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Thevenin Equivalent

  1. Oct 20, 2015 #1
    1. The problem statement, all variables and given/known data
    Find the open circuit voltage V0, short circuit current ISC, and RTH,

    2. Relevant equations


    3. The attempt at a solution
    I found the equivalent resistance to be 20 ohms(just by combining resistors in parallel and series) and V0 to be 30V, which I think might be wrong.

    I did a nodal analysis at VA :
    (VA-30)/10 + (VA)/15 +(VA)/30 =0
    VA=10V
    ISC=(VA)/30 ISC=0.33A

    Then I found RTH =30v/0.33A=90 ohms

    Shouldn't my RTH = Req?
    I'm not really sure what I did wrong.
     

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    Last edited: Oct 20, 2015
  2. jcsd
  3. Oct 21, 2015 #2

    CWatters

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    Inspecting the original circuit shows that the open circuit voltage Vo cannot be 30V due to the potential divider effect of the 10 and 15R.

    When the output is open circuit what's the effect of the 30R?
     
  4. Oct 21, 2015 #3
    Can you neglect the branch with the 30 ohm resistor? Could you then add the 10 and 15 ohm resistors in series?
     
  5. Oct 21, 2015 #4
    Nevermind, adding the resistors in series didn't work I still got V0=30
     
  6. Oct 21, 2015 #5
    Should V0=6.67V?
     
  7. Oct 21, 2015 #6

    gneill

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    jdawg, can you write out the steps of the procedure for finding the Thevenin equivalent of a circuit? It seems that you're trying things at random and hoping to hit upon a correct result, which is never a good approach.
     
  8. Oct 21, 2015 #7
    You caught me! Honestly I don't really know what I'm doing. I've missed the past few lectures and I have to submit this problem in about an hour so I'm desperate to get it finished.
    What I did was take out the independent source and did series and parallel combinations to get RTH=20ohm. I was pretty confident about ISC being correct, so I used this formula to find VOC: RTH=(VOC)/(ISC) and found VOC=6.67
     
  9. Oct 21, 2015 #8

    gneill

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    Staff: Mentor

    Your text should spell out the procedure.
    In this case one method boils down to:

    Thevenin Voltage:
    1. Remove any load so the circuit is open.
    2. Determine the voltage presented at the open terminals.

    Thevenin Resistance:
    1. Remove any load so the circuit is open.
    2. Suppress all sources: Replace voltage sources with shorts, current sources with opens.
    3. Determine the resistance of the circuit between the load connection points (the resistance from the load's point of view).
     
  10. Oct 21, 2015 #9

    CWatters

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    Unfortunately your value for Is and Vo are both incorrect. Follow the procedure gneill posted in #8.

    Regarding the open circuit voltage...

    You can't totally neglect the branch with the 30R but the fact that one end is open circuit does allow you to simplify the circuit. What would you replace the 30R with?

    Then simply adding the 10 and 15R isn't sufficient to work out Vo. Look up potential divider circuit.
     
  11. Oct 21, 2015 #10
    Oops... I completely messed up that assignment. Thanks for y'alls help, I'll have to come back to this problem after I watch the lectures.
     
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