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Homework Help: RL circuit time constant question

  1. Jun 10, 2012 #1

    Was hoping someone could help me.

    I have the following RL circuit:

    20V input
    8.2kohm resistor and 50mH Inductor in series.

    Determine time constant:

    I answered with: T = L/R 50mH / 8.2kohm

    = 6.09 uS

    Is this correct?

    The question I am stuck on is:

    Determine the value of VL after 1.5 time constants?

    I have the formula, VL = E ( e - t / T)

    I cant work out what to put for the t? I tried putting in:

    20 (e - 1.5 / (6.09uS) but cant be right?

    If someone could help I would be much appreciated

  2. jcsd
  3. Jun 10, 2012 #2


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    Homework Helper

    1.5 time constant means that that t/T=1.5. Substitute 1.5 for t/T into the formula of VL, but do not forget using parentheses.

  4. Jun 10, 2012 #3
    Thanks for the quick response!

    I am getting VL = 4.46V, I was wondering if you could please confirm this?

    I am a little confused because the teacher has written the answer as VL = 15.54V

    Does this mean I just need to take 4.46 from 20 to get the answer?

    Thanks again
  5. Jun 10, 2012 #4


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    Homework Helper

    VL(t) is different if the source is switched on or off at t=0. You calculated VL after the battery is switched off. In case of switching on, VL=E(1-e-t/T).

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