Thevenin equivalent w/ dependent current source

Discussion in 'Homework Help' started by mrmr, Sep 15, 2018.

  1. mrmr

    Thread Starter New Member

    Sep 15, 2018
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    I'm looking for guidance regarding finding thevenin resistance and voltage. I was tasked with finding the voltage and current across resistor R2. Using super mesh method I calculated the values included in my picture. I need to create a thevenin equivalent circuit and arrive at the same values across R2. I'm unsure of why I need to insert a input current when looking for thevenin resistance. Previously I considered Rth to be 4 ohms, since I removed the dependent source. As for thevenin voltage, i'm not sure how to incorporate the dependent current value into my Left loop KVL equation.
     
  2. MrAl

    AAC Fanatic!

    Jun 17, 2014
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    Hello,

    Here is a more clear drawing so others can see it better.

    Did you mean R3 instead of R2 or did you just label R2 wrong?
     
  3. mrmr

    Thread Starter New Member

    Sep 15, 2018
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    Yes, I meant R3.
     
  4. MrAl

    AAC Fanatic!

    Jun 17, 2014
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    Hi,

    Ok great, but also, where is the dependent current source and what is it dependent on?
     
  5. mrmr

    Thread Starter New Member

    Sep 15, 2018
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    The dependent current source is marked as I2 in the circuit, it's the middle branch. As for that I don't think that was provided, and I don't understand how how the dependent source affects the circuit.
     
  6. MrAl

    AAC Fanatic!

    Jun 17, 2014
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    Hello again,

    Ok then that brings us to a temporary stand still because a dependent source has a terminal or set of terminals or an algebraic variable that is used to show the dependency.
    For example, if a voltage was marked as "Vx" and the dependent current source was dependent on that voltage, then the dependent source would have a 'gain' marked as:
    Vx*2
    or:
    Vx*3
    or any number like:
    Vx*1.2345

    and that way we know that the current source depends on Vx.
    If it instead depended on another current somewhere else in the circuit, then we would see something like:
    Ix*1.2345
    or whatever.

    Could it be that it is not really a dependent source, but is really just a 2 amp independent current source?

    Also, because that is a current source i think you can easily find Rth because you open circuit a current source to find that, as a hint.
     
    Last edited: Sep 16, 2018
  7. WBahn

    Moderator

    Mar 31, 2012
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    You've already indicated that you meant R3 and not R2.

    Your diagram shows the symbol for a dependent source, but you don't indicate what it is dependent on. You only mark it as an INDEPENDENT 2A source. So you need to find out what it really is -- the problem can't be solved without that because you would be solving a different problem, not the one you were given.

    The reason that you need to use a test source to find the Thevenin resistance when a dependent source is involved is because it's output depends on what is going on in the circuit (whereas the output of an independent source does not). So you can't just turn it off the way you can independent sources.
     
  8. MrAl

    AAC Fanatic!

    Jun 17, 2014
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    Hi there,

    That's the point i was trying to get across too. Doesnt make sense to declare a dependent source and then not show what it depends on. As is it looks like a regular independent source.

    We dont really need a test source in this case, but that is one way to do it.
     
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