How to Find Equivalent Resistance and Current Across Resistor for RL Circuit?

Discussion in 'Homework Help' started by Retrograde, Dec 5, 2018 at 5:03 AM.

  1. Retrograde

    Thread Starter New Member

    Hi everyone! I've never thought that in all my review with RC and RL circuits, I would come across one as difficult as this! So the problem asks me to find the time constant for the RL circuit when the switch closes at t = 0, and to find the current across the resistor R3 in terms of time. For the time constant, I attempted at redrawing the circuit at t = infinity to represent steady state and find the equivalent resistance from there. My experience with RL/RC circuits and circuit analysis has suggested to me that the inductor becomes a short circuit and that I can use superposition with the voltage source, and the current source to find Req. My understanding is that R3 is parallel to the series combination of R4 + R5, followed by R2 being in series with all of that. However, I do not know if this is completely correct. Part of me thinks that R1 is parallel to all of that, so when I attempted at doing this approach, I ended up with Req = 1.59 ohms, which for the time constant, led to tau = 0.000000630 seconds. I honestly think this is very likely wrong, but I could be wrong as well?

    As for finding the current i(3) across the R3 resistor, I was thinking that if I find i3 at t<0 and t = infinity, both of which are at steady state, I can put them into a general solution equation, where i3(t) = i3(t=infinity) + [i3(t<0) - i3(t =infinity)] * e^(-t/tau). I tried using the mesh current method to find i3 for both cases, wherein the voltage source remained intact for t<0, and the current source only being present for t=infinity. I don't know if this was the right approach, but I did end up with i3(t<0) = 2.67 A and i3(t=infinity) = 1.78 A. It took a lot of re-drawing of the circuit, but I'm not sure whether this is a correct approach as well into finding i3(t)?

    I've spent several hours nonstop trying to solve this problem, but I'm more surprised that I've been stuck for so long! Is there anyone that can help me with this, or point me in the right direction?

    AA1 05-Dec-18 10.08.gif
    Last edited by a moderator: Dec 5, 2018 at 5:10 AM
  2. ericgibbs


    Jan 29, 2010
    Hi Retro,
    Welcome to AAC.
    Please post your attempt at solving the problem so that we can guide you.
    Reposted your image.
    Retrograde likes this.
  3. Retrograde

    Thread Starter New Member

    Hello Eric! Oh dear, I believe you're right: just attempted at opening up the attachment only to find it fail. I posted an updated PDF that should include all of previous attachment!

    AA1 05-Dec-18 10.22.gif AA1 05-Dec-18 10.23.gif
    Last edited by a moderator: Dec 5, 2018 at 5:26 AM
  4. WBahn


    Mar 31, 2012
    You know it's a first order circuit, right?

    So you know the form of the equation after the switch closes. For the inductor current, you just need the initial current, the final current, and the time constant.

    For the time constant, you need not the resistance seen by one of the supplies at infinite time, but the resistance as seen by the inductor at all time after the switch closes. Think Thevenin.