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Transfomer question

Discussion in 'Electronic Basics' started by Harry Muscle, Dec 8, 2003.

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  1. Harry Muscle

    Harry Muscle Guest

    I have a large transforme at 250VA. It's got three windings, one for
    the 120AC, one for 14.5AC, and one for 28AC. I hooked up the 120AC to it
    with no load attached to any of the other windings, and they measured 14.4AC
    and 28.4AC. What surprised me is that these readings were so close to the
    printed value with no load attached. I was under the assumption that
    usually the volts are a bit higher until you connect a load rated at what
    the transformer can provide. Does this mean that I should expect the volts
    to drop once I connect a load to it? Or is what I'm experiencing usual for
    transformers this big. I'm just trying to estimate that vol I will
    receive with a 100W load attached. Unfortunately I don't have a way of
    testing right now since I don't have a 100W load handy and to make the load
    that I will be using I need to find out the volts first. Catch 22.

    Thank you for any help,
    Harry
     
  2. Ian Stirling

    Ian Stirling Guest

    A 15A 120V AC heater (or several smaller) will equal 100W at 28V.
     
  3. The regulation of transformers improves as they get larger. If the
    regulation is limited by the temperature rise, and the volume and
    power output go up approximately as the cube of the size, while the
    surface (which dissipates the heat) goes up only by the square of
    size, it is clear that they can't get away with the high losses on big
    transformers.

    Put some managable load on the transaformer and measure the regulation
    with that load, and extrapolate the sag linearly to your expected
    load.
     
  4. default

    default Guest

    Yes transformers are rated at the full load voltage as a rule. You
    neglected to mention what voltage was applied to the 120 volt winding.
    The nominal voltage is 120 but can vary by 10%.

    Large transformers do have better load regulation than smaller ones.
    Size matters.
     
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