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What is load capacitance

Discussion in 'Electronic Basics' started by Wong, Sep 17, 2003.

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  1. Wong

    Wong Guest

    What is load capacitance ? How does it affect the circuit ?
  2. Dana Raymond

    Dana Raymond Guest

    Load capacitance is the capacitance seen by the device driving the load.
    In the case of OPAMPs too much load capacitance can create problems because
    as the device tries to increase the load voltage the capacitance will pull a
    lot of current (remember, capacitance causes current to lead voltage).

    So, when driving capacitive loads from an opamp you need to ensure that its
    within spec or add a series resistor between them to limit that initial
    inrush current.

    Dana Frank Raymond
  3. Mantra

    Mantra Guest

    First, a "load" is an impedance to which power from a circuit is
    delivered (and either dissipated in the load or reflected backed to
    the source circuit). Impedance is, very approximately, "The AC
    equivalent of DC resistance with phase shifting added, both of which
    change of over frequency". Examples could include antennas, loud
    speakers, AC motors and appliances, lights, logic gate inputs, etc.

    If you can assume only linear circuit components are used, you can
    reduce *every* circuit, without approximation to just three
    components: a power source (voltage or current), a source impedance
    and a load impedance. The power source and source impedance
    combinations are also known as Thevenin and Norton sources, thus every
    circuit can be reduced to two components: a Thevenin/Norton source and
    a load impedance. If you can't assume linear, you can still reduce to
    three or two, but with certain limitations. Components like
    resistors, capacitors and inductors are practically linear.
    Semiconductors and vacuum tubes are definitely non-linear, thus there
    are limitations.

    If the load impedance has a primarily capacitive reactance, or
    negative imaginary impedance, then you have capacitive load, which can
    be modeled as a load capacitance. The load could also be inductive
    (positive imaginary impedance), or resistive (zero imaginary
    impedance). You can have combinations also forming
    resistive-capacitive or resistive-inductive impedances. I'm assuming
    you know complex numbers, or at least, trigonometry and right
    triangles: impedance is always the hypoteneuse, resistance one side
    and (inductive - capacitive) reactance the other side.

    The specific effect on a circuit depends on what the circuit is. In
    digital circuits a capacitive load acts to slow down pulse rise times.
    In RF/microwave it creates a phase shifted reflection of incident
    power back into the power source. In audio it can create a low-pass
    filter effect. In power electronics (e.g. AC motors) it creates a
    phase shift for starting a motor or for compensating power factor
    shift caused by the motor. Sometimes these are intentional effects;
    sometimes not. A common thread is that power is not fully used to its
    theoretical maximum: only purely resistive loads dissipate any power;
    "reactive" power is reflected back.

  4. Steve

    Steve Guest

    What is load capacitance ? How does it affect the circuit ?

    geez... talk about opening a can of worms!

    "load capacitance" is simply the capacitive element of a load.

    For example, an amplifier circuit driving an 8 ohm speaker will
    experience not just the 8 ohm resistive load, but some "inter-turn"
    capacitance and some inductance, both due to the coil windings.

    Capacitive loads can affect the rise and fall times, the frequency
    response and phase of a signal etc. If the capacitance is large
    enough, it could even swallow up the signal entirely and just give out
    a DC level!

    Capacitors present less of a load to high freqency than they do for
    low frequency. Thus, a cap to ground acts as an "AC ground" and
    effectivly becomes a low pass filter.

    What is the circuit you are analysing? That would help people to give
    a more precise answer.

  5. Wong

    Wong Guest

    What is the circuit you are analysing? That would help people to give
    I am interesting on load capacitance value for a 32.726KHz crystal.
    That frequency is feed to a microcontroller.
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