Diode Problem

Discussion in 'Homework Help' started by MatthewPerry, Feb 20, 2011.

  1. MatthewPerry

    Thread Starter New Member

    Nov 29, 2010
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    Hi. I've been trying to understand the solution to a practice problem in my book (it only provides the answer), but I cant seem get it. The problem gives the circuit in the attachment and asks the following: Find a value of R such that V0 is equal to 2.4 V. Assume that the diodes available have 0.7 V drop at 1 mA.

    The solution is R = 139 ohms. I keep getting about 160. If someone could tell me how I should go about solving this it would be great! Thanks.
     
  2. Jony130

    AAC Fanatic!

    Feb 17, 2009
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    You forgot to attach the diagram.
     
  3. MatthewPerry

    Thread Starter New Member

    Nov 29, 2010
    3
    0
    Here it is.
     
  4. Jony130

    AAC Fanatic!

    Feb 17, 2009
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    Te get the right answer you need to use Shockley diode equation
     Id = Is * e^{\frac{Vd}{26mV}}
    First you need to find Is current, because you know Id = 1mA; Vd= 0.7V so
    Is = 2.03fA
    Now you want Vo = 2.4V but you have three diodes so Vd = 2.4V/3 = 0.8V
    Id = 2.03fA * e^(0.8V/26mV) = 46.816mA
    So we know Id current which causes 0.8V voltage drop across the diode.
    And now we can find resistor R.
    R = (10V - 2.4V) / 46.816mA = 162Ω

    So I get the same answer as you.
     
    MatthewPerry likes this.
  5. Georacer

    Moderator

    Nov 25, 2009
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    1,266
    I support Jony130's method of solution.
     
    MatthewPerry likes this.
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