Quick Thevinin Question

Discussion in 'Homework Help' started by Digit0001, May 31, 2011.

  1. Digit0001

    Thread Starter Member

    Mar 28, 2010
    89
    0
    Hi
    Can someone tell me how would i do this question
    DSC00066.JPG
    I have tried to use a nodal analysis but still incorrect answer

    P.S
     
    Last edited: May 31, 2011
  2. Jony130

    AAC Fanatic!

    Feb 17, 2009
    3,957
    1,097
    I don't see any picture ??
     
  3. Jony130

    AAC Fanatic!

    Feb 17, 2009
    3,957
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    First you need to find voltage across RL when the load is disconnect.
    The open circuit voltage Vab = Vth
    The second step is to find the current flowing from a to b when a & b are shorted together.
    And then Rth = Vth/Isc

    So know sow as your solution
     
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  4. Digit0001

    Thread Starter Member

    Mar 28, 2010
    89
    0
    ok for Vth i get

    (9-Va)/2 = 3Vx-Va
    2(2Va) = 9-Va
    4Va = 9-Va
    5Va = 9
    Va = Vth = 9/5

    Then when i find In = (33/6)/4

    so R = Vth/In = (9/5)/(In) = 4.4 ohms

    What is wrong?
     
  5. jegues

    Well-Known Member

    Sep 13, 2010
    735
    43
    You wrote your KCL(or nodal analysis) incorrectly,

    You wrote a "current entering the node = the other current entering the node."

    Just write a simple KVL around the loop and express Vx in terms of the current flowing, solve Vab.
     
  6. Jony130

    AAC Fanatic!

    Feb 17, 2009
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    Simply nodal equation
    (9-Va)/2 + 3Vx-Va = 0

    Or we can write one equation like this

    I = (9V - 3Vx) / ( 2Ω + 1Ω) = ( 9V - 3Vx)/3Ω

    I = (9V - 3(2Ω*I))/3 = (9V - 6I)/3

    I = (3A - 2I)

    3I = 3A

    I = 3A/3 = 1A

    And Vx = 3*(1A*2Ω) = 6V

    Vth = Vx + 1A*1Ω = 7V
     
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