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Homework Help: Transistor trouble

  1. Jul 19, 2010 #1
    I need some guidance on this question, and possibly a check over my attempts? I'm not exactly a star Physics student. Help is much appreciated.

    1. The problem statement, all variables and given/known data
    [PLAIN]http://img835.imageshack.us/img835/8124/ques.jpg [Broken]

    6) Label the above transistor.
    7) If switch S is opened, the bulb only lights up for a moment. Explain why this happens.
    8) How to increase the time delay?
    9) Explain how to make the bulb not to light up.
    10) Explain how to change the time delay.

    2. Relevant equations
    None of which I am sure of...

    3. The attempt at a solution
    Before we begin, the whole question seems a bit... uh, screwed up. But maybe I'm wrong. Here's what I did:

    6) NPN (assuming that the emitter is connect to common point, not at bulb)

    7) False statement. When capacitor (C) charges up, transistor turns on. Bulb is lit, and stays lit. The said delay, is between when the switch is opened and the bulb is first turned of.

    8) Increase the value of C.

    9) Remove battery? It sounds like a cheap answer. Any better ones?

    10) ...

    I also made a different attempt afterward:

    6) PNP

    7) During the time C is charging there is a current through, and therefore a voltage across, R. This turns on the transistor until the voltage drops.

    8) R and/or C should be increased.

    9) Remove battery? Sigh.

    10) Should be similar as question 8; R and/or C value is decreased or increased.

    This may be a trick question to begin with, because my teacher seemed pretty confident that none of us in the class would be able to answer.
    Last edited by a moderator: May 4, 2017
  2. jcsd
  3. Jul 19, 2010 #2


    User Avatar

    Staff: Mentor

    I think your answers are fine. I agree that the question is wrong -- with neither the NPN nor PNP topology will the bulb just light briefly at switch opening.

    BTW, to keep the bulb from lighting in the NPN case, you can do something with the value of the resistor that keeps the bulb current very small (too small to light it) -- what can you do with the R value?
    Last edited by a moderator: May 4, 2017
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