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cheap mains stepdown 240-120

Discussion in 'General Electronics Discussion' started by cevin, Jul 14, 2015.

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  1. cevin

    cevin

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    Jul 14, 2015
    I have a simple but specific question.
    Living in the UK, mains voltage is 240

    I have several USA imported items that run on 120v and currently swap my external stepdown converter between them.

    presuming the linear tx supplies inside the equipment has just a single primary winding, can I add another smaller 120v transformer in series to drop the input voltage and just leave the secondary open?

    many thanks for your input
     
  2. Minder

    Minder

    2,922
    615
    Apr 24, 2015
    If I understand your intended connections, any transformer used would have to be the same Va.
    Going to that extent, you might as well substitute the original with a transformer of the correct voltage.
    M.
     
  3. Alec_t

    Alec_t

    2,757
    733
    Jul 7, 2015
    Sadly, no. That will drop the output volts too much, because the added primary isn't magnetically coupled to the existing primary. What you need (according to the Spice simulation attached) is an inductor (or transformer primary) with an inductance about 40% of the existing 120V transformer primary inductance and the same current-handling ability.
    SeriesLDropper.gif
     
  4. cevin

    cevin

    2
    0
    Jul 14, 2015
    Very nice. and fast guys.
    cheers! :)
     
  5. Minder

    Minder

    2,922
    615
    Apr 24, 2015
    It can be done using the buck-boost principle but you might as well replace the existing one.
    M.
     
  6. AnalogKid

    AnalogKid

    2,330
    653
    Jun 10, 2015
    The above ideas work only for items with linear power supplies. OTOH, items with switching power supplies probably have universal inputs, but you won't know unless you check.

    ak
     
  7. Minder

    Minder

    2,922
    615
    Apr 24, 2015
    A SMPS supply will also work with a buck/boost transformer, in fact any lower higher voltage device.
    But SMPS are usually various input voltages, the OP mentioned Linear supply.
    But using the buck/boost principle is a little redundant.
    Max.
     
  8. duke37

    duke37

    5,297
    739
    Jan 9, 2011
    Just use an auto transformer with the input connected across the entire winding and the output connected to a centre tap. Inductances is series with the input will give funny effects and will work properly on only one specific type of load.

    Remember that a 60Hz transformer will have a harder life when running on 50Hz. If your load will stand it, it may be better to use a 100V input to the equipment.
     
  9. cjdelphi

    cjdelphi

    1,096
    104
    Oct 26, 2011
    if it's a computer power supply, replace it with a 240v supply....
     
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