About Transistor NPN, Circuit Problem

Discussion in 'Homework Help' started by johncena, Aug 15, 2010.

  1. johncena

    Thread Starter Member

    Aug 15, 2010
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    This is the problem...

    [​IMG]

    Finding for the Voltage Input. I get 3.23v, but i know it's not the correct answer. pls guide me. thanks
     
  2. mik3

    Senior Member

    Feb 4, 2008
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    Did you include the voltage drop across the LED in your calculations?
     
  3. johncena

    Thread Starter Member

    Aug 15, 2010
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    i think the voltage drop of LED is neglected, as it only just need 20mA on Ic , The problem states that What voltage (Vin) does it need to produce that 20mA on Ic
     
  4. mik3

    Senior Member

    Feb 4, 2008
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    The question doesn't say to neglect the LED voltage which can vary from 1.5 to 4 volts depending on the colour of the LED. Thus you have to include it since Vcc is just 10V and 2V across the LED can be quite significant.

    Do you have the correct answer?
     
  5. Jony130

    AAC Fanatic!

    Feb 17, 2009
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    Hmm,
    Ib_min = 200uA so if safety margin is 2*Ib_min = 400uA
    Vin = Ib*Rb +Vbe = 2V+Vbe
    Or
    Vin = 200uA*5K + Vbe = 1.7V----> 2*1.6V = 3.4v
     
  6. hobbyist

    Distinguished Member

    Aug 10, 2008
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    I got Viin=2.32V.

    But I used IB (min), for the calc.
     
  7. Potato Pudding

    Well-Known Member

    Jun 11, 2010
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    Allowing around 3V for the LED Vf the Rc=300 Ohm and +Vcc of 10V will allow about 22mA with the transistor in saturation.

    To allow exactly 20mA with 100Beta, if you double the volts as instructed Jony has it almost right.

    Double only the volts across the resistor.

    1 Volt across the resistor *2 +Vbe = 2.7 Volts
     
  8. Potato Pudding

    Well-Known Member

    Jun 11, 2010
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    The problem is set up badly, but you know what?

    You will get a lot more poorly thought out questions and proposals when you get out into real work. Know what you need to know and decipher the intended question, even when they don't know what they are trying to ask. It is a like a puzzle game sometimes, and if you learn to have fun with it and have a sense of humor about it you will find things hilariously fun instead of horribly annoying.
     
  9. johncena

    Thread Starter Member

    Aug 15, 2010
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    Thanks for all your replies, and your time to answer my problem. Our instructor solved it with an answer Vin = 3.93v
     
  10. JoeJester

    AAC Fanatic!

    Apr 26, 2005
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    How did the instructor handle the 1.5 - 2V drop across the diode?
     
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