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Current swing in bridge amp

Discussion in 'Electronic Basics' started by Tim, Jan 9, 2005.

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  1. Tim

    Tim Guest

    Can someone please confirm or modify the following, given a bridged
    audio amp outputting a sinewave to an inductive load.

    1. Reading across the load, the _voltage_ will be seen to rise and
    fall as a positive value every full cycle.

    2. However, the _current_ changes direction every half cycle, at the
    peak and trough of the sinewave.

    For example, if two wires were arranged side-by-side so their EMF
    cancels, they would effectively see twice the signal frequency in
    terms of current flow.

    IOW the cancellation frequency, being once in each direction, would be
    twice the conventional sinewave frequency.

    Thank you for any comments or corrections.

  2. Right. The current will change in one direction (decreasing, passing
    through zero and increasing) as long as there is voltage across the
    inductance of one polarity. As soon as the voltage reverses polarity,
    the current stops increasing in that direction and starts to decrease,
    pass through zero and increases in the other direction (all change in
    one direction). The formula that relates inductive voltage to current
    is V=L*(di/dt) with V in volts, L in henries and di/dt in amperes per
    Are you saying the same voltage is applied to both wires, but of
    opposite polarity? If so, their currents would be equal but
    opposite. No new frequencies.
    No. Everything except power occurs at the same fundamental frequency.
  3. CBarn24050

    CBarn24050 Guest

    Subject: Current swing in bridge amp
    No. How could that possibly be? Firstly it should be obvious that the voltage
    across the load must reverse if the current through it reverses, there will of
    course be some phase shift between them. Secondly, to get a doubleing of
    frequency you need to generate the second harmonic somehow, that means passing
    through some non linear element, an inductor is linear, so no way there either.
  4. Ban

    Ban Guest

    The voltage changes signs as often as the current, it might be they are not
    in phase, but of the same frequency, somehow you are imagining something
  5. Jim Denton

    Jim Denton Guest

    Perhaps I did not explain my thoughts well enough.

    To simplify, let's say there are two bridge outputs each driving a
    separate length of wire. There is no reference to earth.

    These wires are then laid side-by-side but in reverse directions, such
    that when the current is following one way in one, it will be moving
    oppositely in the other.

    We then apply a 100Hz sinewave of the same phase to each, which, for
    the purpose of this discussion, can be said to cancel each other out.

    So then, while the applied frequency is 100Hz, there are actually two
    _cancellations_ per cycle due to current reversal. One each
    corresponding to the peak and trough of the sinewave. Hence it occurs
    at 200Hz.

    This is simply a thought experiment. I realize the effect is not

  6. They do not cancel twice per cycle, they cancel continuously
    throughout the cycle. Zero is zero. If the cancellation is not
    perfect, then there is a residual that is a smaller (and possibly
    phase shifted) version of the original waves.
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