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Series connected zener diodes problem

  1. Mar 1, 2016 #1
    I have attached the image with post.

    Answer to question is given as A. I am not getting the explanation to how can one zener diodes be in breakdown and other not. I think if the combined voltage of 150V is applied then only breakdown occurrs in both the diodes simultaneously.

    SvPyxhW.jpg
     

    Attached Files:

  2. jcsd
  3. Mar 1, 2016 #2

    Hesch

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    You are right: The answer is D) as for Vsupplied < 100 V.

    D1 and D2 will conduct some small amount of current, sharing the supplied voltage, but the voltages VD1+VD2 < 100V are not defined as break down voltages.
     
    Last edited: Mar 1, 2016
  4. Mar 1, 2016 #3

    jim hardy

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    Maybe we should ask the question instead as:
    "Does some arbitrarycurrent I100 exist
    where the sum of the diode voltages is 100 volts? "
    Surely there does.

    The diodes see the same current of course ...
    Any given current would be a horizontal line on the graph
    and where it crosses each diode's characteristic curve is the voltage for that diode at that current.
    so let's take a ruler and dividers to it .
    Sliding my red horizontal current line down from zero amps
    it appears D1 breaks down at 80 volts well before D2 reaches even 20 volts
    so if i lay my horizontal current line so it crosses D2's characteristic curve at 20 volts
    i have found one possible I100 .
    zeners.jpg

    It seems strange that those two diodes have such different avalanche currents

    but those are the curves we were given.

    I'd have to pick answer A.

    I think this was more an exercise in graph reading than zener diodes ?

    old jim
     
    Last edited: Mar 1, 2016
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