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How to calculate time constant tau for RL and RC circuits from their graph?

  1. Sep 28, 2009 #1
    I made graphs of simple RC and RL circuits. We were told to mark off at 10% of the rise and 10% away from the max. This gives a delta. We were then supposed to use this information form the graph to calculate tau, and compare it with the theoretical value. I know how to calculate the theoretical value, for RC it's just tau = RC and for RL it's just tau = L/R. But then how do you calculate it from the graph?
     
  2. jcsd
  3. Sep 28, 2009 #2

    kuruman

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    Did you make a plot with linear axes or a semi-log plot?
     
  4. Sep 28, 2009 #3
    I guess I used linear axes, I don't know what that other thing is.
     
  5. Sep 28, 2009 #4

    kuruman

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    Ok, so you have two separate times that you can read off the graph, t1 when you get 0.1 (or 10%) of max and t2 when you get 0.9 (or 90% of max). This gives two equations

    0.1 = e-t1
    0.9 = e-t2

    What do you get if you divide the top equation by the bottom?
     
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