Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Finding the thevenin equivalent using Vo and Io

  1. Mar 21, 2012 #1

    I'm trying to find the thevenin equivalent of a particular circuit so that I can calculate the current across an inductor.

    I've attached a pdf with the circuit and the working out I've done so far.

    When I'm talking about i0 I'm talking about the Thevenin equivalent current and voltage at the nodes near the 1V power source.

    I have done this question over and over (and admittedly I do keep getting different answers).
    I know that the correct i0 is 3A however the closest I've gotten is -5A (the rest have been divisible by 7 or something).

    I have a feeling it has something to do with my signs around the circuit, but it'd be a huge help if someone could look over it and let me know where I'm going wrong/ how to get it right lol.

    The yellow part is attempt 1 with the orange part being attempt 2.
    Anything that is yellow but is circled in orange I've used in attempt 2.

    Also the matrix at the bottom is cut off a bit.
    It should be: R2-2*R1

    Attached Files:

  2. jcsd
  3. Mar 21, 2012 #2
    I have a question, which current control this current control voltage source ?
    And which direction this control current must flow to make CCVS voltage positivity.
    Also I assume that you need Io to find Rth ?
    Rth = Vth/Io
    Last edited: Mar 21, 2012
  4. Mar 21, 2012 #3
    Maybe I'm not getting what the problem is, but isn't

    Voltage across 4 ohm resistor 4 ohm X 3 amp = 12 volt
    Current through 2 ohm resistor 1 volt/2 ohm = 0.5 amp
  5. Mar 21, 2012 #4
    Apologies, the current controlled voltage source is defined by i which is in the location of i0, in the opposite direction i = -i0.
    Carl I'm trying to find i0, would you suggest nodal analysis? I use mesh analysis normally just because I find it easier to understand the sign conventions.
    Last edited: Mar 21, 2012
  6. Mar 21, 2012 #5
    Is this correct?
  7. Mar 22, 2012 #6
    I wouldn't think so, 3i is the value given to a dependant voltage source.
    That is the current i3 is the current passing through the dependent voltage source which = 3i Volts.

    I wouldn't be surprised it my lecturer had put in a voltage source instead of a current source though. If nobody else has any idea's I might just put this down to image error.

    It was an example in a lecture and I'm pretty bad at circuit calculation so I wanted to be able to do it and get the same answer.
  8. Mar 22, 2012 #7
    If so, we can write one equation

    Io = 1V/2Ω + (1V - (-3*Io)/4Ω

    Solving this for Io I get this result

    Io = 3A
  9. Mar 22, 2012 #8
    Jony130, I came up with the same results using

  10. Mar 22, 2012 #9


    User Avatar
    Gold Member

    Using superpostion (open current sources and short voltage sources) You can find currents, voltages and thevinin equivalents....by quick inspection.

    Superposition and thevenin are you friend. Embrace them.
  11. Mar 22, 2012 #10
    But it is impossible to use superposition when we have dependant source in the circuit.
  12. Mar 22, 2012 #11


    User Avatar
    Gold Member

    I don't see why not. Nodal analysis will work too I think. KCL...

    But super position usually requires no math equations, just simple V=IR stuff.
  13. Mar 22, 2012 #12
    Thanks guys, I thought there was a way for me to thank you, but I'm not able to see it.

    This has helped a lot.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook