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Homework Help: Electronic Circuit with 2 diodes

  1. Jun 3, 2017 #1
    1. The problem statement, all variables and given/known data

    I have to find Vo in every diode state possible.
    We know diode Vf = 0.7

    2. Relevant equations
    Va-Vc < 0.7 = diode OFF
    Va-VC > 0.7 = diode ON
    3. The attempt at a solution
    State 1:
    if D1 OFF and D2 OFF
    Vo will be equal to Vi

    State 2:
    if D1 OFF and D2 ON
    Vo will be equal to Vi

    State 3:
    if D1 ON and D2 OFF
    Vo will be equal to 0.7 since the diode is limiting component it at 0.7

    State 4:
    if D1 ON and D2 ON
    Vo= impossible since D2 will not receive enough volt to activate, it will receive less than 0.7 as the resistance will take some.
  2. jcsd
  3. Jun 3, 2017 #2


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    What is your question?

    To what extent are the diodes to be considered to be ideal ?
  4. Jun 3, 2017 #3


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    Only if there is no load (see state 3).
    I don't understand that argument, and the conclusion is wrong. This state is only possible if there is a load at V0.
    I don't understand that argument and the conclusion is wrong.
  5. Jun 3, 2017 #4
    Each state is independent from each other, its like 4 different problems.
    In state 1, both are off so its like there is no diode at all and the second resistor is out as well.
    which means Vo = Vi

    for state 2, I honestly have no idea how to do it. It confuse me a lot. Can you give me some tips ?

    For state 3, I came up with a different solution : Vo = Vi - 0.7

    For state 4, I came up with the same solution as state 3, Vi -0.7 = Vo
  6. Jun 3, 2017 #5
    They are perfectly ideal, with vf = 0.7
    question is to find Vo in every state as stated in the first post.

    Each state is an independent problem.
  7. Jun 3, 2017 #6


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    Only if there is no load at V0. But if there is no load, you never have current through D1!
    If diode 2 is conducting current, where does the current go? What does that mean for the voltages? There are two things you can say.
    Only with a load. Otherwise: Where does the current go?
    Again, that needs a load, and the way you answer state 1 there is no load.
    You can make an additional statement in this case, based on the conducting diode 2.

    Can you have a load attached yes or no?
  8. Jun 4, 2017 #7
    Not really sure how to answer the load attached question, this is all I know about the problem. I believe there is load.
    I forgot to mention that it is also possible that some state are impossible.

    For state 1, that would mean Vo = R*I

    For state 2, I would say current would go to Vo, as D1 cant receive current in that direction. only from Anode to cathode.
    so Vo would be -0.7 - R*I = Vo

    I wanna clear state 1 and 2 before trying again with 3 and 4, one step at a time so I understand the problem correctly
  9. Jun 4, 2017 #8


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    Okay, with possible loads then.
    That is the voltage drop at the resistor, the difference between two potentials - it is not Vi or V0
    But the resistor can.
    Right, assuming I is the current flowing through the lower part.
  10. Jun 7, 2017 #9

    rude man

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    Staes 1, 3, 4 OK
    State 2 wrong.
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