Transformer primary / secondary windings

Discussion in 'General Electronics Chat' started by PackratKing, Oct 17, 2010.

  1. PackratKing

    Thread Starter Well-Known Member

    Jul 13, 2008
    OK. a search gave me a bunch of old threads to bump.........

    A 600w "minuteman" UPS, 1:1 isolation transformer, has a single 120vac primary, [ blue / white ] and three secondary choices - other colors, all coils read equal ohms.......there is grounding between coils to disrupt capacitive coupling.

    Is it possible to re-tie the groups of coils, so as to provide a center-tapped 240 vac primary, or is this going to cause issues with phasing.......among other things

    The goal here, is to employ the #8 winding that would normally become the [ battery driven ] primary when power failed, so as to reap a rectifiable source of 240 volts High amp DC
    Last edited: Oct 17, 2010
  2. DonQ

    Active Member

    May 6, 2009
    There's a little confusion of primary/secondary input/output, but here is some general info that you may have to invert/modify if I misunderstood your intent.

    If it has a single primary, and it is not already center-tapped, it will not be possible to 're-tie' the coils to give a center-tapped coil on what the transformer labels the primary, at any voltage.

    It is possible to 'back-drive' transformers, i.e. using the secondary as the primary and vice-versa. The secondaries having equal ohms does not necessarily mean that they have the same number of turns. You can determine this by simply running it and measuring voltages.

    I would strongly suggest using some lower AC voltage while testing, just for the safety aspect of it. Using a 12VAC wall-wort type power supply (or some equally low current AC supply) will show the same ratios, but at a survivable 'oops' potential. This won't be able to drive any load, so don't apply one except for a multimeter/scope yet.

    If the secondary windings have the same output voltage, you should then either go by the phasing info available for the xformer, or determine it yourself with a scope, or by series connecting separate windings. If they are in phase, the AC voltage will add, if they are out of phase the voltages will subtract. This is the point where the 12VAC comes in handy. Depending on the 'grounding between coils', connecting in series may create a short. This is manageable with a low-current 12 VAC input, but frighting and dangerous connected directly to the line.

    There are a number of possible combinations available depending on what you find, the easiest would be with two 120V coils in series on the secondary side, backdriving what used to be the non-center-tapped primary but is now used as the output. This is for 240VAC input, 120VAC output.

    The third secondary could be used as another output, depending again on that grounding thing. Figure it out at 12VAC, then try it with a fused 120V. Best would be to parallel, in-phase, with the other 'output' to allow twice the current, at half the voltage of the input (watts in=watts out).

    Then, regardless of how you wind up wired, don't go above the current rating of any of the individual windings. Ground all cases, fuse all inputs/outputs, keep your hands in your pockets, and all the other usual precautions when working with line voltages.
  3. PackratKing

    Thread Starter Well-Known Member

    Jul 13, 2008
    Thank you Mr. Quixote !!!!!!!

    I was getting a little impatient :( for an answer on this, and yours hit the need right on

    It was indeed my intent to backdrive the unit, in an attempt to determine what was what, tho' could not find any reference [ as yet ] to the process you set forth.

    Shall post results of research in a later contribution 'da Man !!! :D

    Long story such rotten luck. My monster ain't wired the way I had hoped. I did determine that the heavy wire secondary puts up 32 volts........ that will be more than adequate for the task at hand, and a wee less dangerous.

    Thanx again. :p
    Last edited: Oct 17, 2010