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Transformer 240V to 110V how many watts do I need?

Discussion in 'Misc Electronics' started by EF, Apr 11, 2005.

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  1. EF

    EF Guest

    Hi I'd appreciate some help.

    I have a NiMh battery charger which has AC input of 100~120VAC
    50~60HZ, 1.2A.

    It outputs at 36VDC, 1.8A.

    How many Watt transformer do I need to use this charger in the UK
    where we have 240V? The output of the charger itseff is only 65W, but
    the input is more, but I'm not sure how to take the phase into

  2. Power = volts x amps so its about 65 watts. Given allowance for less than
    100% efficiency, 100 to 150 watts would be my guess.
  3. Ken Taylor

    Ken Taylor Guest

    If the input spec's are correct then the input could be up to 144W so,
    assuming inefficiencies as you suggest, I'd be looking to maybe 175-200W.
    Seems ridiculous for the job but there you go.

  4. EF

    EF Guest

    Yeah, it seems over the top when the charger only outputs 65W.
    Normally I wouldnt worry and just go for the larger transformer, but
    in this instance weight is important as I need to carry the charger
    and transformer daily.

  5. EF

    EF Guest

    Many thanks for everyone's input.

    How many watt is 144VA? I ask because some transformers here have
    their rating listed in watts, not VA.

  6. Jim Gregory

    Jim Gregory Guest

    Check inside charger to see if there is a mains i/p voltage selector setting
    (series/parallel switch for primaries), as there may be the opportunity of
    setting it to 230V.
    If you are bringing the charger to the UK, you may source the step-down
    in the UK. You could buy a 200VA, 240V to120V model from, for example,
  7. CWatters

    CWatters Guest

    Can we ask what it's for?
  8. EF

    EF Guest

    It's to charge a 36V 8Ah battery pack for an electric motor. I'm using
    the motor on a bicycle, so have to carry charger with me on the bike,
    which is why I wanted to see if a 100VA transformer would be
    sufficient, as they are a lot lighter. But from everyone's advice it
    looks like it would have to be 200VA.

    But I'll follow Jim's suggestion first and open it up to see if it is

  9. Jim Gregory

    Jim Gregory Guest

    *If* it proves switchable, ie, two primaries arrangeable either in parallel:
    115V or in series: 230V, and you adjust it for the higher input, when you
    come over here you'll need to fit a new 3-pin plug, 13A (max) type but with
    a 2 or 3 Amps 1" fuse inserted... this connector carries an optional ground
    connection, as well.
    Any electrical shop, DIY store, pound shop, ironmongery sells them.
  10. If I understand it correctly the charger has 1.2 A at 100 VAC (and maybe
    1.0 A at 120 V). Here in Europe we often have wide range devices with
    110...230 V input. The input current noted on the device nearly always
    referes to the lowest input voltage.

    So you need a transformer with output 100 V / 1.2 A with is 120 VA. You
    don't need any additional wattage.

  11. CWatters

    CWatters Guest

    Interesting. I was also going to ask if the battey is 3 lead acid cells in
    series?. If so you might be able to rewire the batteries in parallel for
    charging and use a 12V car battery charger.

    If they aren't lead acid then the problem is harder. You can buy chargers
    capable of charging 36V's worth of NiCad/HiMH cells (eg competition model
    aircraft battery chargers) but they tend to be very expensive and most (all)
    use a 12V battery as the power source to allow charging on the flying field.
  12. blazeinferno

    blazeinferno Guest


    The output is 65W. Does the charger specify how many watts on the
    input side or maybe how much current it consumes on 120V?
    If it does specify the current then: Power (Watts=Volts *Amps) and that
    should tell you the minimum power your transformer should be able to handle.
    If nothing is specified for the input side, then I would estimate the
    efficiency of the charger to be at worst 75%. With this approximation, the
    input power should roughly be about 65W/0.75=87W. A transformer between 85W
    and 100W should be fine. The power for 120V or 240V will still be the same.
    The fundamental difference is that current is halved at 240V.

    Hope this helps


  13. One more thing to worry about when a transformer powers anything with a
    rectifier if it also has a filter capacitor: The waveform of the current
    drawn by the device. The RMS current can be higher than that of a
    resistive load drawing the same amount of power and a sinewave current
    waveform - sometimes a lot higher. The RMS current is what will heat up
    transformer windings.

    Try putting a low value resistor in series with the input of the
    charger, and looking at the current waveform on an oscilloscope. Draw it
    on graph paper and calculate an approximated RMS value.
    Or, you may be able to find an automotive lamp that, when placed in
    series with the input of the charger, glows without dropping a lot of
    voltage nor burning out. Find out how much current it takes to make the
    lamp glow the same way, and that's the RMS current drawn by the charger.
    Multiply RMS current by voltage, and that's what the transformer has to be
    rated for (often in volt-amps rather than watts).

    - Don Klipstein ()
  14. The waveform's peak value divided by the rms value is called crest
    factor. For pure sine waves the crest factor is 1.414 but input currents
    into filter capacitors can easily reach crest factors of 10.

    The crest facor is very important if you run uninterruptable power
    supplies. For the reason mentioned they have to deliver high peak
    currents to your computer's power supplies.

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