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Homework Help: Help with thevenin - dependent sources

  1. May 10, 2010 #1
    1. The problem statement, all variables and given/known data

    It is not so important for my question

    2. Relevant equations

    So first of all, sorry to just barge in and post this question, but I am not sure how to solve it. I have attached images of the circuits. The task is to find the equivalent thevenin or norton generator. My problem is that I do not know how to do this when there are dependent voltage sources in the circuit. On the other hand I do know how to solve with dependent current sources.

    What to do with them, do I calculate its voltage and ad it to the resistors voltage or it determines the total voltage between its two nodes?

    http://img69.imageshack.us/gal.php?g=img4308r.jpg

    Thank You in advance for helping me with this


    3. The attempt at a solution
     
    Last edited: May 10, 2010
  2. jcsd
  3. May 10, 2010 #2
    First consider the closed loop with nothing between A and B. Find the currents using Kirchhoffs law. To find the thevenin voltage, do as you normally would, taking the dependent source into consideration, ie count the voltage gain/loss across it.

    To find the thevenin resistance, consider the current that would flow between A and B if a wire were placed there (again, kirchoff). The thevenin resistance is then simply V/I, where V is the calculated thevenin voltage.
     
    Last edited: May 10, 2010
  4. May 10, 2010 #3
    So basically it is the same when writing KVL equations?

    ie first picture KVL

    When going from the upper node of resistor R1 towards the upper node of resistor R3 it is
    -Rmi1+(-)ixR2?

    Only this part I do not understand. with the rest I'm ok.
     
  5. May 10, 2010 #4
    Yes correct.
     
    Last edited: May 10, 2010
  6. May 10, 2010 #5
    Thank you very much. Then how does the controlled voltage source affect the current in that branch?
     
  7. May 10, 2010 #6
    It affects the current like any normal voltage battery would by supplying a constant voltage. Just include it in your KVL when you solve for the currents. Did I answer your question?
     
  8. May 10, 2010 #7
    Yes, once again thank You.
     
  9. May 10, 2010 #8
    One more question regarding this. When calculating thevenin resistance I switch off the independent sources. In this case short circuit, the voltage source. That would mean i1=0. It makes the dependent source an open loop then because Rm times 0 is 0. Correct?
     
  10. May 10, 2010 #9
    Don't switch off any sources just connect A and B with a wire and look for the current flow. No current flows through R3 because it is in parellel with a wire. So you have two loops to solve.
     
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