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Homework Help: Power dissipated across a transistor regulator

  1. Dec 12, 2017 #1
    1. The problem statement, all variables and given/known data
    upload_2017-12-13_10-41-38.png



    2. Relevant equations
    For the 1st part
    Ohms law and resistance across zener is 10V

    I cant get 2nd

    3. The attempt at a solution
    For 1st part I took Vi/p = 18V

    Putting KVL for left loop of source-->collector-->base-->zener-->ground
    18 = Vcb + 10
    So Vcb = 8V
    And Vbe = 0.7V
    So Vce = 8.7V Correct answer.

    For second question,
    P dissipated in transistor = Vce times total current = Vce times Ie
    Ie = 10 - 0.7 divided by 1 K
    =9.3mA
    So P dissipated by transistor = 8.7 times 9.3 = 80.91 mW
    Book answer is A.
    Where am I wrong?
     

    Attached Files:

  2. jcsd
  3. Dec 13, 2017 #2

    CWatters

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    I haven't checked the numbers as I'm on my phone but I suspect an error in the question. Try calculating the maximum power dissipation rather than the minimum. The max power dissipation is usually a much more important value to calculate as it determines the heat sink requirements for the transistor.
     
  4. Dec 13, 2017 #3

    CWatters

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    PS In this circuit you may not need a heatsink but the power dissipation in the transistor is the main reason why other circuits are sometimes preferred for higher power applications.
     
  5. Dec 14, 2017 #4
    Yup you're right. I get the correct answer if the consider the second question to have typo and be maximum. Thanks.
     
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