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12VAC to 24VAC transformer needed

Discussion in 'Electronic Basics' started by [email protected], Jun 13, 2005.

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  1. Guest

    I need to find a transformer (converter?) that will allow me to
    upconvert from 12VAC to 24VAC in order to drive a small motorized
    device (50 watts) for continuous duty. I tried using a 110V/220V
    'travel' transformer -- it gave me the correct voltage but not enough
    output current (probably not enough turns on the windings). I need a
    transformer rather than an electronic converter since I am driving an
    AC motor... isolation isn't an issue so I could go with an
    Autotransformer. All of the transformers I see out there are for high
    voltage primaries (110V or more). Thanks.
     

  2. The method I suggest below will require a slightly larger
    transformer than one designed for your purpose, but it
    would only leave about half the copper unused. For a
    one-off application, that may be better than trying to
    find an unusual transformer.

    If you can find any transformer with a 24 VAC center-tapped
    secondary rated for your load current (which appears to be a
    bit above 2 A), then you can drive the center-tap and one end
    with your 12 VAC and take 24 VAC from the ends. Be sure
    to cover the primary lead ends as they will (likely) have a higher
    voltage. This will function as an auto-transformer, with about
    2 A flowing thru each half of the 24 V winding for a total of
    4 A applied to the center-tap. If the center-tap is brought out
    as a single wire, you need be sure it is good for 4 A. (If the
    secondary is split, as is common, the wires should be large
    enough for the rated current.)
     
  3. ehsjr

    ehsjr Guest

    A 110 to 220 transformer will work with 12 vac on its primary
    to give you 24 vac output on its secondary.
    Ed
     
  4. Guest

    Thanks Larry -- seems like an obvious solution once you mentioned it
    :) and MUCH better than finding a something special.
     
  5. Guest

    Hi Ed ... that was exactly the approach that I tried before (with the
    'travel' transformer)... it just didn't give me enough current on the
    output -- presumably it had a low number of turns. I think Larry
    (above) gave me something to run with but thanks for the feedback!
     
  6. John G

    John G Guest

    But to supply the OPs load the 110 to 220 volt transformer will need to
    be rated at about 500 va to have heavy enough wire to cope with the 2
    amps needed.
     
  7. Bob Monsen

    Bob Monsen Guest

    The number of turns defines the voltage ratio. For a 110 to 220
    transformer, the ratio is 1:2. This is also true of a 12 to 24 transformer.

    I'm guessing your source is not keeping up it's part of the bargain, and
    drooping under load. That is, unless there is something sinister about
    the travel transformer that we don't know about.

    You may not know that you'll have to supply a bit more than twice the
    current at 12V than the 24V device needs. A transformer will keep
    constant volts * amps (except for losses). Thus, you'll need to supply
    at least 4.2A at 12V to keep up with your 50W motor.

    Larry's suggestion also discards any isolation between the load and the
    source. This may not be an issue for you, but then again, it may.
     
  8. Losses are a big issue when a transformer is used at a
    fraction of its design voltage. The OP's 2.1 A load, if
    taken from a 110 to 220 transformer secondary, will
    require a 460 VA transformer. The fact that the load
    is only 50 VA means a much larger transformer would
    be required, about 9 X. By using the secondary of a
    50 VA transformer as I suggested, there is much less
    iron and copper put to waste. (Copper is wasted due
    to having 9 X as many turns as are needed for the core.
    Iron is wasted to hold all that extra copper.) There is
    some waste with my suggestion too, but not so much.
    A dedicated 12 to 24 VAC autotransformer could be
    built without the wasted copper of a 110 VAC primary.
    But that waste nominally only about half the copper.
    When the OP stated "isolation isn't an issue so I could go
    with an Autotransformer", I took him at his word, assuming
    he understood the loss of isolation.
     
  9. Chris

    Chris Guest

    Elegant. I like it.

    But if the OP desires isolation, he might also want to get two
    transformers of at least 70VA rating (12VAC * 6A or more secondary,
    24VAC * 3A or more secondary) and backfeed the first transformer to
    supply the second like this (view in fixed font or M$ Notepad):
    `
    ` |
    ` T1 T2
    ` o------. ,-----|-----. ,-------o
    ` 12VAC In )|( )|( 24VAC Out
    ` )|( | )|(
    ` o------' '-----------' '-------o
    ` Sec. Pri. | Pri. Sec.
    `
    ` |
    `
    ` T1: Pri. 120VAC Sec. 12VAC 6A
    `
    ` T2: Pri. 120VAC Sec. 24VAC 3A

    created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de

    T1 is set up backward (the 12VAC is supplying the secondary of the
    transformer, and the 120VAC primary is the output). This is called
    backfeeding a transformer. The output of T1 is used to supply T2,
    which outputs a standard 24VAC.

    Good luck
    Chris
     
  10. ehsjr

    ehsjr Guest

    <snip>

    Your solution not only saves on the very real "technical"
    losses, it also avoids the "wallet" losses incurred with
    the 110:220 transformer my post mentions. I just looked at
    the prices for the kind of transformer I had in mind - ouch.
    We're talking the 100 dollar neighborhood. Should be able to
    get something at 24V ct around 5 amps for around $25.

    Sounds like your idea is a winner.

    Ed
     
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