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Homework Help: Transistor Switch Circuit

  1. Oct 15, 2012 #1
    1. The problem statement, all variables and given/known data

    I am given a problem for my electronics class that asks to design a transistor switch that will switch a 10-Ohm load powered from a +5V supply. The transistor beta is about 50.

    It also asks which current is needed to turn the transistor fully on (saturate it so that the collector-emitter voltage is less than 0.3V)

    2. Relevant equations

    3. The attempt at a solution

    My biggest problem is that the book we use SUCKS. I am simply lacking material and, maybe, some examples. I have tried approaching the instructor, but his explanations do not "speak" to me.

    I know what the circuit looks like. I then proceed to substitute the transistor with a diode+current source model. After that, I am befuddled.

    Any help and preferably hints are appreciated!
    Last edited by a moderator: Oct 15, 2012
  2. jcsd
  3. Oct 15, 2012 #2


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    Staff: Mentor

    Can you show us the circuit diagram that you would use? If you have about 5V across a 10 Ohm load, what will the collector current of the NPN transistor be?

    Now get a more accurate number for Ic by including the Vce,sat number in the calculation of the collector current. What is it now?

    How is Beta defined? Using the definition of Beta, what can you say about the base current required to support the Ic value that you calculated?
  4. Oct 15, 2012 #3
    I am not really sure how I could show you. Your hints were helpful though :) I was able to "see" it. This has been a bizzare class. I can do QM and Solid State of diodes and transistors alright. However, when it comes to their applications, I am lost.
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