example problem about capacitor

Discussion in 'General Electronics Chat' started by PG1995, Oct 27, 2011.

  1. PG1995

    Thread Starter Active Member

    Apr 15, 2011
    753
    5
    Hi

    Please help me with the problem in the attachment. It would be kind of you. Thanks.

    Regards
    PG
     
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    Last edited: Oct 27, 2011
  2. jimkeith

    Active Member

    Oct 26, 2011
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    99
    Q1: Vo(t) simply means that voltage is a function of elapsed time (after switch is closed)

    Q2: calc the Thevenin equivalent resistance (resistors are left to right)

    Requiv = r1 * (r2 + r3) / (r1 + r2 + r3)

    T = Requiv * C

    Hope this is helpful
     
    Last edited: Oct 27, 2011
    PG1995 likes this.
  3. PG1995

    Thread Starter Active Member

    Apr 15, 2011
    753
    5
    Thanks, Jim.

    But I'm getting wrong answer. I have solved the problem the end. You can have a look on the attachment to see the procedure I have followed. Please help me. Thank you.

    Regards
    PG
     
    Last edited: Oct 27, 2011
  4. ErnieM

    AAC Fanatic!

    Apr 24, 2011
    7,394
    1,606
    Your procedure to calculate tau was correct, you just didn't use uf for the cap:

    tau = R*C = (3K * 6K)/9K * 100uF = 2E3 * 100E-6 = 0.2

    However I don't track you when you say:

    Vc(t) is 8-8e^(-t/0.6)

    Where did you get that 0.6 factor? This does not seem to be the correct answer. I think it is:

    Vc(t) is 8-8e^(-t/0.2)

    That should be calculated using the same R computed as the Thevenin resistance. You find that when you "look" into the circuit where the cap is, short the voltage source and compute the resistance. jim got it right.

    I'm not sure what "what is the resistance across the capacitor" means, the question is a bit vague. Does it mean just R2 and R3 or does it also include R1?
     
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