Zener Dioide Problem

Discussion in 'Homework Help' started by Digit0001, Nov 8, 2011.

  1. Digit0001

    Thread Starter Member

    Mar 28, 2010
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    0
    Hi

    How would you do the following problem:

    A Zener diode is connected in a voltage regulator circuit.
    zenerdiode.JPG
    The Zener voltage is Vz = 10V and the Zener resistance is assumed to be rz=0. (a) Determine the value of Ri such that the Zener dioide remains in breakdown if the load current vari4es from Iload =50 to 500mA and if the input voltage varies from V1 = 15 to 20V. Assume Iz(min) = 0.1I(max).

    P.S
     
  2. t_n_k

    AAC Fanatic!

    Mar 6, 2009
    5,448
    782
    The constraints are minimal.

    The minimum zener current would presumably occur when the input voltage is at its lowest and the load is at its maximum. You are advised to set Iz(min) to 0.1Iload(max). That should be sufficient information to solve the problem.
     
  3. Digit0001

    Thread Starter Member

    Mar 28, 2010
    89
    0
    i have tried doing this:

    I1= Iz+Il

    Iz = Vz/rz = 0

    so I1 = IL

    now (Vin(min) - Vz)/ Ri = I1 = IL(min)

    (20 - 10)/ Ri = 50m

    10/50m = Ri

    so Ri = 200ohms

    checked with answer this is wrong, what is wrong with this??
     
  4. MrChips

    Moderator

    Oct 2, 2009
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    At max Iload, Rload = 10/.5 = 20 ohm
    At max Vi Ri = 10/.5 = 20 ohm
    If you need to include Iz, Rz = 200 ohm
    Then Ri = 20//200 = 200/11 = 18 ohm
     
  5. t_n_k

    AAC Fanatic!

    Mar 6, 2009
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    It might be worth double checking that Ri=18Ω actually works.
     
  6. jimkeith

    Active Member

    Oct 26, 2011
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    99
    Ri = ERimin /IRi
    ERi min = 15-10 = 5V
    IRi under above conditions = 0.5 + 0.1*0.5 = 0.55A
    Ri = 9.09Ω

    A better question is: Specify a zener that will do this without smoke
     
  7. t_n_k

    AAC Fanatic!

    Mar 6, 2009
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    782
    Agree with your value for Ri - hence my query.

    Excellent point about the practical implications.
     
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