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Parallel resonance on oscilloscope

Discussion in 'General Electronics Discussion' started by herbkanis, Jun 23, 2013.

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  1. herbkanis


    Jun 23, 2013
    Hi everyone,
    I’m looking for help on how to set up an experiment to view the current flow of parallel resonance with an oscilloscope.

    My resonate setup is the same as the example on (or attached image)
    I’m using a 0-50 mhz sin/tri/sqr wave generator and the same components as in the example. 40.1hz, 1R, 1H, .15u

    What would be the best way to view the current and voltages flowing back and forth between the coil and the capacitor?

    In the example, the resonance seems to be nailed to 40.1 hz and any deviation causes the example to change noticeably. In my experiment, I put a 10k resistor in series with the coil and the capacitor and thought I’d measure the voltage drop across it. However, the scope image doesn’t seem to change much as I alter the frequency unless I go really far off the “resonant” frequency.
    Also I don’t see much change in the signal when I alter the inductance. (I’ve made a variable inductor, .5H to .998H)

    I’m sure I’m just unfamiliar with how to set this all up. Any help would be appreciated.
    Thanks in advance, Herb

    Attached Files:

  2. duke37


    Jan 9, 2011
    There seems to be some errors in you post.

    The generator is likely to be 0 to 50 MHz rather than 0 to 50 mHz.

    The reactance of 1H at 40.1 Hz is about 250 ohms.
    In order to get resonance at 40.1 Hz, the reactance of the capacitor will also be needed to be 252. This means the capacitor will be 15uf, not 0.15 uF as in the text.

    To measure the current flowing between the inductor and capacitor, insert the 1 R resistor between the two and measure the voltage across that. The resistor will change things but not a lot as it is much lower than the reactance.
  3. herbkanis


    Jun 23, 2013
    Hi Duke,
    Yes, i did say .15 incorrectly. However when I setup the test, I was using a 15uf cap.
    While the generator was 0-50mhz, it does a decent job from 0-100hz, so I don't see why that would be a problem.

    I had already tried to measure the drop across a resistor on the coil side, but did not see much change on the scope. I rather thought it would be a clear sine showing a fluctuating voltage. ??
  4. duke37


    Jan 9, 2011
    Please forgive me, I am grumpy today.

    You should post things correctly, you were out by a factor of ten for your capacitor and a factor of one thousand million for your signal generator. There is a lot of difference between 50 mHz and 50 MHz. Note that I have used a capital H for Herr Hertz, I think he would like that.

    A resistor placed in series with either the inductor or the resistor will have a sine wave voltage across it as you expected. Its amplitude will be low. Please post the circuit.
  5. davenn

    davenn Moderator

    Sep 5, 2009

    for the sake of good learning get used to using the correct symbols

    for example ....

    H = Henries
    Hz = Hertz
    m = milli
    M = mega
    uF = microFarad

    many of these definitions are named after guys who discovered various electrical/electronic principals, hence we use capital letters
    H = Mr Henry
    Hz = Mr Hertz
    F = Mr Faraday
    not only is it good to use capitals for people's names it also saves a lot of confusion as in you above posts when you have a m instead of a M :)

    if you are going to play with electronics and physics in general, its just one of the rules of the game :)

    Last edited: Jun 26, 2013


    Sep 23, 2012
    It's Mr.Faraday to you!
  7. davenn

    davenn Moderator

    Sep 5, 2009
    oops true

    edited haha too early in the morning

  8. CluQu


    May 22, 2013
    I'm confused as all hell now. I cannot tell from other posters if you want the reactance, the resonant frequency, or how to put the scope probes on the circuit to see the waveform, or some combination of the three?

    I'm going to assume that since the title is about oscilloscope that you want to know where to connect the probes. Just put it on the 2k resistor, and observe.
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