Thevenin equivalent voltage

Discussion in 'Homework Help' started by chadchoud, Dec 21, 2008.

  1. chadchoud

    Thread Starter Member

    Dec 18, 2008
    11
    0
    Hello

    In this question, I have the voltage between A and B and I need to find Vin.
    How do I do that?

    I tried setting the ground at B, and I called V1 the node above A and V2 the node below A.

    I did a node voltage:
    V1/5 + (V1-VAB)/4 = 0
    and same for V2

    then I tried Vin = V1 - V2 but I did not reach the desired value.
    Can somebody help me?
     
  2. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    It would be helpful for you to show a bit more detail of your work in arriving at the answer you chose.


    hgmjr
     
  3. chadchoud

    Thread Starter Member

    Dec 18, 2008
    11
    0
    Sure :)

    I attached an image showing where I chose V1 and V2. The ground is on B.

    so:
    V1/5 + (V1-VAB)/4 = 0
    V1(1/5+1/4) = 10.46+6.04j
    so V1 = 23.24+13.12j V

    (V2 - VAB)/(-4j) + V2/(8.66j) = 0
    V2(1/(-4j) + 1/(8.66j)) = -6.04+10.46j
    so V2 = 77.73+44.88j

    Now I want Vin which is equal to V1-V2 (right?) = -54.5-31.68j
    = 63.03 < -149.83

    Which is different from the correct answer.
     
  4. The Electrician

    AAC Fanatic!

    Oct 9, 2007
    2,286
    331
    Your very first equation is wrong. Those two currents don't add up to zero. What about the current in the + line of the Vin source?

    The rest of your calculations suffer from the same problem.

    I would solve the problem by letting the - input be the reference and calculate two voltage dividers to nodes A and B, take the difference, and go from there.
     
  5. chadchoud

    Thread Starter Member

    Dec 18, 2008
    11
    0
    Amazing! After a lot of trials, I reached the correct answer :)

    Vin-VA = Vin*4 / (4-4j) ..... (1)
    Vin-VB = Vin*5 / (5+8.66j) ..... (2)

    Then do (2) - (1)
    Isn't that what you meant?
     
  6. The Electrician

    AAC Fanatic!

    Oct 9, 2007
    2,286
    331
    Then you have to do:

    (48.3<30) divided by ((2) - (1)).
     
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