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Average Current

Discussion in 'Electronic Basics' started by Jack, Oct 16, 2004.

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  1. Jack

    Jack Guest

    hi all

    For a 1n540x diode average forward current is given 3amps, read in the data sheet.

    Is this average current the same what i measure in my dmm.

    Thanks for reply
  2. Almost certainly. But I never use a component at its absolute maximum
    rating, voltage, current, or power.
  3. Jack// ani

    Jack// ani Guest

    Thanks John

    I probably knew that, just wanted to get assured of it.

    Ok now it will be wise to ask, suppose i want to get 3amp dc out of it
    using bridge rectifier, what should be the transformer amps rating.

    By calculation it should be 3.33amps,


    so, Irms=3.33amps

    It that correct? And will it depend upon full/half wave rectifier?
  4. The diodes have an average current rating because they drop a nearly
    constant voltage over a wide range of current. So the heat produced
    is fairly closely proportional to the average current, as long as you
    don't get into extreme cases (of very large pulses widely spaced).
    Transformers are heated by the square of current, like resistors,
    because their windings are resistive. So they have an RMS current
    rating. RMS stands for square root of the mean (average over a
    representative time period) of the square of the instantaneous
    current. The proportion of the RMS current from the transformer to
    the average DC rectifier output current depends a lot on what follows
    the rectifier. If the load is a resistor, then the RMS load current
    is also the RMS transformer current. In this case, the RMS load
    current is 1.11 times the rectifier average current.

    If the load is primarily capacitive, all the rectifier current is
    confined to the brief periods when the transformer voltage is higher
    than what is stored in the capacitor, so the RMS current can get quite
    a bit higher than the average current. This affects the current
    rating required in the transformer.
    It is correct for a resistive load and a full wave rectifier (load
    current proportional to absolute value of instantaneous transformer
    voltage). Half wave rectifiers pose additional problems for
    transformers that involve core saturation that don't occur with full
    wave rectification. And the ratio of RMS to average current is also
    different (higher).

    What load will your rectifier be driving?
  5. Jack// ani

    Jack// ani Guest

    Just two more questions, John

    1. Reactance offered by a capacitive load is always higher that a
    resistive load, then why should it extract more current?

    2. Why do capacitor charges to peak ac voltage if placed across a
    bridge rectifier? Without a cap i measured 11v, and it increased to
    16v, after hooking a cap?

    Thanks for your help
  6. Reactance is a linear concept that applies to sine waves. To get your
    mind around a capacitive input rectifier filter, you have to think in
    the more general differential description of capacitance. I=C*(dv/dt)
    current equals the capacitance times the time rate of change of
    voltage across the capacitor.

    As long as the capacitor voltage is equal to or greater than the
    transformer voltage, the rectifier isolates the two. But the moment
    the transformer wave rises above the capacitor voltage, the rectifier
    is essentially a short circuit, and the voltage on the capacitor must
    rise as fast as the transformer wave is rising, regardless of how much
    current that takes. So the current into a capacitive filter is narrow
    sort of half sine wave pulses that occur on the part of the
    transformer voltage wave just before the peak voltage. Since the
    transformer windings are heated by the RMS current, a pulse waveform
    like this has a much higher RMS value than the average of the current
    in those pulses. This is what the 'squared' part of the RMS does. It
    is not unusual to have to double (or more) the transformer RMS current
    rating relative to the DC output average current when using a
    capacitor input rectifier filter to take care of this higher winding
    RMS current. The exact ratio depends on how much leakage reactance
    there is between primary and secondary windings that tends to spread
    out the charging pulses, by sagging the waveform a bit while the cap
    is charging, lowering the slope a bit. If you look at the transformer
    waveform with a scope, you can see the flattened spot on the wave
    where the cap is charging just before peak voltage.
    The diodes act as check valves, pumping the cap up all the way to the
    transformer waveform peak voltage and then turning off, leaving that
    voltage trapped in the capacitor. A resistor load on the rectifier
    keeps it on the whole waveform so the resistor voltage is the same as
    the transformer waveform (except for the inversion of one half). your
    meter reads the average of the rectified transformer waveform instead
    of the peak value.
  7. Jack// ani

    Jack// ani Guest

    Thanks a lot.
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