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RC with limiting current supply

Discussion in 'Electronic Design' started by Efthimios, Jan 6, 2009.

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  1. Efthimios

    Efthimios Guest

    Let imagine a RC circuit that requires 20 uAmp to charge in 0.2 sec.
    But the electric source can supply only 1uAmp. Voltage is not a
    problem. We have enough to pass through the resistor.

    The question is what will happen with this limited current???

    I think it will take more time to charge the capacitor compared with
    the theoretical RC value. Is this correct????

  2. Tim Shoppa

    Tim Shoppa Guest

    Purposefully limit the supply current, and you have just invented the
    linear ramp generator. One of the textbook definitions of capacitance,
    after all, is I = C dV/dt.

    I don't think the RC value means what you think it means, unless the
    charging cure is at least mostly exponential. For example, look at
    tiny portions of the exponential curve but not the whole thing: you'd
    come up with entirely different dV/dt's. In fact limiting the
    application to a tiny portion of the curve is a time-proven method for
    waveform generation.

  3. Efthimios

    Efthimios Guest

    I thought that the time a capacitor take to charge and discharge
    dipends on the RC constant??? Half charge time 0.693RC.

    Although i don not know what is a linear ramp generator (google did
    not help much) i have not understood what will happen if the supply
    current is limited.

  4. Tim Shoppa

    Tim Shoppa Guest

    Yeahbut that only matters if the R is limiting the current. In the
    example you gave, it doesn't. So R becomes irrelevant.
    Again, a textbook definition of the relationship between charge rate
    (dV/dt), current, and capacitance is:

    I = C dV/dt

    You'll see that as long as the current remains constant, dV/dt will
    remain constant and that's the definiton of a linear ramp.

    Now in real life, keeping I constant, or for that matter keeping C
    constant, can become a challenge :).

  5. Jasen Betts

    Jasen Betts Guest

    yeah, it'll take much longer
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