Current of base and collector in shunt regulator

Discussion in 'Homework Help' started by jiangiang, Jul 24, 2012.

  1. jiangiang

    Thread Starter New Member

    Jul 24, 2012
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    Hi, I'm doing on a assignment and stuck on this question.
    [​IMG]
    How can i find the zener diode current and the collector current?
    Very appreciate for anyone help:)
     
    Last edited: Jul 24, 2012
  2. mlog

    Member

    Feb 11, 2012
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    What has you stuck? Can you find any of the answers? Try finding VRL at the load resistor. That is easy and should get you started.
     
  3. jiangiang

    Thread Starter New Member

    Jul 24, 2012
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    I'd found the VRL and IL, and the question need me to find the Iz and Ic, this two current i have no idea how to start
     
  4. Jony130

    AAC Fanatic!

    Feb 17, 2009
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    The first Kirchhoff's law should help you find answers

    [​IMG]


    And also Ib = Ic/β come in handy
     
  5. mlog

    Member

    Feb 11, 2012
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    You were told Is and you now know IL. You should be able to calculate the combined current of Iz and Ic.

    Notice that Iz is also your base current. The transistor's current gain, β will provide the relationship between the base and collector currents.

    You will probably have to make an assumption about the β or current gain. Look it up in a 2N3904 data sheet.

    Does this make sense to you? Let us know what you find.
     
  6. #12

    Expert

    Nov 30, 2010
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    Here are some helps.
     
  7. jiangiang

    Thread Starter New Member

    Jul 24, 2012
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    Thanks everyone, now my question is, am I correct if I find the β value from this graph ?
    [​IMG]
     
  8. mlog

    Member

    Feb 11, 2012
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    You're making this more difficult than it should be. I would assume the transistor is at room temperature, say 25°C. So follow that curve over to the collector current on the x-axis. What did you find for Ic? What do you read on the y-axis for hFE?
     
    Last edited: Jul 25, 2012
  9. jiangiang

    Thread Starter New Member

    Jul 24, 2012
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    Thank you very much.
    But in your opinion, what is the easier way to find beta from the datasheet ?
     
  10. #12

    Expert

    Nov 30, 2010
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    The "base-emmitter on voltage" vs "collector current" graph also works on this one.
     
  11. jiangiang

    Thread Starter New Member

    Jul 24, 2012
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    thank you everyone :D
     
  12. crutschow

    Expert

    Mar 14, 2008
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    Look in the data sheet "Electrical Characteristics" table for the Hfe value at the collector current closest to the current in your circuit. Generally you want to use the minimum Hfe value column which gives you the worst-case maximum base current.
     
  13. jiangiang

    Thread Starter New Member

    Jul 24, 2012
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    Thanks again, i will try it :)
     
  14. mlog

    Member

    Feb 11, 2012
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    The collector current is ~50 ma. Follow the vertical line upwards from 50 mA on the x-axis. Find the point where it intersects the 25°C curve and read across to the left on the y-axis. It looks like ~200 to me. If you expect the transistor to get hotter or colder, then use the other temperature curves.

    The other way is to use the table near the front of the data sheet. It will specify a few points that might be close to your operating current or voltage. It can give you a rough idea of the β. You want to be conservative though as someone else mentioned.
     
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