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Kirchoffs Laws and Thevinen Equivalent

  1. Apr 22, 2005 #1
    I have a question here which asks me to find the Voltage and Current of RL in two different ways: using Kirchoffs Laws and then Thevinens Theorem.

    I have tried it a couple times and get different answers using each method.

    Perhaps somebody could tell me where I have gone wrong or if somebody that has access to multisim or some other circuit emulator could enter in the circuit and tell me what the answer is that would be a huge help.

    Attached is my workings and a picture of the circuit.
    P.s. sorry for the handwriting (its my quick working writing).

    Many thanks

  2. jcsd
  3. Apr 22, 2005 #2
    Your Vth is correct in the thevenin example, it is indeed 76 Volts. I'm guessing you messed up a sign somewhere in the Kirchoff example--- being the nice person I am I'll try to find it for you :D

    Also I'll check your Thevenin Rth by using a short circuit current method.
  4. Apr 22, 2005 #3
    Whoops nm i didnt know that they gave you the resistance of the load! With that in mind, with your Thevenin voltage you can easily find the current through the load.
  5. Apr 22, 2005 #4
    For your Kirchoff example, I found your mistake--- although your method is on the right track, you would do well to make use of Mesh Currents... that is, instead of labeling i1, i2, i3 etc... you should label each in terms of mesh currents... that is--- i1 would equal the current ia thru first mesh minus ib current through second etc...

    Do that, use matrices to solve your system of equations, and you should get it.

    And might I add, don't use a supermesh in this problem!
  6. Apr 22, 2005 #5
    Thanks heaps.
    I knew one of them would be right, and the thevinen is the simpler one to do, but i couldnt see where I went wrong in the kirchoffs one. I will have another crack. Cheers
  7. Apr 22, 2005 #6
    Oh, did you put the circuit in multisim to make sure it is correct? Thanks
  8. Apr 22, 2005 #7
    I don't have Pspice or any other relevant software installed on my laptop, and I'm not at home unfortunately!

    I don't doubt that it's correct.

    Check your EE dept's IEEE student branch or local EE student organizations to see if they hold Pspice, Hspice, Multisim, Matlab, etc tutorials, you'd definetely benefit from them when you take the harder courses in ckt theory and digital design!
  9. Apr 22, 2005 #8
    I still cant find the error in my kirchoffs one. Could you point me to the right part?
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