universal time constant formula

Discussion in 'Homework Help' started by ra1ph, Jan 8, 2010.

  1. ra1ph

    Thread Starter Active Member

    Jan 5, 2010
    31
    0
    Hi i need your help , new to this forum.

    I'm a first year electronics student with semester 1 examination tomorrow.
    I'm trying to understand the universal time constant formula for capacitors. I understand the formula when given: time = >0 my problem is when time = Zero.
    I have read and understand the example given on this site

    Volume I - DC » RC AND L/R TIME CONSTANTS »

    Could someone help me or point me to an example where t = 0?

    Thanking You
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    Look at it this way - voltage can only change as the time changes.
     
  3. Boo

    Member

    Oct 27, 2009
    40
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    the expression for charging or discharging always e^-t/RC. Where Rc is the time constant. If 0 time has passed we get e^0 which is simply 1, in other words, nothing has changed.
     
  4. ra1ph

    Thread Starter Active Member

    Jan 5, 2010
    31
    0
    Ok thanks, My problem was with the (final-start) voltage or current part of the formula. I get it now- nothing has changed!

    Thanks
     
  5. Boo

    Member

    Oct 27, 2009
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    0
    Remember that voltage does not change instantaneously in a capacitor!
     
  6. Wendy

    Moderator

    Mar 24, 2008
    20,765
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    And current doesn't change instantaneously in an inductor. Same principal, similar math.
     
  7. ra1ph

    Thread Starter Active Member

    Jan 5, 2010
    31
    0
    Thanks!
    so the following formula is valid for when t = 0

    change=(final-Start)(1-1/e^(t/RC))

    because change = 0 always when t = 0

    It does not matter what the final and start voltages are. I expected to get what the actual voltage or current were are time = 0 when using the formula mentioned.

    So what formula should I use to get the actual values when t = 0 or is just a understanding that you need?
     
  8. beenthere

    Retired Moderator

    Apr 20, 2004
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    If the problem starts at T = 0, then the initial states are as given.
     
  9. hitmen

    Active Member

    Sep 21, 2008
    159
    0
    Just remember the magic formula

    X(t) = x(∞) + x(0)e^-T/tau

    This is what I remembered if I remembered it correctly:D
     
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