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Diodes & rectifiers

  1. Jan 21, 2017 #1
    1. The problem statement, all variables and given/known data
    I am practicing problems like this:
    upload_2017-1-21_15-45-27.png
    where Vs=10V, R=2kohm, they say it is like a battery + ideal diode with VD=0.7V
    I have to sketch the transfer characteristic Vo=f(Vs) and draw the waveform of Vo(t)
    2. Relevant equations


    3. The attempt at a solution
    I am really unsure how to go about these problems. I have the answer, I just don't know they got to it. Can someone explain please?
    upload_2017-1-21_15-49-10.png
    I am also unsure of how to draw the waveform, but I think it could be something like:
    upload_2017-1-21_15-50-17.png
    Is this correct?

     
  2. jcsd
  3. Jan 21, 2017 #2

    mfb

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    VI = Vs?

    Where did you take the voltage drop at the diode into account?

    The problem statement asks for V0(t), not VD(t).
     
  4. Jan 22, 2017 #3
    I do not understand, I just found this graph in my textbook. Can you explain even the first part, since it too is just a drawing from the book.
     
  5. Jan 22, 2017 #4

    mfb

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    Your first plot has an axis label VI. What is that?
    Where did you find that graph? The symbols should be introduced somewhere.

    The diode won't conduct until there is a 0.7 V potential difference between its sides.
     
  6. Jan 22, 2017 #5
    VI is supposed to be Vs, sorry the example in the book had different variables than my question. I don't understand what a transfer characteristics is. I think if it is forward then you put a short circuit and if it is reverse you put an open circuit? But I don't know how that translates on the graph
     
  7. Jan 22, 2017 #6

    cnh1995

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    Is your voltage source ac or dc? You have shown it as dc but you have drawn the waveforms assuming it to be ac.
     
  8. Jan 22, 2017 #7
    I don't know, I guess it is DC?
     
  9. Jan 22, 2017 #8

    cnh1995

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    Well, go with dc since you have shown it that way.
    So, your dc source is variable, say from 0V to 10V. The diode drop is assumed to be 0.7V. What can you say about the i-v characteristic of the diode from this? Can you plot the diode voltage first, as a function of the input dc voltage?
     
  10. Jan 22, 2017 #9
    Would it be forward biased and therefore a short circuit ? I'm not sure about the plot
     
  11. Jan 22, 2017 #10

    cnh1995

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    images (4).jpg
    Now for input voltage Vs≤0.7V, what is the diode voltage? For Vs>0.7V, what is the diode voltage?
     
  12. Jan 22, 2017 #11
    When it is less, it is 0. When it is more it is 0.7?
     
  13. Jan 22, 2017 #12

    cnh1995

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    We are talking about "voltage" across the diode. If Vs=0.5V, what is the voltage across the diode? Use the above graph. You are right about Vs>0.7V.
    It seems that the input is supposed to be ac, since the output voltage is a function of time. With dc input, you can draw the transfer characteristic but I don't think they are expecting you to draw Vo(t) for dc input.
     
    Last edited: Jan 22, 2017
  14. Jan 22, 2017 #13
    I am unsure on how to draw transfer characteristics and the Vo(t), it was never explained to me in class.
     
  15. Jan 22, 2017 #14

    cnh1995

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    Ok. We can work that out here. We will take the voltage source as AC, with magnitude 10V.

    You are right about Vs>0.7V.
    What is the volage across the diode when Vs<=0.7V? Refer the graph in #10. Once you know that, you can draw the transfer characteristic easliy.

    Or answer this: If Vs<=0.7V, what is the voltage across the 2k resistor?
     
  16. Jan 22, 2017 #15

    mfb

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    There is still the resistor.

    At V0=-10 V, analyze the circuit. What is the voltage at the resistor?
    At V0=-5 V, analyze the circuit. What is the voltage at the resistor?
    At V0=0 V, analyze the circuit. What is the voltage at the resistor?
    At V0=+5 V, analyze the circuit. What is the voltage at the resistor?
    At V0=+10 V, analyze the circuit. What is the voltage at the resistor?
    With those values (and suitable intermediate values), you can plot the voltage at the resistor as function of the source voltage. That is the curve you need in the first part.

    I would expect your voltage source to be AC, otherwise plotting things as function of time is useless.
     
  17. Jan 23, 2017 #16
    Ok thank you, I understand now.
     
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