Simple RC circuit question

Discussion in 'Homework Help' started by secondhandloser, Oct 26, 2008.

  1. secondhandloser

    Thread Starter Member

    Sep 30, 2008
    Question attached in PDF form.

    As far as what I've done, I wrote up the condition required while the voltage across the SCR is less than 5v

    after the switchover, I'm having difficulty writing the equation, as I don't understand what the voltage difference would be across the SCR at t=∞.
    At t=0, I know it will be 5v, because that's what it is before the changeover.

    Any help on even how to set up the equation will be appreciated- I know it's simple, and I'm just blanking on it.
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    Do you have the values for R and C?
  3. secondhandloser

    Thread Starter Member

    Sep 30, 2008

    That's what I'm trying to solve for.
  4. Ratch

    New Member

    Mar 20, 2007

    I will give it a try, but the hour is late and I might make a mistake.

    The voltage across C when first powered up is (6/s)(1/Cs)/(R+1/Cs) . Cleaning it up and finding the inverse Laplace we get 6(1-exp(-t/RC)) . Equating the expression to 5 volts and substituting t=1 sec we get a RC constant of 1/ln6 . Now we write a node equation (V-6)/R +VCs-5C+50E-6 = 0 . Cleaning it up and separating out V we get V = (5RC - R*50E-6 +6)/(RCs+1) . Finding the inverse and substituting t=0.1, and RC = 1/ln6, and equating the voltage to 0.1, I get R = 174 kohms. Then C = 3.2E-6 farads. I hope I did everything correctly.