Cap charging & discharging

Discussion in 'General Electronics Chat' started by aamirali, Apr 10, 2012.

  1. aamirali

    Thread Starter Member

    Feb 2, 2012
    415
    1
    I have a voltage source connected to resistor, as in figure. Lets say i have 1.5V dc, 10K & 1nF.
    1. Cap will charge >90% in 5*R*C.
    2. Cap will also get 1.5v. Rough approx.
    3. Now if my voltage source starts to fall & get 1V. So now cell=1V, cap=1.5v . What will happen now, how to analyse the ckt.
     
  2. #12

    Expert

    Nov 30, 2010
    16,298
    6,810
    Vo = dV x (e^ -t/RC)
    Volts out equals change in voltage times e to the negative time over (resistance times capacitance)

    In this case, Volts out = (Volts on capacitor at the start) minus? change in volts times e to the -t/RC

    unless I flopped a minus sign. If you calculate more than the battery voltage, switch a minus to a plus
     
  3. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    The math works, but so does the graphical approach...

    [​IMG]

    5TC = 99.3%, give or take.

    This is from a thread I had about a year ago on RC time curves.

    RC oscillators like the 555 live or die by this math.
     
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  4. crutschow

    Expert

    Mar 14, 2008
    13,016
    3,235
    Just remember that the capacitor will always charge (or discharge) to the new voltage with a exponential change between the old and new voltages based upon the RC time-constant.
     
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  5. ErnieM

    AAC Fanatic!

    Apr 24, 2011
    7,389
    1,605
    One formula that continues to come in handy is to compute the time between two arbitrary voltages in an exponential charge. This formula always works for an RC network:

     T= RC ln\frac{(Vss - Vi)}{(Vss - Vf)}

    Where:

    T = time to change between Vi and Vf
    R = resistance
    C = capacitance
    ln = natural log
    Vss = Steady State voltage (what the voltage goes to if you wait forever)
    Vi = initial (starting) voltage
    Vf = final voltage
     
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  6. #12

    Expert

    Nov 30, 2010
    16,298
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    Thanks. That form covers the scenario when Vi is not zero.
     
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