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Capacitor current

Discussion in 'General Electronics' started by Patrick Cobb, Jul 3, 2003.

  1. Patrick Cobb

    Patrick Cobb Guest

    I need to know how to find the number of amperes a .3 farad capacitor can
    put out if it is charged to 7 volts.
     
  2. Robert Baer

    Robert Baer Guest

    Use I = E / R for initial current, if the resistance is large compared
    to the ESR of the capacitor.
    Remember that the discharge current drops exponentially.
     
  3. Robert Baer

    Robert Baer Guest

    Sorry, that is incorrect.
    Connect a resistor of 7 meghoms across the capacitor, and read an
    initial current of 1 microampere.
    Use a 7K resistor, same initial charge, and read 1 milliampere.
    You have something missing.
     
  4. Yes you are correct, but I think time factor for both the resistances
    will remain the same so same amount of current will flow. What do you
    think about this?
     

  5. Yes you are totally correct I was somehow misleaded. I simulated the
    circuit and verified that you are right also my above discussion was
    wrong as time of discharge (exponential current decay) is directly
    proportional to the value of the resistance.
    Although this is a discussion one can not always be right, but I
    apologize for my wrong discussion.

    Animesh Maurya
     
  6. Robert Baer

    Robert Baer Guest

    No; 1uA is *not* 1mA.
    However, the total *charge* is the same (Q=c*v) but after that
    everything else changes.
    The 7Meg load will take 1000 times longer to reduce the voltage to 37%
    (one time constant, T=R*C) of initial as compared to using a 7K load.
     
  7. unitron

    unitron Guest


    Since the formula for a capacitor's time constant is T=RC any change
    in resistance (R) will change the the time constant. As with any
    other circuit, the more resistance the smaller the current. The
    smaller the current the longer it can flow at that smaller rate before
    the capacitor is discharged. Decreasing the resistance will allow
    more current to flow, but, since the capacitor holds a finite amount
    of charge, the time over which that greater current will flow before
    the capacitor is discharged will be shorter.
     
  8. Robert Baer

    Robert Baer Guest

    Aie-Yup!
     
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