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110 -> 240 step down converter

Discussion in 'Electronic Basics' started by Beeble, Sep 9, 2004.

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  1. Beeble

    Beeble Guest

    apologies if this is the wrong group for this question.

    i live in australia and i'm looking to import a soundcard from the US that
    comes with an AC powered breakout box. it's described as:

    "9v AC 500 mA 'wall wart' power supply"

    i'm a little confused as the step down converters aren't described as
    having similar stats. two that i imagine must be able to do the trick
    (based on their price) are described as:

    240v-115v (24VA) -


    240v-115v (65VA) -

    i'm hoping that first one (24VA) would do the trick.. although i'm not
    even sure if these will be sufficient. i really have no clue

    any advice appreciated. thanks
  2. John G

    John G Guest

    Buy a 9VAC 500mA supply locally.(Jaycar .Tricky Dicky etc.)
    It will take 240 volts input, have the correct plug and be approved and
    be cheaper.
    Much better than trying to use 2 transformers, 1 to get 110 volts and
    then 1 to get 9 volts.
  3. andy

    andy Guest

    VA is a way of saying 'Watts' (roughly - I think there's some subtle
    difference in the way its used).

    9 volts at 500mA is 4.5 watts power output from the wall wart. Allow 50%
    efficiency (probably better than that), and it's going to be taking no
    more than 9 watts. So either of them should do.

    The only problem might be the supply frequency - someone posted a week or
    so ago saying that running transformers on a different frequency to the
    one they were designed for can make the magnetic core saturate. If this is
    right, I would appreciate an explanation as well, because I don't
    understand this bit.
  4. Beeble

    Beeble Guest

    i initially thought that's all i'd have to do, but after doing some
    searching i found a lot of people in a similar situation talking about
    converters so it made me think that perhaps there were some internal
    electronics in the breakout unit that would require 110v. obviously, i
    am totally clueless about these things.

    thanks for the info.
  5. Externet

    Externet Guest

    Your question is confusing.
    What you need is a wall mounted power adapter transformer, from your
    230VAC net to 9VAC, capable of 500mA or 5VA, with the same size plug,
    for sure available at many places at your location. And nothing else.
    No 110-->240 nor 240-->110 anything to keep things simple.
    Choosing any of your 24VA or 65VA mentioned transformers will be more
  6. Jamie

    Jamie Guest

    i don't see any reason why you can not simply go to your
    local radio shaft or hardware store and get a wall wart
    designed for your 240 system down to 9 vac?
    in any case the first one should work find.
  7. Don Kelly

    Don Kelly Guest

    There is a relationship between voltage and maximum flux density.
    For a sinusoidal voltage Vac =4.44(frequency)(core area)(turns)(flux
    density). The magnetising current is whatever is needed to reach the
    resultant flux density.
    A device designed for 100V at 60 Hz will be designed so that the flux
    density is just at or below the knee of the saturation curve of the core. If
    the voltage is too high, the core saturates and there will be a high
    magnetising current (change from being a small fraction of the total current
    to being larger than the load current is possible)-extra heating and
    possible funny smells and waveforms. Cheap devices don't allow much slack
    for this.
    Now if the frequency is changed to 50Hz, as the core and turns are fixed, at
    120V the peak flux will be 1.2 times what it was at 60Hz- roughly equivalent
    to running the device at 144V, 60Hz.. Not good. 100V,50Hz is OK. Also a
    50Hz device on 60 Hz will not run into this problem.
    Don Kelly

    remove the urine to answer
  8. John G

    John G Guest

    You told us it runs from a 9VAC from a wall wart, then it runs on 9
    volts with no 110 volts inside it.
    Whatever you do you will get 50HZ and not 60HZ unless you spend heaps on
    an inverter converter of some sort.
    I silll think my original suggestion is the easiest.

    Anyway what is a Breakout Unit? So we might have some idea what we are
    advising about .

    By the way I have worked in electronics service in various fields for 50
    years in Australia and this 120v/60hz american thing has been a PITA
    for ever.
  9. Beeble

    Beeble Guest

    It's basically just an external controller box for an internal
    soundcard. It sits outside your PC connected to your soundcard and you
    can run cables (guitar, microphone, etc) in and out of it.

    That's the specific unit.
  10. andy

    andy Guest

    The point being that

    - it's easier to plug cables in and out of
    - if it's well designed, there's less problem with EM interference with
    the analog signal before it's digitised.
  11. John G

    John G Guest

    Yes I still think a local wall wart is the way to go. The american one
    will not be approved for use in Aus any way.
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