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Equivalent Circuits

Discussion in 'General Electronics Discussion' started by Mohammad Al-Hiyali, Oct 18, 2018.

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  1. Mohammad Al-Hiyali

    Mohammad Al-Hiyali

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    Apr 8, 2015
    I am studying Thevenin's Theorem. In one of the slides introducing the theory. The slide states that circuits below are equivalent.
    How are the circuits below equivalent?


    upload_2018-10-18_13-15-42.png
     
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    What is their open circuit voltage and short circuit current of each?
     
  3. Cannonball

    Cannonball

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    May 6, 2017
    One circuit will have current flow and the other won't.
     
  4. BobK

    BobK

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    How do you figure that?

    Bob
     
  5. duke37

    duke37

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    V2 (48V) has 18Ω across it and supplies 3A.

    The ouput will be equivalent as long as the circuit does not overheat.
     
    Cannonball likes this.
  6. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    For the purpose of the question, equivalence is determined by what happens at the output.
     
  7. Kabelsalat

    Kabelsalat

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    Jul 5, 2011
    [​IMG]

    Image you have a load that have a certain resistance (you are to decide it's value).

    Then ask yourself this question:
    - Will the voltage over the load be the same in both cases?
    - Will the current through the load be the same in both cases (?)
    - If the terminals is shorted, will there flow the same amount of current in both cases
    - If you remove the load and let the termnals open, will there be the same voltage in both cases.

    If yes is the answer on all those questions, then the circuits are equivalent.

    Normally, the two first questions can be omitted, but by answering those - espechially if you take your time to actually calculate on a couple of examples - that should prove the concept for you.
     

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  8. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    That is no help here. If the open circuit voltage and the short circuit current of a linear source are equal, the sources are equivalent. For any linear passive load the voltage across the load (aka output voltage of the source) and the current through the load (aka output current of the source) will be the same.
    @(*steve*) 's question to the op is to check whether this is the case here. The answer to this question is also the answer to the op's original question.
     
  9. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    In fact, since this is a linear circuit, showing that the output of both circuits is equal at any two different points is sufficient to show that it is the same at all points.

    Short circuit and open circuit are the conventional points to test because they are typically simpler. There's no reason you couldn't use 43Ω and 270kΩ.

    In this case the results for open and short can be determined by inspection (i.e. no more than simple math you can do in your head).
     
  10. AnalogKid

    AnalogKid

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    Jun 10, 2015
    Math check.

    ak
     
    duke37 likes this.
  11. duke37

    duke37

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    My fingers do not always go where they should.:)
     
  12. BobK

    BobK

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    It is worse when they do go where they shouldn’ t.

    Bob
     
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