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Resistive vs. Capacitve Loads

Discussion in 'Electronic Basics' started by max w., Feb 10, 2007.

  1. max w.

    max w. Guest

    I see these terms often, but I haven't seen a description between the
    two that I
    am comfortable with. Or better yet an example an example of each.
    Any help would be appreciated,
    max w.
     
  2. Tom Biasi

    Tom Biasi Guest

    First do you understand the properties of resistance and capacitance?
    A load is the part of the circuit that is dissipating the power.
    You say you see these terms often, where?
    AC power generators, opamps?

    Tom
     
  3. max w.

    max w. Guest

    Yes, I understand the properties of resistance and capacitance.
    I am trying to find out what they mean when it is applied to digital
    electronics.
    Thanks
     
  4. Pretty well self evident. If you are driving a long wire for example, co-ax
    or the like, it is likely to have significant capacitance. You need to allow
    for this.
     
  5. max w.

    max w. Guest

    I understand that part. Let me explain it like this, and maybe my
    conceptual idea is
    wrong. A digital driver can drive so many pf. The input pin of the
    receiver has an input
    capacitance. The total capacitance of the input pins cannot exceed the
    strength of
    the driver. Are these considered capacitive loads? The more loads you
    have slows
    the driven signal down. I assume this is the capacitve aspect. If this
    assumption is
    correct, what is an example of a resistive load on a PCB? (aside from
    a resistor)
    Thanks
     
  6. John Fields

    John Fields Guest

    ---
    Since there's always _some_ series resistance inherent in the output
    of any driver, the time it will take for the output to rise to a
    certain voltage will depend on the resistance of the source (the
    driver), the resistance of the load, and the load capacitance.

    Assuming your circuit looks like this: (View in Courier)


    VCC>------->\ <---GND
    \
    O
    |
    [Rs]
    |
    +----+--->Vout
    | |
    [RL] [CL]
    | |
    GND>----------+----+--->GND


    If RL is >> Rs, then, since:

    T = Rs CL

    you can see that the time it takes for Vout to rise to a particular
    voltage will increase as CL increases.


    An example of a resistive load would be a coaxial cable terminated
    with a resistive load equal to the source resistance and the cable
    impedance:

    VCC>-------->\ <---GND
    \
    O
    |
    [Rs]
    |
    GND>---------+ | |
    | | |
    | | |
    | | |
    | | |
    | | |
    | | |
    GND>---------+ | +----->GND
    |
    [RL
    |
    +------->VOUT
     
  7. mg

    mg Guest

    For a back-of-the-envelope calculation, capacitances in parallel add
    together. So, for example, if you had 2 loads of 15pf, the total would
    be 30pf. Drivers have a characteristic output impedance. Transmission
    lines (board traces) have a characteristic impedance. Loads
    (receivers) have a characteristic impedance. Impedance is defined as
    the ohmic resistance plus the net reactance, which can be either
    capacity or inductive, or can equal zero. In the case where the net
    reactance is zero, you would have a purely resistive load.

    In situations where the source, line and load impedances are not
    equal, there will be a reflection of the signal. In other words, when
    the source sends a signal to the load, the signal will bounce back to
    the source and distort the signal (i.e. make it less square). The
    exception to this rule is when the length of the transmission line is
    small compared to the wave length of the signal.

    For information on impedance and impedance matching, you could do a
    search on Google, or find a book on the subject.
     
  8. A resistive load passes current in proportion to the
    instantaneous voltage. If there is voltage across the load,
    it is passing current, regardless of time.

    A capacitive load pases current when the voltage across it
    changes, and in proportion to how fast it is changing. Any
    steady voltage drives no current through a capacitive load.

    So resistive loads draw current throughout the time when the
    source applies voltage to it. The capacitive load draws a
    spike of current when the source tries to step the voltage,
    and slows the rise time of the step, if the source has
    series resistance.

    If the load is a resistance and capacitance in parallel,
    then the source must supply both currents simultaneously.

    If the load is a series combination of resistance and
    capacitance, the capacitor blocks any DC current, and the
    resistor limits the current that can be passed during any
    fast rate of rise or fall, but extending the time the
    current passes, after the step in voltage has stopped
    changing. There will be and exponential decay of current as
    the resistor drop shrinks, and the capacitor sees the fill
    applied voltage gradually, instead of as the source changes it.
     
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