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Inductance, volt drop and time constant? Help

  1. Aug 30, 2014 #1
    Good evening all, I am looking for some clarity on a question I am answering at the moment.

    An inductor of negligible resistance and an inductance of 0.2H is connected in series with a 330ohm resistor to a 12v d.c. Supply determine:

    A - the time constant of the circuit
    B - the voltage drop across the inductor after two time constants
    C - the voltage drop across the inductor after three time constants
    D - the resistance of a 0.2H coil used to replace the inductor if the circuits time constant falls to 0.55s

    Attempt so far
    I = V/R = 12/330 = 36.36mA
    A - t=L/R = 0.2/330 = 606microseconds
    B - I'm trying to use Ve exp -Rt/L but it does seem to be giving me any answers that make sense?!
    C - same as B
    D - R= L/t = 0.2/0.00055 = 363.64 ohms

    Any help would be greatly appreciated.

  2. jcsd
  3. Aug 30, 2014 #2


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    Staff: Mentor

    I moved your thread to our homework section.

    What do you get?

    At (D), your value for t in the formula does not match the value given in the problem statement. I guess that should read 0.55ms?
    Last edited: Aug 30, 2014
  4. Sep 1, 2014 #3
    Hi sorry for the delay i had a busy weekend.

    When using the above equation i get as follows;

    B - 12 exp -(330 *0.0012)/0.2 = 12 exp -1.98 = 1.66v
    C - 12 exp -(330 *0.0018)/0.2 = 12 exp -2.97 = 0.62v

    Regarding D yes sorry that should have read 0.55ms
  5. Sep 1, 2014 #4


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    Staff: Mentor

    Apart from rounding errors, it is correct. Note that you can directly plug in -2 and -3 in the exponents, without calculating the time.
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