Simple Thevenin Question

Discussion in 'Homework Help' started by jegues, Mar 2, 2011.

  1. jegues

    Thread Starter Well-Known Member

    Sep 13, 2010
    See figure attached for problem statement as well as my attempt.

    We are supposed to find Req for the conditions given in the circuit.

    I calculated an answer of Req of 45kohm but my answers disagrees with the answet listed. Does anyone see where I went wrong in my solution?
  2. The Electrician

    AAC Fanatic!

    Oct 9, 2007
    In your image, it appears that Req is the resistance seen at the Vx node, but you are calculating some other resistance. You need to calculate the current into the Vx node,

    You have calculated the current through R5, and that is not the current into the node Vx.
  3. mjhilger


    Feb 28, 2011
    You need to set two loop currents i1 & i2. i1 is the current for the loop of r1, r3 & r5; i2 is for the lower loop r2,r5,r4. You know the sum of the voltage drops across the resistors in the loop equations is 0. Try that approach.

    And by inspection, since all the resistors are near 1k and because of the setup you should expect the answer close to 1k; so if you are far from there, you know something is wrong.
    Last edited: Mar 4, 2011
  4. Vahe


    Mar 3, 2011
    If you take a look at your first circuit at the top of the page where Req is defined as the resistance looking into the port, you have to realize that Req = vx / (current injected into that terminal). You are trying to to that in the rest of the problem by calculating a current ix; however, the ix you are calculating is not the current injected into the terminal at the top that you need. Since you have simplified the circuit, you have eliminated that terminal. Best way to do this problem is to put a test source (for example of value vx) at the top; write nodal analysis equations and determine the current leaving the + terminal of vx. The ratio of vx/ix is then Req.