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Homework Help: Confusion about how KCL and KVL are used for diode circuits

  1. Jan 27, 2017 #1
    1. The problem statement, all variables and given/known data
    This is just one of the example problems in my book which is already solved for me, but I don't really understand their solution which I'll post here:
    They start out with assuming Vin is very negative, which makes D1 turn on and makes Vout=VD,on+Vin. THen they solve for the currents.

    2. Relevant equations

    3. The attempt at a solution
    What I am confused on is that if the diode is turned on, then the branch with the diode is a short circuit, so why is there current going in the R2 branch?
    The book solves all the problems like this one the same way, but I don't really get it. Are we assuming the diode is actually on the brink of turning on/off, so it allows current through it, but it's actually not a short circuit yet?
  2. jcsd
  3. Jan 27, 2017 #2


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    Staff: Mentor

    What does VD.on represent? Is the diode treated as ideal or non-ideal in these problems? :wink:
  4. Jan 27, 2017 #3


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    Homework Helper
    Gold Member

    No. You are asked to use the constant voltage model for the diode.
    images (4).jpg
  5. Jan 27, 2017 #4
    Oh wow, not sure why I didn't realize it until you guys said it, but now it makes sense, thanks!
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