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Thevenin equivalent

  1. Oct 27, 2011 #1
    For the circuit below, can someone please help me understand why Zeq = 0 (in words...not by solving):
    jt6jw4.png

    I know Vth = 50<0° since this is the voltage drop across the parallel elements. I don't understand why the equivalent impedance would be 0. I know when you shut off a voltage source, elements in parallel with it also go away. does this have anything to do with it?
     
  2. jcsd
  3. Oct 28, 2011 #2

    Zryn

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    Gold Member

    http://en.wikipedia.org/wiki/Th%C3%A9venin%27s_theorem" [Broken]

    So you know the Thevenin Voltage already. In step two you can short all voltage sources, open all current sources (as per step 2a.) and then comment on the resistance of the remaining circuit.
     
    Last edited by a moderator: May 5, 2017
  4. Oct 28, 2011 #3

    NascentOxygen

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    What is the element represented by a circle with an arrow in it?
     
  5. Oct 28, 2011 #4

    NascentOxygen

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    What is the element represented by a circle with an arrow in it?
     
  6. Oct 28, 2011 #5
    it's a current source
     
  7. Oct 28, 2011 #6
    so is it because when you short the voltage source the elements in parallel are shut off, too?
     
    Last edited by a moderator: May 5, 2017
  8. Oct 28, 2011 #7

    Zryn

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    It's only a matter of calculating the parallel resistance as per normal:

    1/R(t) = 1 / R(short) + 1 / R(open) + 1 / R(C) + 1 / R(L) + 1 / R(R)

    R(t) = 1 / [1 / 0 + 1 / inf + 1 / -j10 + 1 / j20 + 1 / 30]

    R(t) = 1 / [(1 + 0 + 0 + 0 + 0)/0] (put all 5 fractions over a common denominator)

    R(t) = 1 / [1 / 0]

    R(t) = 1 * [0 / 1]

    R(t) = 0

    You could have just 'seen' that by knowing that the short circuit from the voltage source will bypass all the other elements in the circuit, since current will follow the path of least resistance and split down parallel paths in inverse proportionality to the magnitude of the resistance of each path. I guess 'shut off' is correct, but you could word it better :smile:.
     
  9. Oct 29, 2011 #8

    NascentOxygen

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    Are we treating this whole 5 element thing as a voltage source? So we seek the equivalent impedance as seen between the top rail and the bottom rail?

    Or are we determining the thevenin impedance of a voltage source comprising an ideal voltage source in parallel with a current source?

    (Please don't say it doesn't matter. I can see that! I'd still like to know the question before I state the answer. :smile: )
     
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