Thevenin Reduction to find Time Constant

Discussion in 'Homework Help' started by bluetooth tamer, Mar 20, 2015.

  1. bluetooth tamer

    Thread Starter New Member

    Jan 16, 2015
    I'm working on coursework and we recently started learning time constants. For an RC circuit the time constant is C x Req where they are in parallel.

    This problem throws in a dependent current source which I don't have a clue with how to work with other than that Req = Voc/Isc. I tried this and got Req to be 6k however this doesn't get me the provided solution when I take T = RC and plug in 6K*5μ. My answer then becomes 30ms but doesn't match up with the provided solution of 20 ms. It's not like for my professor of 20 some years of teaching this course to make a mistake so I choose to believe that I'm missing something.

    Am I missing some fundamental rule(s)?

    Any help will be tremendously appreciated Capture.PNG .
  2. WBahn


    Mar 31, 2012
    If you show your work for how you got 6kΩ for Req, then we can look it over and help you see where you went wrong.

    But you might ask yourself some sanity check questions first.

    If K=0.5, that means that half of the current that goes through the 1kΩ resistor goes through the dependent source and the other half goes through the 6kΩ resistor.

    If I told you that I have two parallel branches and half of the current goes through each branch and that one branch was a 6kΩ resistor, what would that tell you the other branch looks like?
    bluetooth tamer likes this.
  3. bluetooth tamer

    Thread Starter New Member

    Jan 16, 2015
    Thank you thank you! Your wording helped jog my memory about mesh analysis. I ended up equaling the voltage across the capacitor to V and and then divided V by i1, found with mesh analysis, to get resistance which turns out to be 4k. When I multiplied 4k by 5u I get 20ms which is the right answer. From now on I'll post my work for any problem I don't understand.