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Capacitor Discharge Time: How can I calculate it?

Discussion in 'General Electronics Chat' started by Splash1, Feb 6, 2009.

  1. Splash1

    Splash1 New Member

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    Capacitor

    Voltage: 400 Volts
    uF: 14

    If I want to run a device with that capacitor... How can I calculate the time?
    (Device: 250 Watt)
    I know the time is in MiliSecs, but I don't know how to calculate it.

    Second question:
    if I have 3 capacitors like the first one (connected in serie)
    A) What is tht total voltage?
    B) Total uF?
     
  2. phoenox

    phoenox New Member

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    The time constant (tau) capacitor circuit is resistance * capacitance. This is the amount of time it takes the voltage to fall to 1/e (37%) of the original value.

    in your case ..

    P = V^2 / R
    R = 400^2 / 250 = 640 Ohms

    tau = 640 X 14 e-6 = 9 ms = .009 seconds

    so after 9 miliseconds the voltage will be 400/e = 147 V
    after 18 miliseconds .. 147/ e = 54 ms

    Question 2 :

    The total voltage will still be whatever voltage you apply to the circuit. The voltage across each capacitor will be 1/3 of this.

    The total capacitance will be1/( 1/C1 + 1/C2 + 1/C3)

    = 1/(1/14 + 1/14 + 1/14) = 4.7 uF
     
  3. OutToLunch

    OutToLunch New Member

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    there will be a time constant if the load is resistive. if it is a constant current load, then just use dv/dt=i/C

    if you want to use more caps to get a longer time period of operation, put them in parallel.
     
  4. dave

    Dave New Member

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  5. Splash1

    Splash1 New Member

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    Sorry, I meant parallel instead of serie.

    How affect the parallel caps connection, to the voltage and the uF?
     
  6. OutToLunch

    OutToLunch New Member

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    total capacitance of parallel capacitors is the sum of the capacitances.

    i have no idea what you mean by affect on the voltage - since the caps are in parallel, they will have the same voltage across them.
     
  7. Splash1

    Splash1 New Member

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    Example:
    parallel connections of 4 capacitors:
    Each one: 400 Volts 16 uF

    I think the total is: 400 Volts 64 uF, right?
     
    Last edited: Feb 6, 2009
  8. GCA

    GCA New Member

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    Looks right to me.
     

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