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DC transformer amps.

Discussion in 'Electronic Basics' started by hartly, Apr 29, 2005.

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  1. hartly

    hartly Guest

    Why does my 9v D.C. transformer only give a max. amps. of 0.22A?.
    I've looked inside it and it doesnt seen to have any resistor in
    series to restrict the current.
    A 6v. battery charger can give out 4 amps.,which proves its nothing
    to do with the voltage of input or output.
  2. Andrew Holme

    Andrew Holme Guest

    It'll be the rating of the transformer and/or the rectifier diodes.
    The transfomer core can only handle so much magnetic flux before it
    satures. The windings have resistance and dissipate heat. There are
    Eddy currents that also generate heat. The bridge rectifier diodes
    will be of a similar rating to the transformer. I should imagine your
    battery charger is larger and heavier.
  3. Hi,
    But everything to do with losses its iron core and the copper
    wire that it is wound with.

    Cheers - Joe
  4. The output current has to pass through the resistance of the secondary
    winding. This uses up voltage and produces heat. The output current
    (transformed by the turns ratio), in addition to the magnetization
    current has to pass through the primary winding. This also drops
    voltage and produces heat. A transformer is rated for output current
    based on acceptable total voltage drop (called regulation, expressed
    as %) and temperature rise. If they build the transformer with a
    larger core that has a larger window opening, they can use larger wire
    that drops less voltage and produces less heat at a given current,
    raising the current rating. It turns out that the rated power (output
    voltage times rated current) is roughly proportional to the weight of
    the transformer.

    You can get more current out of any transformer, if you accept the
    lower voltage, and either do this briefly enough that the transformer
    doesn't overheat, destroying the insulation, or are willing to put the
    fire out.
  5. John Larkin

    John Larkin Guest

    Its price or its weight are pretty good indicators of how many watts
    it can deliver.

  6. No, core saturation does not act to limit the load current.
    In fact, load current tends to reduce core saturation.
  7. Jamie

    Jamie Guest

    size of the core, wire, etc...

    the wire in the transformer is small, thus creating resistance of its
    Core saturation with inductance etc. etc. etc....
  8. hartly

    hartly Guest

    Thanks Andreww, and others.
    My low current transformer does have 2 diodes in the circuit, but I
    hadnt realized that they also restrict amps like a resistor does.
    So that must be why it gives only 0.22 A

  9. John Fields

    John Fields Guest

    No. The reason you can only get 0.22 amps out of it is because of (to
    a first approximation) the resistance of the primary and the
    secondary. The diodes will drop a _voltage_ somewhere between 0.5V
    and 1V depending on the current going through them. Read John
    Popelish's reply again.
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