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CT transformers - how is 180 deg out of phase achieved?

Discussion in 'Electronic Basics' started by billg, Apr 12, 2004.

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  1. billg

    billg Guest

    I've got a your typical small wall wart transformer that takes 110 VAC
    and produces 13 VDC. The secondary winding is center tapped and uses
    diodes/capacitor to provide DC - something like this

    How is it that the legs/coils off the center tap on the secondary
    winding are 180 degrees out of phase? Is it the exact placement of
    the center tap, turn ratio, or transformer construction? Does the
    same apply for multitapped transformers?

    Thanks in advance.

  2. Each turn on the transformer produces essentially the same AC
    voltage. The phase of that voltage depends on which direction that
    turn uses to surround the common magnetic field. If the windings
    continue to wrap around the core in the same direction after the
    center tap connection point is reached, than the voltage continues to
    develop in the same direction. Instantaneously, this is similar to
    having a stack of cells (batteries) all turned the same way. If you
    make a connection to the middle of the stack, then one end of the
    stack is positive with respect to that point and the other end of the
    stack is negative. That is just another way of saying that the two
    ends of the stack are 180 degrees out of phase with each other.
    Another analogy for a center tapped winding is a see saw. With
    respect to the center point, one end has to go up (positive height)
    when the other end goes down (negative height) if the see saw is rigid
    (constant slope versus length = constant volts per turn).
  3. John - G0WPA

    John - G0WPA Guest

    i.e The winding doesnt change direction at the centre point, from end to end it
    may be 100vac. So the center tap will be at the 50v point. measuring center to
    one end will thus give +50vac, and to the other end, -50Vac. Try it with four
    batteries, negative probe of your meter between batteries two and three, and
    youll see it measure +3v at the top end, and -3V at the bottom end. Your
    negative then is at the 0v point!! ...a 6 volt center tapped supply, without
    you reversing the direction of any batteries, windings are the same in a

    Drop the QRM from my addy to mail me...
  4. billg

    billg Guest

    Thanks for the replies. I did come across the following link that
    further clarifies this thread

  5. Rich Grise

    Rich Grise Guest

    They're not. They're simply the inverse of each other. There is no
    phase shift, just a polarity inversion.

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