1. We will be in Read Only mode (no new threads, replies, registration) starting at 9:00 EDT for a number of hours as we migrate the forums to upgraded software.

Thevenin's Equivalent Circuit

Discussion in 'Homework Help' started by wheeltiger, Nov 17, 2007.

  1. wheeltiger

    Thread Starter New Member

    Mar 19, 2007
    :confused: How can I find the Thevenin's equivalent of this circuit?

    I have replaced the voltage source with short circuit...
    Z_{TH}=(5\Omega+j\Omega) PARALLEL j3\Omega

    :(but I don't know what I need to do with the 4\Omega

    :( and I don't know how to find V_{TH} too...:confused:

    Please help!
  2. sohcahtoa


    Nov 7, 2006
    I'm not sure what they're getting at with the 4j either. I'd say ignore it.

    I have to run to class, so I'll try to answer as quick as possible.

    I did the following to figure out the Thevenin equivalent:

    1) Remove the load (Zl)

    2) Make the input voltage a short.

    3) Now you are looking for Rth seen from the right side of the circuit. In redrawing the circuit, you can see that 5ohms and j(ohms) are in parallel. So far we have (5 || j). Considering this to be one impedance (say, Zparallel), and redrawing the circuit, you can see that Rth is actually Zparallel in series with the 3(j) inductor.

    Therefore, Rth = 3j + (5 || j)

    4) Now Vth is the voltage seen at the load terminal with the load removed. So we have a voltage divider between the 5ohm resistor and 1(j) inductor, as the voltage seen at the far end of the 3(j) inductor is the same as the node between 5 and (1)j since there's no load Zl.

    The voltage at the voltage divider node is then V(1j/(5+1j))

    So your Thevenin equivalent is V(1j/(5+1j)) in series with 3j + (5 || 1j) in series with the original load impedance, Zl.

    Does that sound right?
  3. abbas_6007

    New Member

    Sep 7, 2009
    I will answer to your problem 2moro. i am a new user of this site.
  4. The Electrician

    AAC Fanatic!

    Oct 9, 2007
    You can't ignore it. The two inductors are coupled and the 4Ω is a measure of the mutual inductance between the two. You must take it into account to get the Thevenin equivalent.
  5. Perfectboy

    New Member

    Sep 9, 2009
    Right! You can't ignore it as said. I have got the right answer.