
     
  2. alfacliff

    Well-Known Member

    Dec 13, 2013
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    the resistance does not change with frequency. the inductive impedance is a combination of inductance and resistance at a frequency. are you including the capacitive reactance in your calculations?
     
  3. MaxHeadRoom

    Expert

    Jul 18, 2013
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    The resistance of the inductor is considered in series with the Inductive reactance and does not change with frequency.
    The resistance influences the total Q of the coil.
    Max.
     
  4. Teslafan

    Thread Starter New Member

    Mar 8, 2011
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    Thank you for the response. My question is how can i calculate the inductor series resistance so i can be able to calculate the total resistance of the circuit (ohmic resistance and inductive resistance)
     
  5. Teslafan

    Thread Starter New Member

    Mar 8, 2011
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    Alfacliff thanks for the response. Lets say i have an RL circuit containing only a resistance and an inductor. When i simulate that with LTSpice the total impedance is changing with frequency. My question is if i can have the impedance data for 10 Hz using field measurement, could i possibly calculate the impedance at 50 Hz using some kind of relationship formula between frequency and impedance or resistance?
     
  6. alfacliff

    Well-Known Member

    Dec 13, 2013
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    use the formula for inductive impedance..
     
  7. Teslafan

    Thread Starter New Member

    Mar 8, 2011
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    the formula only gives me the imaginary part and not the real one.
     
  8. Jony130

    AAC Fanatic!

    Feb 17, 2009
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    Here you have

    Z = sqrt{R^2 + XL^2}

    XL = 2 * pi * F * L
     
  9. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    When you enter an Inductor into LTSpice, you can decorate the model with all sorts of parasitic series and shunt resistance, interwinding capacitance, etc. Read the help file... it goes on for several pages...

    ind.gif
     
  10. Teslafan

    Thread Starter New Member

    Mar 8, 2011
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    Let Z=R+jX. That formula is for converting in polar coordinates and give the magnitude of impedance.
     
  11. Teslafan

    Thread Starter New Member

    Mar 8, 2011
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    Yes indeed MikeML i tried to eliminate all parasitic resistances in order to see if the results are still the same. And the total resistance of the impedance kept changing with frequency..
     
  12. MikeML

    AAC Fanatic!

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    Post your .asc file. I cant tell from your description what is not meeting your expectations.
     
  13. #12

    Expert

    Nov 30, 2010
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    Z^2 = R^2 + L^2
    Z^2 - L^2 = R^2
    Insert the value of L and the value of Z
    Run the numbers through your calculator to get to R^2
    Square root the R^2 result and you will find the full amount of resistance, which includes the internal resistance of the inductor. Subtract the resistor you know about and the results will be the resistance that is in the windings of the inductor.

    If you are already familiar with algebra, please forgive my intrusion.
     
  14. Teslafan

    Thread Starter New Member

    Mar 8, 2011
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    That is my file.
     
  15. Teslafan

    Thread Starter New Member

    Mar 8, 2011
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    Ok the resistance that is in the coil is constant or frequency related?
     
  16. #12

    Expert

    Nov 30, 2010
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    Constant. Consider it to be an imaginary resistor in series with the inductor. It's really a distributed resistance, but it works like a single resistor in series.
     
  17. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Constant, the resistance of the coil depends on material wound with, the diameter and number of turns, i.e. length of the conductor.
    Or measured in a traditional way with DC resistance meter.
    Max.
     
  18. MikeML

    AAC Fanatic!

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    Here is an LTSpice sim of a simple RLC network. I show that the Real part of its input impedance is constant with frequency, while its Imaginary part (and sign) varies with frequency.. Note that inductive reactance cancels capacitive reactance at only one frequency (resonance), leaving only the resistance...

    Note how I found the impedance and that it would normally be a vector (magnitude & angle). Note what I did to plot the Re and Im parts of the impedance separately.

    Ain't LTSpice great? Hey Cruts... Lets see you do this with Multisim

    226.gif
     
    Last edited: Nov 17, 2014
  19. Teslafan

    Thread Starter New Member

    Mar 8, 2011
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    MikeML thank you for the sim info it was very helpful. If you try and do that for the asc i attached you will see that the magnitude is not the same and varies with frequency. That is my original question
     
  20. MikeML

    AAC Fanatic!

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    That is a consequence of the parallel branches in the network.

    Even this trivial network shows a variation in both the Magnitude of Zin and the Real term of Zin...

    226a.gif
     
    Last edited: Nov 18, 2014
    Teslafan likes this.
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