Transformer with three out pins

Discussion in 'General Electronics Chat' started by vladtess, May 10, 2011.

  1. vladtess

    Thread Starter Member

    Jan 5, 2011
    43
    1
    Hi there! I have a transformer that has an input of two wires (120 vac) and an output of 15 vac in yellow colored wires. But there is also another, back wire output, that I don't really need, since I get my desired 15 vac, but just curious what is that for? I read somewhere that it is for the 'negative voltage' or somethhing. Can someone enlighten me please? Or a link to some page, cuz google didn't really come up with right results. (maybe i search wrong keywords).

    Thanks much!!
     
  2. DerStrom8

    Well-Known Member

    Feb 20, 2011
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    I would assume the black wire is the center tap. Your two yellow wires are "hot", out of phase, so you get 15 volts between them. If you tested between a yellow wire and the black wire, you would probably get about 7.5 volts.
    Der Strom
     
  3. vladtess

    Thread Starter Member

    Jan 5, 2011
    43
    1
    Thank you!!

    Sir, when you say out of phase, what do you mean? Now I see that center tap is a wire in the center of the coil. What would be the purpose of the center tap? Thanks!!
     
  4. ErnieM

    AAC Fanatic!

    Apr 24, 2011
    7,386
    1,605
    The center tap has two common uses I can think of when making a DC supply.

    First is when you can use a simpler bridge rectfier only using 2 diodes.

    Next is you can use a full wave bridge (4 diodes) and if you use the center tap as ground you get both positive and negative voltages out.
     
  5. studiot

    AAC Fanatic!

    Nov 9, 2007
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    Whilst it is most likely to be a centre tap, some manufacturers supply an increment connection that allows the output to be increased by a few volts.

    This caters for situations where you are connecting to lower mains voltage for some reason, but still need the exact output you would get from normal mains.
     
  6. DerStrom8

    Well-Known Member

    Feb 20, 2011
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    Out of phase means that while one wire has a positive voltage, the other has a negative (to "receive" it, so to speak). Then when the first is negative, the other is positive. It keeps switching back and forth (hence "AC" alternating current).
    The question about the center tap seems to already have been answered :)
    Hope this helps!
    Der Strom
     
  7. vladtess

    Thread Starter Member

    Jan 5, 2011
    43
    1
    Thanks you all, very useful!!
     
  8. marshallf3

    Well-Known Member

    Jul 26, 2010
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    A lot of transformers come with a CT secondary that's never used.
     
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