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Transformers

Discussion in 'Electronic Basics' started by Rodney, Aug 13, 2005.

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  1. Rodney

    Rodney Guest

    Hi,

    I've noticed that a transformer with, say, a voltage rating of 10 volts can
    sit at a considerable higher voltage than 10 volts if it is not under load.
    Once a current is being drawn from it the voltage will drop to that
    specified. I was thinking that a high wattage transformer, even though it
    may be specified at 10 volts could sit at a much higher voltage if, even
    under load, too small a current were being drawn from it. That is, if the
    load were too small. The higher voltage could possible damage the load
    circuit in some cases, I think. Please correct me if I'm wrong.

    Rod
     
  2. Andrew Holme

    Andrew Holme Guest

    Are you talking about the AC voltage across the transformer secondary, or
    the DC output after rectification and smoothing?

    The former depends on "Load Regulation" which is quoted by the transformer
    manufacturer as a percentage = 100 * (V_no_load - V_full_load) /
    V_full_load. Larger transformers have better load regulation than small
    transfomers.

    The AC output voltage is specified in volts RMS. The rectified and smoothed
    DC output will be almost a factor sqrt(2) higher (peak voltage less the
    diode drops) under no load.
     
  3. Rodney

    Rodney Guest

    I'm talking about the DC output. I was thinking of a wall transformer in
    particular. The load would be a device rated at 10 volts but requiring
    little current. The transformer would be rated at 10 volts DC output but
    having a fairly large current capacity at the output. I've measured the no
    load output of some and it was too high until I put a load on it to bring it
    down to the rated output voltage.

    Rod
     
  4. John Fields

    John Fields Guest

     
  5. Ralph Mowery

    Ralph Mowery Guest

    The small unregulated DC supplies will have a higher unloaded voltage. The
    low voltage AC comming out of the transformer in it is rectified to DC and
    maybe a small filter capacitor in it or the device it powers will charge to
    a voltage about 1.4 times the AC voltage. As the load is increased the
    capacitor can not charge to the peak value and hold it. If the supply was
    regulated this would not hapen.
     
  6. Kitchen Man

    Kitchen Man Guest

    Others have noted that you are using incorrect terminology - you are
    referencing an unregulated plug-in DC power supply. A lot of people
    call this a wall-wart, 'cause it tends to look like your wall is
    growing a wart when you plug one in. I think it's a silly name, but
    to call it a transformer is simply wrong.

    Now, on to your question. I have also noticed the same phenomenon,
    and I have wondered the same thing about what would happen if the
    rating of the supply was grossly in excess of the load attached. The
    following article is a very good reference for the question:

    http://www.glitchbuster.com/wallwart.htm

    Enjoy.
     
  7. Rodney

    Rodney Guest

    Any "wall adapters" that I've dismantled didn't have capacitors, just
    rectifiers, but thanks for the information. But my original question was
    would this higher voltage, in some cases, damage a load that was too small a
    current drain to
    bring the voltage down to the proper level. If you had a 10 volt DC device
    which did not draw
    enough current to bring the supply level down to the 10 volt rating and the
    DC supply was sitting at 14 volts it could be possible to damage DC device.
    That would be in the case that the AC/DC adapter had a high current
    capability. I think it would require a sufficient load to bring the voltage
    down. So if someone wanted to buy an AC adapter for their video game,
    for example, they would buy one with the right voltage rating and it might
    not matter if the current rating were higher but then if the current rating
    were too high and the adapter thus had a high wattage it could be
    theoretically
    possible to damage the video game, or whatever it may be, if the video game
    did not draw enough current to bring the AC adapter down to the rated
    voltage. In
    this case you would be supplying a 10 volt device with about 14 volts. This
    is mostly theoretical since I think most things have internal voltage
    regulators but personally, to be on the safe side, I think I would buy a
    replacement adapter with a current rating that was not a great deal higher
    than that of
    the original AC adapter.
    Thanks,

    Rod
     
  8. Rodney

    Rodney Guest

    Any "wall adapters" that I've dismantled didn't have capacitors, just
    rectifiers, but thanks for the information. But my original question was
    would this higher voltage, in some cases, damage a load that was too small a
    current drain to
    bring the voltage down to the proper level. If you had a 10 volt DC device
    which did not draw
    enough current to bring the supply level down to the 10 volt rating and the
    DC supply was sitting at 14 volts it could be possible to damage DC device.
    That would be in the case that the AC/DC adapter had a high current
    capability. I think it would require a sufficient load to bring the voltage
    down. So if someone wanted to buy an AC adapter for their video game,
    for example, they would buy one with the right voltage rating and it might
    not matter if the current rating were higher but then if the current rating
    were too high and the adapter thus had a high wattage it could be
    theoretically
    possible to damage the video game, or whatever it may be, if the video game
    did not draw enough current to bring the AC adapter down to the rated
    voltage. In
    this case you would be supplying a 10 volt device with about 14 volts. This
    is mostly theoretical since I think most things have internal voltage
    regulators but personally, to be on the safe side, I think I would buy a
    replacement adapter with a current rating that was not a great deal higher
    than that of
    the original AC adapter.
    Thanks,

    Rod
     
  9. Actually you'll see the same thing with cheap and small transformers,
    be they in AC adaptors or standalone transformers.

    The manufacturers want cheap (and small), so they design for it. They
    use a thin wire, and that wire has higher resistance. Load it down,
    and of course you'll get a lower voltage, the same thing as when
    you put a load on a high impedance audio source. Think voltage divider.
    The manufacturers know this, and design for a closed system. They
    know the load and the needed voltage at that load, and so design
    the transformer accordingly. It works fine.

    It only falls apart if someone starts using it for some other purpose.
    Take a transformer out of a clock radio, a nice small transformer and
    at first it seems like a decent voltage. But start using it for much,
    and you see that the voltage may be too low for your use.

    As discussed here recently (or in one of the newsgroups in the hierarchy)
    it isn't always a sales gimmick to tell the consumer to use the
    manufacturer's AC adaptor. They may have good reason for you to
    use it, because then they know it will supply what is needed. Pick
    another adaptor that seems to be right, and it's anybody's guess
    whether or not things will work as expected.

    Get into decent transformers, and specifically more universal
    transformers, and the problem goes away to a large extent. A higher
    current transformer, or even one where small size and cost is
    not a factor, and the windings will use thicker wire to handle
    the current. That thicker wire will have lower resistance, and
    so the load will have to be greater to see the droop under load.
    I can pull a nice 6.3v filament transformer out, and it will be
    only a small percentage over 6.3v with no load. It is expected
    to supply some amps of power, and hence the design can't have
    a higher output impedance. Thus it is designed to supply the
    needed voltage in the first place, rather than at a certain load.

    If you load it enough, then of course there will be droop. But
    it will be a much higher load.

    Michael
     
  10. John Fields

    John Fields Guest

     
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