Calulating time constant

Discussion in 'Homework Help' started by kevinh111, Jan 19, 2012.

  1. kevinh111

    Thread Starter New Member

    Jan 19, 2012
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    0
    Hey guys, have a question on a homework assignment related to RL time constants.

    "A constant voltage is applied to a RL circuit at time t=0. The voltage across the inductance is 20 V at 3.46ms and 5 V at 25ms. Obtain R if L = 2H."

    Given the equation T(tau/time const.) = L / R, how do I first calculate the time constant of the circuit in order to calculate the resistance?
     
  2. t_n_k

    AAC Fanatic!

    Mar 6, 2009
    5,448
    782
    If the DC supply is Vd

    Then

    V_L(t)=V_de^{\frac{-t}{\tau}}

    at t=3.46ms

    V_L(3.46ms)=V_de^{\frac{-0.00346}{\tau}}=20

    at t=25ms

    V_L(25ms)=V_de^{\frac{-0.025}{\tau}}=5

    Solve the two equations to find Tau
     
  3. kevinh111

    Thread Starter New Member

    Jan 19, 2012
    2
    0
    Awesome, thanks for the quick response.

    I am having some difficulties solving for Tau, the answer I obtained is that tau = 0.00465, using mathematica.
     
  4. t_n_k

    AAC Fanatic!

    Mar 6, 2009
    5,448
    782
    The correct value for the time constant is 15.54ms.

    Divide the two equations and take the natural logarithm of the result.
     
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