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Homework Help: Thevenin Equivalent

  1. Mar 18, 2009 #1
    1. The problem statement, all variables and given/known data
    Find the Thev Eq circuit for a DC power supply that has a 30-volt terminal voltage when delivering 400mA and a 27-volt terminal voltage when delivering 600mA.

    2. Relevant equations
    KVL: Vs=IsR1+IsR2

    3. The attempt at a solution
    Assuming that each 3volt step is a 200mA change, than Voc would be 36v. I determined the load resistance by checking both current EQs [6vload/.4A] and [9vload/.6A] and determined the solution of both to be 15ohms. I did a KVL utilizing the 30v and 400mA figures and determined that Rth is 75ohms. I then did a KVL with the other figures and received a 30ohm answer. Where have I strayed???
  2. jcsd
  3. Mar 19, 2009 #2


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    Science Advisor

    The Thevenin voltage and Thevenin resistance are fixed for a particular black box and don't change when you hook up different loads to it. The load resistance most certainly changes, but you've basically assumed that they're constant (how else would the first circuit only draw 400 mA while the second draws 600 mA?) while the Thevenin Resistance is changing.
  4. Mar 19, 2009 #3
    As you can see by now MATLABdude, Edward Lawry Norton is my arch nemesis. But I thank you for the help!
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