R thevinen help

Discussion in 'Homework Help' started by jaymo3141, Feb 22, 2014.

  1. jaymo3141

    Thread Starter New Member

    Feb 16, 2014
    How in the world does this book get Rth = 1ohm. It is essentially saying the two 2 ohm resistors are in parallel but they aren't. Help is appreciated.
  2. anhnha

    Well-Known Member

    Apr 19, 2012
    How would you calculate Rth?
    To find the Thevenin resistance, set all independent sources sources (Vs1 and Vs2) to zero and find the resistance of the resulting circuit.
  3. mjakov

    New Member

    Feb 13, 2014

    Because it says the circuit is in steady state at t- and since the current on the inductor is continuous we can assume that i_L at t- is the same as i_l at t+. When calculating Rth we calculate the resistance between the terminal points and set sources to 0. With voltage sources this means to replace them with a short circuit. So we end up with the following circuit at t=0+: terminal + ..... 2Ohm par. (2Ohm +.2H) ...... terminal -
    Since the inductor is in the steady state and provides for no voltage drop we can short circuit it and we get:
    terminal + ........ 2 Ohm par 2 Ohm ....... terminal -
    This gives as the Rth of 1 Ohm.

    Best regards!
  4. jaymo3141

    Thread Starter New Member

    Feb 16, 2014
    Thanks for your help guys. I didn't realize that the two 2ohm Resistors are considered in parallel since Voc is between them. I was calculating Rth not using Voc/I but using the look back method. So I turned the sources off which gives you two resistors in a square. They would be in series if not for the Voc terminals.