I purchased a power transformer with dual primary windings. Reading up on the correct way to hook up the primary coils in parallel for 115V, the proper way is to connect the coils by tying the positive with the positive and the negative to the negative. This is indicated by the dot on the diagram as well.

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However, on the datasheet for the power transformer I bought, the diagram looks different. Each of the terminals is labeled with a voltage.

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How do these two diagrams map to each other?

Is the correct connection for parallel to connect 115V to 115V and 0V to 0V, (1 -> 4 and 2 -> 3), or is it connecting the 0V terminals to the opposite 115V terminals (1 -> 3 and 2 -> 4)?

Here is the product listing in question.


  • \$\begingroup\$ I read the data sheet and, on that information alone, I would not trust that transformer at all. I mean, how can two terminals be called 0 volts (on the primary) given that there is a coil between them that MUST produce an emf? \$\endgroup\$ – Andy aka Aug 11 '17 at 10:08

The datasheet drawing is misleading and/or incorrect, as it shows a single primary winding with two "Zero Volt" taps.

If you have the transformer in hand, you should check the primary side with an ohmmeter to see if it is really connected as the drawing, or in fact has two separate windings, with no connection between the two "Zero Volt" terminals.

Edit: If the primary is two separate windings, you should determine what the correct phasing is before connecting the windings in parallel - connect the two "zero volt" terminals together, apply power between them and one "115 V" terminal, then measure the voltage between the two 115 V terminals - if it is zero, the phasing is correct, and the 115 volt terminals can be connected together to parallel the windings. If the voltage between the 115V terminals is 230 V or so, then the phasing is wrong, and the 115 V terminal of each winding should be connected to the Zero V terminal of the other winding for parallel operation.

  • \$\begingroup\$ Then waht do you do? Do you trust that wiring it 115 to 115 and 0 to 0 is correct and it won't melt down when you apply 115 volts because they have labelled the phasing incorrectly LOL? \$\endgroup\$ – Andy aka Aug 11 '17 at 17:10
  • \$\begingroup\$ @Andyaka: If the primary is two separate windings, you should determine experimentally what the correct phasing is to use the windings in parallel. \$\endgroup\$ – Peter Bennett Aug 11 '17 at 17:45
  • \$\begingroup\$ I'm assuming when you mean "apply power" you mean line voltage? \$\endgroup\$ – Ashton Snelgrove Aug 12 '17 at 2:38
  • \$\begingroup\$ @AshtonSnelgrove: yes, line voltage would be fine, although a lower voltage might be a bit safer if you have something handy... \$\endgroup\$ – Peter Bennett Aug 12 '17 at 3:31
  • \$\begingroup\$ I was able to determine that the primary side is two windings. Using the method @PeterBennett describes, I determined that the phase matches the other drawing. Talking through why this would work, when the one primary is connected to power, the other primary behaves like a secondary. The first coil induces current in the second coil, and you can measure the voltage. If that's right, it might be good to put some details in the answer. \$\endgroup\$ – Ashton Snelgrove Aug 14 '17 at 15:52

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