RLC in parallel, resonance and multisim

Discussion in 'Homework Help' started by ninjaman, Dec 16, 2014.

  1. ninjaman

    Thread Starter Member

    May 18, 2013
    306
    1
    hello

    I have this
    upload_2014-12-16_10-13-6.png
    the values are
    1uF
    151mH
    4700 ohm

    I put this together on multisim to find resonance using the ac analysis option but all that happened was a high pass filter. does the capacitor short circuit as the frequency increases. I have to calculate the resonant frequency and im not sure how to do that.
     
  2. JoeJester

    AAC Fanatic!

    Apr 26, 2005
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    Why do you need to put it togerher on multisim to find the resonant frequency? If associative learning kicked in, you already know the resonant frequency.

    Start your bode plot at 1 hz or 0.1 hz

    If your starting it higher it will look like low pass filter.

    You have another fine example of low Q circuit.
     
    Last edited: Dec 16, 2014
  3. ninjaman

    Thread Starter Member

    May 18, 2013
    306
    1

    thanks Joe!
    what I meant to say was, I have a nice picture from the AC analysis function on multisim that shows bandwidth and resonant frequency. I cant get the bandwidth or resonant frequency. I know that at resonant the impedance will be maximum so it should look the opposite of what I have for the series circuit. I think I know how to calculate the resonant frequency. I already know the values, I think the formula is different than series but there is a resistor in there. I am thinking, work out the RL circuit first, then that in parallel with the capacitor?

    cheers!!!

    simon
     
  4. JoeJester

    AAC Fanatic!

    Apr 26, 2005
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    The current will be minimum at resonace in this circuit. In my youth, this was an antiresonant circuit while the series RLC was called resonant .... With maximum current flow.
     
  5. ninjaman

    Thread Starter Member

    May 18, 2013
    306
    1

    hello Joe

    thanks for your help!

    I have put this in multisim and tried to get an AC analysis, this is what I get. so this is a low pass filter?
    upload_2014-12-16_16-52-28.png
     
  6. Jony130

    AAC Fanatic!

    Feb 17, 2009
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    Please start your AC analysis from 0.1Hz
     
  7. crutschow

    Expert

    Mar 14, 2008
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    That circuit has a damping factor much greater than 1 so will show no resonant peak. R must be less than 777Ω to reduce the damping below 1, to where you can see a peak (reduction) in the generator current at resonance.
     
  8. ninjaman

    Thread Starter Member

    May 18, 2013
    306
    1
    hello

    tried 0.1Hz and 0.001Hz, the same result
    change resistance to less than 777 ohms and same problem

    shouldn't it look like a "U" shape when I have magnitude and frequency?
     
  9. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    Here is the "resonance" of your network as a function of the values of R1 = 10, 100, 1000, and 4700Ω. What happens to the Q between 100Ω and 1000Ω?
     
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    Last edited: Dec 16, 2014
  10. crutschow

    Expert

    Mar 14, 2008
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    Are you looking at the source current?
     
  11. WBahn

    Moderator

    Mar 31, 2012
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    You haven't given any indication of what you are using as the parameter that you are trying to find the resonance for? Is it a voltage? If so, what voltage? Is it a current? If so, what current?
     
  12. JoeJester

    AAC Fanatic!

    Apr 26, 2005
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    One thing, by experiment, your oscillator or signal generator will have an impedance ... probably 50 ohms.

    Attached is the sweep using a signal generator with an impedance of 50 ohms and the inductor has zero resistance.
     
  13. ninjaman

    Thread Starter Member

    May 18, 2013
    306
    1

    hello joe

    would I have to include the 50 ohms in the calculations then. I seem to get voltage readings that are a little off. I put this down to poor test equipment.
    is the 50 ohms in series with the circuit?

    thanks

    simon
     
  14. JoeJester

    AAC Fanatic!

    Apr 26, 2005
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    You need to publish in the forum your recorded readings. It would be best if you annotated the schematic and indicate your reference point (i.e. ground or across the component.)

    Help the members help you.
     
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