RL Circuit Help

Discussion in 'Homework Help' started by Digit0001, Mar 24, 2011.

  1. Digit0001

    Thread Starter Member

    Mar 28, 2010
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    0
    Can someone help me solve this RL Circuit. My problem is how would i get the Resistance value?

    I simplify the circuit using thevenins theorem and then i calculated two equations when i have two meshes, however this did not work.



    P.S
     
  2. Georacer

    Moderator

    Nov 25, 2009
    5,142
    1,266
    Care to post your equations? We can help you better if we can see where you go wrong.

    Also are you sure you want to short-circuit that inductor? It's ok if you want to study the DC state of the circuit, but if you want to find the transients, you aren't allowed to do it.
    Please clarify.
     
  3. Digit0001

    Thread Starter Member

    Mar 28, 2010
    89
    0
    i didn't bother posting up the equations because even if i solved them it gave me zero.

    Here it is anyways..

    (1)
    3I+I-I1-2(3I) = 0
    -2I-I1 = 0

    (2)
    5I1-2(3I1)+I1-I = 0
    -I = 0


    Well i am trying to find the resistance so then i can find the time constant τ

    where it is τ = L/R.
     
  4. t_n_k

    AAC Fanatic!

    Mar 6, 2009
    5,448
    782
    The equations are incorrect. The problem requires a transient analysis method - either in the time domain by the use of differential equations or in the 's' domain by the use of Laplace Transform.

    The Laplace transform approach is simpler, provided you are acquainted with the method - particularly with respect to the handling of initial conditions.
     
  5. Digit0001

    Thread Starter Member

    Mar 28, 2010
    89
    0
    can someone tell me if the equations are correct. If they what should be the next step in solving?

    V = 1/6 di/dt + 3i

    (V-2x) + V/5 + i = 0
     
  6. t_n_k

    AAC Fanatic!

    Mar 6, 2009
    5,448
    782
    I think your second equation should read

    V-2Vx + V/5 + i =0

    You need a third relationship, namely

    Vx=-3i

    With your three equations you can then eliminate V to give a first order differential equation in the variable 'i'.

    Re-arrange the top equation in this post to give

    V(1+1/5)=2Vx-i=2(-3i)-i=-7i

    So

    V= -35i/6

    Equate this with the top equation in your last post, viz

    V = 1/6 di/dt + 3i

    Hence,

    -35i/6=1/6 di/dt + 3i

    Then re-arrange and solve your differential equation ... which I'll leave to you.

    The necessary boundary condition will be

    i(0)=5A
     
  7. Digit0001

    Thread Starter Member

    Mar 28, 2010
    89
    0
    thanks for the help
     
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