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Homework Help: CIRCUIT ANALYSIS: FInd the Thevenin equivalent of the circuit. 2 res - 1 IVS - 1 VCVS

  1. Jan 29, 2007 #1
    1. The problem statement, all variables and given/known data

    Find the Thevenin equivalent at terminals a-b of the circuit below.

    http://img153.imageshack.us/img153/6116/chapter4problem40ef4.jpg [Broken]

    2. Relevant equations

    v = i R, KCL, KVL, [itex]R_{TH}\,=\,\frac{V_{OC}}{I_{OC}}[/itex].

    3. The attempt at a solution

    To get [itex]R_{TH}[/itex], I removed the independent voltage source and I added a test voltage of 1V, an unknown test current [itex]I_{OC}[/itex] and two KVL loops.

    http://img156.imageshack.us/img156/1597/chapter4problem40part2ne1.jpg [Broken]

    [tex]V_0\,=\,-1\,V,\,\,4\,V_0\,=\,-4\,V[/tex] <----- Right?

    For KVL Loop 1)



    For KVL Loop 2)



    KCL at node between two resistors)



    Now I use the equation mentioned above for [itex]R_{TH}[/itex] to get the Thevenin equivalent resistance)


    Now I need to get the Thevenin equivalent voltage at the terminals a-b. I redrew the circuit to include the 70V independent voltage source and added two node voltages.

    http://img249.imageshack.us/img249/6549/chapter4problem40part3rj2.jpg [Broken]

    [tex]V_0\,=\,70\,-\,V_1,\,\,V_2\,=\,4\,V_0[/tex] <----- Right?


    KVL Loop 1)



    KVL Loop 2)



    Now I have two equations in two variables and I can solve. I get [itex]V_1\,=\,87.5\,V[/itex] and [itex]V_{TH}\,=\,17.5\,V[/itex]. But wouldn't the Thevenin equivalent voltage include the contributions from both sources? (i.e. - [itex]V_{TH}[/itex] is actually [itex]V_1[/itex], [itex]V_{TH}\,=\,87.5\,V[/itex] and not [itex]V_{TH}\,=\,17.5\,V[/itex])
    Last edited by a moderator: May 2, 2017
  2. jcsd
  3. Jan 29, 2007 #2
    Does the [itex]R_{TH}[/itex] look correct? Shouldn't [itex]V_1[/itex] and [itex]V_{TH}[/itex] be equal?
  4. Sep 13, 2008 #3


    User Avatar

    Re: CIRCUIT ANALYSIS: Find the Thevenin equivalent of the circuit. 2 res - 1 IVS - 1

    In the equation for loop 1 you have made a wrong substitution. You should have
    instead of
    Of course this leads to [itex]V_{TH}=V_1[/itex]
    Last edited by a moderator: May 3, 2017
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