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Homework Help: Electrical Engineering (Nodal analysis problem)

  1. Aug 4, 2011 #1
    1. The problem statement, all variables and given/known data

    Using Nodal analysis, find Vo in the circuit of Fig. 3.56

    http://img233.imageshack.us/img233/7080/74830694.png [Broken]

    2. Relevant equations
    1. KCL and KVL
    2. Supernode???

    3. The attempt at a solution

    http://img560.imageshack.us/img560/8250/16133010.png [Broken]

    Hi, I'm stuck with this question, I'm not sure to use supernode to solve it or not. Pleaae help me. Thanks in advance. :)
    Last edited by a moderator: May 5, 2017
  2. jcsd
  3. Aug 4, 2011 #2


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    Staff: Mentor

    Hi sykoh2. Welcome to Physics Forums.

    You should be able to complete the problem with the nodes you've chosen, provided that you make sure to include all the branches! (In the "@Node 1" equation you started with, you failed to include in the sum the current through the branch with B1 and R4).

    You might also note that V2, the voltage at node 2, is identical to Vo which you're trying to find.
  4. Aug 4, 2011 #3
    Hi gneill, just to confirm with you what is the current through the voltage of the 3V source and the 1 ohm resistor, is it (3V - 0V) / 1 ohm?
  5. Aug 4, 2011 #4


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    Staff: Mentor

    Nope. The voltage at the top of R4 will be V1 - B1. Think of a branch as a stack of component from the ground (reference) node up to the node that you're writing the node equation for. Essentially you're doing a KVL pass through those components with an assumed current value and direction for the branch. So for this branch you have:

    V1 - B1 - I*R4 = 0

    which gives I = (V1 - B1)/R4 for the branch current.
  6. Aug 4, 2011 #5


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    Science Advisor

    Hi sykoh2. It usually makes nodal analysis much easier if you replace the voltage sources with their Nortons equivalents. Doing this makes your problem quite a lot easier.
  7. Aug 4, 2011 #6
    Last edited by a moderator: May 5, 2017
  8. Aug 4, 2011 #7


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    Staff: Mentor

    In your Node 1 equation you're not being consistent in your current directions.

    When you write (V2 - V1)/3 that's the current flowing from node 2 INTO node 1. When you write (4V2 - V1)/5 that's the current flowing from the controlled source INTO node 1. When you write (V1 - 3)/1, that's the current flowing OUT of node 1 to the ground node. Surely this latter current must EQUAL the sum of the other two.

    If you want to make the equation consistent and sum to zero, change the sign of the the last term.
  9. Aug 14, 2011 #8
    okay, Thanks for the guidance, I get a clearer pictures of how to solve it. Thanks. :)
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