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transformer 110 to 240

Discussion in 'Electronic Basics' started by mystraightroad, May 6, 2005.

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  1. Hello

    total novice at this so I'm hoping someone can help. I have a USA
    appliance and the spec is given as:

    INPUT 110 volt fuse protected
    OUPUT 12 volt DC as 3 amps, circuit breaker protected

    the plug is a USA 3 pin type (2 thin plugs at bottom, one round plug at
    top). Am I right in thinking that the round one means the appliance is
    grounded/earthed?

    I can't find anything that relates to watts. Do I calculate this by 12
    (volts) * 3 (amps) to give me 36?

    Also, if I buy a transformer which is too low (Maplins do one at
    45watts) will I blow the transformer, the appliance, or both!?

    Many thanks
     
  2. Guest

    mystraightroad wrote:
    :
    No. But why buy a transformer to power an adapter? Just but a 12V 3A
    adapter for 230V input. Maplins sell 'em.
     
  3. Tom Biasi

    Tom Biasi Guest

    Yes, the round pin is earth ground, it should be connected to the chassis of
    your device.
    Your output can supply 36 Watts.
    You didn't say what the device is. I suspect a power supply for something.
    Without any more information I can only guess that your 45 watt adaptor will
    be fine.
    Regards,
    Tom
     
  4. John Fields

    John Fields Guest

    ---
    Yes, but the round pin is, arguably, on the bottom.
    ---
    ---
    Yes. That's the power your appliance can supply to a load designed
    to operate on 12 volts DC.
    ---
    ---
    More than likely, just the transformer. But, before you buy a
    transformer you need to determine what your appliance draws from the
    mains. It should be on the nameplate somewhere and will either be
    identified as "A" or "amperes", or "VA" (Volt-Amperes). If it's
    specified in VA, then all you need to do is to get a transformer rated
    to supply that, or a greater number of Volt-Amperes. However, if it's
    specified in amperes, then multiply 120V by that number of amperes (to
    get Volt-Amperes) and get a transformer which can supply at least
    that.

    If you can't find out how much current your appliance draws from the
    mains, then what I'd do is assume that it's running at about 50%
    efficiency under full load and get a transformer that can supply twice
    the output power rating, or about 75VA.

    If you're a total novice, though, the best thing to do would be to
    take the thing to an electrical shop and have them measure the mains
    current into it while it's fully loaded. That is, with a 4 ohm
    resistor on its output. The resistor will be dissipating 36 watts, so
    it'll get hot if the measurement takes much time at all to do. If you
    have to buy one, a 50 watt resistor would be a good choice.
     
  5. thank you so much for your replies.

    Ok, I have a feeling this is a stupid question but here goes....

    If we assume that the draw is twice the max output (75w) would it hurt
    if I were to use a transformer rated at 300w rather than one at 100w.
    A friend has said they will lend me a 300w transformer, I'm just
    concerned it might be too strong.
     
  6. ryan wiehle

    ryan wiehle Guest

    you said the appliance at 12v draws 3 amps = 36watts
    if you now supply it with a 300watt supply you have enough power to
    connect about 8 of these devices.
     
  7. Dwayne

    Dwayne Guest

    Just buy a power adapter with a 12 Volt output capable of at least 3 Amps.
    For example if you buy a 48Watt adapter with a 12V, it can suppy a maximum
    current of 48/12= 4A. So you can connect any 12V device, and draw a MAX of
    4A. There is no minumum draw, your 3A device will work fine, so will a
    device rated at 12V and 2A or 1A or 500mA, etc...

    Dwayne
     
  8. Kitchen Man

    Kitchen Man Guest

    A transformer cannot be too strong, but it can be too weak. In the
    trade, acquiring items with greater ratings than minimum is called
    "pro-rating." If you use the 300VA - it's more correct to use the
    term "Volt-Amps" (VA) when speaking of AC power - your design will be
    just fine.

    Here's a very good article that discusses just the subject you are
    concerned with:

    http://www.toddsystems.com/geninfo.html

    Have at it, sparky!
     
  9. Jamie

    Jamie Guest

    go to your local electronic supply store and get your self a
    AC(240 Vac) to DC (12volts DC)converter that will supply 3 or more amps.
    in the long run, it will be cheaper and easier for you.
     
  10. It will work just fine, however, power losses (nothing works with 100%
    efficiency) will be slightly higher. For something used only
    sporadically that's nothing to worry about.
     
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