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A bit of trouble with Thevenin equivalence with dependent sources

  1. Dec 14, 2011 #1
    This really isn't one specific problem per se as it is more of a conceptual issue, so I apologize for breaking away from the given format.

    I've worked through three problems involving a circuit where a thevenin equivalence circuit is required between two points, and all sources are dependent.

    The technique seems to be to apply either an independent voltage or independent current source between the two points, in order to consider the dependent source. My reasoning behind this technique is that it is essentially the same as when I'd short circuit or open circuit between the two points, as I would do normally if I had purely independent sources. With independent sources, I'm applying either an independent voltage source of 0V (to short circuit and find I_SC, the current of the short circuit) or an independent current source of 0A (to open circuit and find V_0C, the voltage of the open circuit).

    I'm pretty sure my reasoning above is correct, but please correct me if I'm wrong.

    The trouble is coming from figuring out what type of source to apply in the first place. The three circuits I worked out previously have been such that:

    1. For a dependent voltage source that relied on the voltage of a resistor, a 1V source was placed.
    2. For a dependent voltage source that relied on the current through a resistor, a 1A source was placed.
    3. For a dependent current source that relied on the voltage of a resistor, a current source was placed.

    I'm trying to figure out the logic behind these choices. My guess would have to be that the source placed is placed in such a way to clarify the dependent source, but some examples that illustrate which source to place and when would be very illuminating.

    Thanks.
     
  2. jcsd
  3. Dec 15, 2011 #2

    vela

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    You're looking at two different aspects of the problem. When you find the short-circuit current or the open-circuit voltage, you're determining the Norton current or Thevenin voltage. When you apply a source to the terminals, you do so to find the equivalent resistance when looking into the two terminals.

    It doesn't matter, in principle, what type of source you apply. The choice is just to make the calculations easier. In some instances, knowing the current flowing across the terminals is more useful in making calculations than fixing the voltage; in others, it's the opposite.
     
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