I'm stuck on this one!

Discussion in 'Homework Help' started by jjw09141978, Jan 29, 2011.

  1. jjw09141978

    Thread Starter New Member

    Jan 29, 2011
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    0
    Stuck on a homework question in my Solid State Electronics class. I've come up with an answer just not certain it's right and wanted to get a second opinion to see if i'm performing calculations correctly. The question is...
    Determine the minimum input voltage required for regulation to be established in Figure 3-68. Assume and Ideal zener diode with Izk=1.5mA and Vz=14V

    The answer i've come up with is 14.84V

    and the second half of the question is....

    Repeat above problem with Zz=20Ω and Vz=14V at 30mA.


    I made several attempts at the second one with no luck. So I finally looked in the back of the book for the answer which is 14.3V. And again i've tried several times yet cannot come up on my own with an answer of 14.3V. Any help, tips or pointers would be very much appreciated:)
     
  2. Adjuster

    Well-Known Member

    Dec 26, 2010
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    300
    Zz represents the slope (dv/di) of the zener regulation characteristic. This slope tells us how changes in the zener current produce changes in the Zener voltage.

    What will happen to the voltage across the less perfect zener when the current through it reduces from 30mA to 1.5mA?
     
  3. jjw09141978

    Thread Starter New Member

    Jan 29, 2011
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    Ok let me see if I got this right. If I take the change in Iz 30mA-1.5mA= 28.5mA and then multiply the difference in current by the Zz of 20Ω 28.5mA x 20Ω= 570mV, then subracting the 570mV from my answer for the first part of the question 14.84V - 570mV= 14.27V, which if I round up that would give me 14.3V Is this correct?:)
     
  4. Adjuster

    Well-Known Member

    Dec 26, 2010
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    300
    I think so.
     
  5. jjw09141978

    Thread Starter New Member

    Jan 29, 2011
    3
    0
    I've tried it a few other ways and the above is the only way I can come up with 14.3 Volts which is the answer in the back of the book. So I can only assume I've performed the calculations correctly.
     
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