Step Response Problem

Discussion in 'Homework Help' started by xz4chx, Nov 13, 2012.

  1. xz4chx

    Thread Starter Member

    Sep 17, 2012
    Circuit Problem

    My work I have shown on two different images
    Work of Problem Part 1

    Work of Problem Part 2

    Sorry the images are sideways. I just noticed that right after i posted them on ImageShack

    The right answer is -2.70 mA and I have -1.50 mA

    Am I working the process correctly?

    So part 1 I found that the voltage of the capacitor is -9.9V at t=0^- since it is a open circuit voltage since Switch A was closed for a long time and Switch B was open for a long time before t=0

    Part 2 I used an integral to find the voltage which I have a feeling is the part where i messed up for t=0^+ in order to find the initial voltage at t=100[ms] in order to find the function of voltage for t>=100[ms]

    For part 3 which is the second picture, I used the formula to find capacitor V(t)= Steady State Voltage + (Initial Voltage (V(200[ms]) - Steady State Voltage)e^(t-t_intial)/τ [V] for t>=100[ms] and τ=CR_thevinin
    So I found Thevinin Resistance to find τ (Time constant)

    I feel besides part two that everything is sound but I have no idea.

    Thank you for the help