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Thevenin Equivalent Circuit

  1. Dec 1, 2016 #1
    1. The problem statement, all variables and given/known data
    See attached image
    Screen Shot 2016-12-01 at 22.11.06.png

    2. Relevant equations
    Thevenin Equivalent


    3. The attempt at a solution
    a) Find the total resistance between A and B by short circuiting the voltage sources:

    Rt = 100Ω in parallel with the two 150Ω

    Two 150Ω in series: R = 300Ω
    1/Rt = 1/100 + 1/300

    Rt = 75Ω

    I am having difficulty finding the thevenin equivalent voltage.

    b) Don't entirely understand the question so I am unable to make an attempt

    Any help would be greatly appreciated.
    Thanks
     
  2. jcsd
  3. Dec 1, 2016 #2

    gneill

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    Staff: Mentor

    What have you tried? What's the usual procedure?
    Hint: Maximum power transfer theorem.
     
  4. Dec 1, 2016 #3
    I have no idea so I have not tried anything. Usually for a system with a single voltage source I would try to find the open circuit voltage between A and B, however the two voltage sources are confusing me.

    Oh so would I take the Rload to be 50Ω, and since for maximum power the thevenin resistance = load resistance, change one of the resistors to give me a thevenin resistance of 50Ω?
     
  5. Dec 1, 2016 #4

    gneill

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    Staff: Mentor

    You can always do a "KVL walk" from A to B and sum up the potential changes along the way.
    Yes.
     
  6. Dec 1, 2016 #5
    I don't quite understand what you mean by a "KVL walk", but i tried going round the top of the circuit and noting how the potential would change I also wasn't sure if the current in the entire system could be calculated as I = 12/400 . Doing that would mean I get a voltage of 0V at A and then 0V at B which can't be correct.
     
  7. Dec 1, 2016 #6

    gneill

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    Staff: Mentor

    :smile: You did well, and a "KVL walk" is just what you did.

    0 V is a perfectly valid result.
     
  8. Dec 1, 2016 #7
    Haha thanks so much!
     
  9. Dec 11, 2016 #8
    I took a different approach to this question, calculating Vb from 0 = (Vb-9)/300 + (Vb-(-3))/100, so Vb = 0. Is there a way to get a nodal equation for Va?
     
  10. Dec 11, 2016 #9

    gneill

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    Staff: Mentor

    You've used A as the reference node for your first equation, so it's potential is defined to be zero. That is, Va = 0 is the node equation for A :smile:
     
  11. Dec 11, 2016 #10
    What do you mean by reference node?
     
  12. Dec 11, 2016 #11

    cnh1995

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    Homework Helper

    A node whose potential is taken as 0V for reference. All the voltages in the circuit are measured with respect to the reference node. It is also called as the "circuit ground".
     
  13. Dec 11, 2016 #12

    gneill

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    Staff: Mentor

    When you do nodal analysis you are determining the potentials of various nodes of the circuit with respect to some reference node. The first step is always to choose the node that will be the reference node for all the potentials.

    There's often an obvious or convenient node to choose for this role (what constitutes "obvious" or "convenient" and how to recognize a good choice for the reference node is something that comes with experience). In this case node A is suitable since it not only forms one of the "output" terminals of the Thevenin equivalent, but is located at the junction of the only two voltage sources in the circuit.
     
  14. Dec 11, 2016 #13
    Ok. Cool. Thanks Guys. Any time we did a problem in lectures, the ground was always marked in, so I never realised you could 'pick' a ground.
     
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