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BJT Switch - Circuit Simulation

  1. Jan 27, 2016 #1

    dwn

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    I am reading The Art Of Electronics, and I realize this is more of an encyclopedia rather than a textbook. I have referenced other sources as this has mainly served as a compass for me to learn about each subject area.

    My question is fairly simple and straightforward. I'm using multisim to replicate a circuit that was provided in the book. The author used a simple transistor switch to light an LED, to explain the function of a BJT. When I went to simulate the circuit, the LED lights up, even though it should not, at least until the SPST is closed.
    What is causing the transistor to act like a short?

    As far as I understand it, the collector is an open circuit, until the point in which the VB ≈ VE + VBE, at which point the collector would be saturated.
     

    Attached Files:

  2. jcsd
  3. Jan 27, 2016 #2

    davenn

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    hi there
    there is no LED in your circuit
    also, because you have no resistance in either the collector or emitter circuit of the transistor, it's possible you have killed it with excess current
    and it has failed in short circuit mode

    so redo your circuit with the LED and it's current limiting series resistor, for a 10V supply it should be around 1k Ohm



    cheers
    Dave
     
  4. Jan 27, 2016 #3

    davenn

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    As a general rule the bjt transistor will go into conduction with ~ 0.7 - 0.8V on the base
    for any voltage higher than that, it will be in saturation
    ( caveat ... higher power bjt transistors require a bit more base voltage)


    Dave
     
  5. Jan 27, 2016 #4

    berkeman

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    Staff: Mentor

    Here are a couple of BJT LED drive circuits that you can start simulating...

    http://i.stack.imgur.com/OnVwa.png
    OnVwa.png

    And a Common-Emitter amplifier stage like above will have a V-I characteristic like this:

    http://sub.allaboutcircuits.com/images/quiz/02244x01.png
    02244x01.png
     
  6. Jan 27, 2016 #5

    dwn

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    Thank you Dave, silly mistake thinking that the light probe was multisims version of the LED. After changing that and adjusting the components to their original values everything worked as expected.
     
  7. Jan 27, 2016 #6

    davenn

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    cool :smile:

    look at the variations that berkeman also posted
     
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