I'm trying to build a dual power supply. I can find mains transformers that give me two (presumably unconnected) secondary windings of equal voltage, and if I connect them in series, I should be able to make myself a transformer that should behave like a center-tapped transformer.

Obviously, if one of the secondary windings has terminals A and B and the other has C and D, I could connect either A to C (equivalently B to D) or A to D (equivalently B to C). If the first winding has the dot at A and the second has the dot at C, my intuition tells me that I need to connect A to D (or B to C), and that the other connection (A to C or B to D) would be wrong, correct?

If you want a specific example, take a look at this datasheet: https://www.mouser.ch/datasheet/2/410/FP34_1400-781326.pdf

This transformer has two secondary windings of 17VAC each, and one winding on pins 5 and 6 has the dot at 5, and the other on pins 7 and 8 has the dot at 7. So I think I could connect 5 to 8 or 6 to 7, but not 5 to 7 or 6 to 8. Is that correct?

  • 4
    \$\begingroup\$ Just make a random connection. If the voltage from end to end is zero, you've made the wrong one, swap it. \$\endgroup\$
    – Neil_UK
    Jun 4, 2021 at 15:57
  • 1
    \$\begingroup\$ The logical connections are obvious but the physical layout is not so obvious perhaps to improve Hipot isolation on each side. In theory connecting the closest pins in the middle should result in a series inductance of zero and minimal crosstalk. It is interesting that they avoid discussion on this illogical change and is contrary to historical diagrams and even their videos which show some that are similar in 1234 order. So you should inquire to Triad why they did this and perhaps Japanese patents may reveal the secret why they chose this unconventional layout dictated by the construction. \$\endgroup\$ Jun 4, 2021 at 16:47
  • \$\begingroup\$ This is such a non-question. \$\endgroup\$
    – Andy aka
    Jun 5, 2021 at 9:08
  • \$\begingroup\$ @Neil_UK In practice, I'll probably do exactly that. I was just trying to understand what ought to happen before I do it. \$\endgroup\$ Jun 7, 2021 at 7:16

1 Answer 1


enter image description here

Here is the picture you are referring to.

You are correct: 5 to 8 (or 6 to 7) gives you a center tap. If you connect 5 to 7 and 6 to 8 the windings would be in parallel for higher current.

  • \$\begingroup\$ I built that circuit yesterday and it worked as advertised. Thanks! \$\endgroup\$ Jun 22, 2021 at 9:03

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