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Low Cost PS for 12 Volts, 250ma for a small DC motor

Discussion in 'Electronic Design' started by Fred, Sep 7, 2005.

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  1. Fred

    Fred Guest

    Hi All,

    I'm looking for a means to inexpensively implement subject PS from a
    240/120 mains supply without using a switch. Yep, I know that a SMPS
    would do the job but can the job be done at less cost?

    What I've looked at so far:

    1) Capacitive Transformer. It looks like it might work except that I end
    up needing large 22uF high voltage caps on the front end which are
    expensive. Needs a switch.
    2) Zener supply. Way too much power dissipated in the resistors. Needs a
    3) CT transformer. Looks good but needs a switch.

    Any bright ideas or is SMPS the solution??


  2. Joerg

    Joerg Guest

    Hello Fred,
    SMPS is usually the answer here. Unless you are facing a gazillion units
    per year I would look for off-the-shelf wall wart type supplies or small
    power bricks that are rated for wide input voltage (90V to 260V or so).

    Regards, Joerg
  3. Fred

    Fred Guest

    Hi Joerg,

    Thanks for the reply. There could well be a large quantity of these
    supplies needed.


  4. Joerg

    Joerg Guest

    Hello Fred,
    Look at power supplies like these: Inc/Web data/DMS120085-P5-IC.pdf

    About $15 if you buy 100 and IIRC they even come with adapters for
    several countries.

    There are many more companies that make such supplies. Astec etc.

    Regards, Joerg
  5. Rich Grise

    Rich Grise Guest

    When you say, "without a switch", do you mean, without having to
    manually switch a toggle switch or slide switch?

    I knew a guy once who made 120/240 battery chargers, and he used
    a 240V relay. At 120V line voltage, it didn't pull in, so the
    primaries were in parallel. At 240, the relay pulled in, and put
    the primaries in series. A relay _does_ do switching, but I've
    never heard a relay called a "switch". :)

    Other than that, probably an SMPS, sorry.

    Good Luck!
  6. ISTR that the famous legal firm, Dolby Labs had a product that had a
    suitably rated light bulb in series with the transformer primary for
    just this function. It helps if you have a constant power requirement

  7. Dolby Labs made a laboratory noise-weighting filter, with lower power
    consumption certainly, that did 120/240 V by putting a filament lamp in
    series with the primary of the mains transformer. The primary voltage
    stayed at about 90 V irrespective of the input voltage.
  8. Ah, The light bulb, probably the worlds' simplest XOR gate

  9. John Fields

    John Fields Guest

    Why can't you just use a wall-wart, and what's this "switch" you
    keep referring to?
  10. Joerg

    Joerg Guest

    Hello John,
    Guess he means the 120/240 selector. Anyway, wall wart or power brick is
    the way to go. Much easier during the agency approval.

    Regards, Joerg
  11. Pooh Bear

    Pooh Bear Guest

    He wants a 'universal' PSU without the need to change voltage taps. See "
    240/120 mains supply without using a switch ".

    For all practical purposes therefore, an SMPS.

  12. Pooh Bear

    Pooh Bear Guest

    Probably about $6 or less if it comes from China.

  13. Pooh Bear

    Pooh Bear Guest

    3 watts makes a simple switcher just about viable.

    I googled "power adaptor china".

    One interesting hit was this.

    There's *tons* of outfits making this stuff out there.

  14. John Fields

    John Fields Guest

  15. Fred

    Fred Guest

    Hello Again,

    Thanks to everyone that has contributed to this thread. I do appreciate
    the input. To confirm a question back a post or two, yes, we are looking
    for a PS that adapts to 120/240 mains input.


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