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240V Control Transformer issue

Discussion in 'Electronic Basics' started by none none, Feb 9, 2007.

  1. none none

    none none Guest

    i have a Control Transformer with 240V on the primary, with 24VAC out the
    secondary. My question is what would happen if i try to use this as a power
    transformer using only 110/120V on the primary?
     
  2. John Fields

    John Fields Guest

     
  3. Jamie

    Jamie Guest

    you get 12 volts
     
  4. none none

    none none Guest

    Sweet.
    I got this transformer from Fry's and intend to make a power supply with a
    1-12V 1-3 amp spec using digital volt & ammmeter for the displays and have
    it variable current and voltage. Seems much more fun than buying one.
     
  5. Eeyore

    Eeyore Guest

    You'll get 11/12V out.

    Graham
     
  6. Jamie

    Jamie Guest

    Well lets see now..
    your average peaks should be around 16.9 volts at the
    filter caps.
    Don't expect to source must current when looking for
    12 volts.
    I don't know what kind of regulator you plan on using
    but!, you should have some more voltage than that if
    you plan on a linear type.
    I suppose you could make you're self a smps (switch mode power
    supply), with proper regulation you could most likely get it stabilized
    some what in the ripple region.
     
  7. Rich Grise

    Rich Grise Guest

    With enough capacitation, 16V is quite comfortable headroom for a 12V
    regulator, if it's designed right.

    I must point out, however, that spec'ing it at 1.2-12V would make the
    design considerably simpller, if you're going to use an LM317. :)

    Good Luck!
    Rich
     
  8. ehsjr

    ehsjr Guest

    Good point. To get 1V out, he could adjust it to
    around 1.6 and put a diode in series. And that ~16.9
    volts is AC peak before rectification, not the voltage
    across the cap, so the 16 volts you mentioned is the
    correct approximation for a single diode drop. (A
    typical 1N540x will drop about .9V at 3A)

    But he's not out of the woods. The LM317 won't handle
    3A. He could use an LM350 instead. Still, the 16 volts
    won't be there if he uses a bridge. It will be ~ 15.1
    volts at 3A. Add in line voltage variation, transformer
    sag, and ripple, and he'll have problems.

    I think for most cases the supply would work - but most cases
    are at well under the max current and with the line voltage
    closer to 120 than to 110. For a solid 12V design, he needs
    a different transformer.

    Ed
     
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