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RLC circuit v(t)

  1. Apr 6, 2015 #1
    http://imgur.com/hAzf2zP RLC ciruit quiz.PNG

    So I found the circuit is series, w0 and a0 equal 20 so it is critically dampened. For V(inf) I used current divider to get the equation (.1 A *120 ohm)/40 ohm = .3 A. Then I used ohms law to find v(t) = .3*120 = 36 V.

    I then wrote down the circuit at t<0, t>0, and t=inf. I am stuck at this point.
  2. jcsd
  3. Apr 6, 2015 #2


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    so t<0 will give you your initial conditions
    t inf will give you your steady state value.
    T>0 will give you the differential equation that you are looking for.

    write out the differential equation first and solve it, either in time or frequency domains
  4. Apr 6, 2015 #3


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    Your V(inf) calculation is wrong. Substitute the two resistors to the right and the current source with a Thevenin equivalent: 40Ω in series with a 4V voltage source.
    So V(inf) = 4V.

    The current through the switch, I(0-) will be (4V - (-6V) ) / ( 40Ω + 40Ω ) = 0.125A (from right to left ).

    So V(0-) = -6V + 0.125A*40Ω = -1V.
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