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Note on transformer winding

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Technical Editor, QST:

The excellent article in November QST by W2VLA on the subject of home-built transformers(1) prompts me to write, mentioning a couple of points that I hope will prove helpful to readers.

The information provided by Maresca (and by Coats in the earlier article)(2) is equally applicable to the rewinding of burned-out or otherwise unusable transformers. By doing this you are saved the trouble and cost of obtaining a new core. If one of the original windings is still recognizable, and if you know its original voltage rating, the new windings can be easily designed by counting the number of turns on the old winding. The number of turns required is given by equation (4) of Maresca's article, and the wire size selected according to his instructions.

If this method cannot be used, then Maresca's calculations will yield the required results. Unless the old core is known to be of grain-oriented steel it is best to assume a lower value of peak flux density, B.- say, 12,000 gauss. A bonus benefit of using an old core is that you have all of the accessories - clamping bolts, and bells, terminal strips, etc.

A possible pitfall that Maresca does not mention is that of a shorted turn. Even one short-circuit between adjacent turns will prove fatal to the transformer because of the very heavy currents that will flow. The shorted turn will be burned up, and the transformer with it, so every care is necessary to avoid a single short. For this reason it is never a good idea to re-use wire from old windings. However sound it may look, its insulation is almost certainly brittle and will be damaged by the process of unwinding it. By the way, don't handle the wire any more than necessary. It doesn't help the enamel any, and it will sometimes work-harden the larger sizes and make winding harder.

The use of scramble-wound pies for the high-voltage winding is a stroke of genius - it removes the one difficult phase of hand winding. The winding should not be too scrambled, however. The wire should be laid on in reasonably uniform layers so that turns that lie close together are not widely separated electrically. Otherwise the voltage between them may be large enough to break down the wire insulation. Further, the more scrambled the winding is, the poorer its space factor and the greater the danger of shorted turns caused by wires crossing one another.


  1. Maresca, "More on homemade transformer design," QST, November, 1960.
  2. Coats, "A cool kilowatt plate transformer," QST, September, 1959.

Philip H. Byrne, VE3AXX