Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

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.


    Attached Files:

  2. jcsd
  3. Mar 1, 2016 #2


    User Avatar
    Gold Member

    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

    User Avatar
    Science Advisor
    Gold Member

    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 .

    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
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted