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Homework Help: Diodes homework (electronics)

  1. May 30, 2017 #1
    1. The problem statement, all variables and given/known data
    A picture of the problem is provided in the attachment

    2. Relevant equations
    there is no special relations except
    I= Is x e^(vd/Vt)

    3. The attempt at a solution
    i have a solution for from the solution manual but i do not understand it
    like the I2 equation i don't get reasonable values when i substitute VD for any value > 1 or even 1
    i attached the solution that i do not understand
    if someone can help please

    Attached Files:

  2. jcsd
  3. May 31, 2017 #2


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    Science Advisor
    Homework Helper
    2017 Award

    Can you be bothered to provide a clear description ? Of the problem and also of the part you don't understand.
    Your pictures are so fuzzy my eyes hurt after discerning 0.025 which is 1/40 which I recognize as kT for room temp, so I don't think there is much wrong with that diode equation. What is it that goes wrong for you ? (Note that with 5 k##\Omega## you don't expect currents exceeding 1 mA anyway).
  4. May 31, 2017 #3
    i am sorry for the fuzzy picture. it is the pic of the solution which i don't understand. leave it alone
    the first pic is about the question and it is clear
    what i know about the problem: that the diodes have 0.7 voltage each as VD
    diodes 3 and 4 only will conduct if the input voltage VA is positive. in this case there will be an output voltage at both C and B ( C is that output before the 1k ohm R )
    for a negative voltage diodes 1 and 2 only will conduct and for this case there will be output voltage at B only

    how much???
    this is what i am asking for

    note: these diodes are working as voltage regulators
    Last edited: May 31, 2017
  5. Jun 1, 2017 #4


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    Staff: Mentor

    The output voltage at B is unable to go more negative than -2V. If you try to send it more negative than this by giving A a large negative voltage, then increased current flows through the 5k resistor via the diodes, and this results in more voltage dropped across the 5k resistor—leaving still 2V across the pair of diodes.

    This assumes the diodes are within their current rating and don't overheat or burn out, and the voltage across each when conducting can be approximated as 1V.

    Note: if it is not possible to provide a clear image, we expect you to remedy this by typing out the relevant equations and text yourself, preferably using LaTex. Blurred images help no one.
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