back to back transformer

Discussion in 'The Projects Forum' started by j4e8a16n, Apr 1, 2014.

  1. j4e8a16n

    Thread Starter Member

    May 1, 2013
    33
    0
    HI,

    I bought 2 transformers:
    120 volts to 36volts output with a center tap.
    I connected the 2 36volts together..
    The problem is I get only 98volts output for a 120 volts input.

    Jp
     
  2. #12

    Expert

    Nov 30, 2010
    16,313
    6,817
    You just discovered the internal losses of a transformer.
     
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  3. inwo

    Well-Known Member

    Nov 7, 2013
    2,433
    315
    That doesn't seem right.:confused:

    Are they identical transformers?

    Try it the other way.
    Leave connection the same and reverse in and out connections.
     
  4. #12

    Expert

    Nov 30, 2010
    16,313
    6,817
    This got my curiosity up, so I dragged out a Radio Shack transformer rated as 120 V in to 25.2 V out, .45 A out.

    That's a ratio of .21 to 1
    Backwards should be 4.762 to 1
    I ran it backwards with 8.83 volts and 14% of the rated load. (560 ohms)
    The results were 29.92 volts when it should have been 42.05 V
    That's off by 28.85%

    The O.P is only experiencing 18.333% loss, maybe because he did not load the transformer.

    Interesting how far off they look when running backwards.

    I wouldn't claim that's a power loss, but it sure is the wrong voltage.
     
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  5. inwo

    Well-Known Member

    Nov 7, 2013
    2,433
    315
    And now I'm curious. :D

    I had two identical 120/10V/10A transformers.

    120vac in one end and 119 out the other. No load.
     
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  6. j4e8a16n

    Thread Starter Member

    May 1, 2013
    33
    0
    Same result. The 2 trans. Are exactly the same.
     
  7. j4e8a16n

    Thread Starter Member

    May 1, 2013
    33
    0
    I put de 120v on the ohmmeter side andvice versa. I get:
    an output of 139volts!

    I ear a hum.

    No hum was on the first test.
     
    Last edited: Apr 1, 2014
  8. GopherT

    AAC Fanatic!

    Nov 23, 2012
    6,059
    3,823
    I'm guessing that your transformers are not as identical as you would like them to be. I also thing one is shorted to the core somewhere. I'm guessing on a primary side. You shouldn't have a hum with no load.
     
  9. daviddeakin

    Active Member

    Aug 6, 2009
    207
    27
    You're not using the centre tap, are you? Don't use the centre tap.
     
  10. inwo

    Well-Known Member

    Nov 7, 2013
    2,433
    315
    One transformer must be mislabeled or tap miswired.

    Driving an 18 volt winding with 36 volts will hum.

    Measure them individually connected to 120vac. Then connect the 18v or the 36 volt (same) connections from one to the other. Ignoring the original labeling.
     
  11. j4e8a16n

    Thread Starter Member

    May 1, 2013
    33
    0
    Dam I have one that is 43 volts and the other 36.

    The center taps read half the voltages for both.

    The 45v ouput one is the one with a humm.

    I dont use the center tap.
     
    Last edited: Apr 2, 2014
  12. #12

    Expert

    Nov 30, 2010
    16,313
    6,817
    Well, that clears up the mystery.
     
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