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I understand that if a center-tapped transformer is connected in series-opposition as in the image below, there would be no voltage across both circuit terminals. But can the individual voltage induced on a single secondary winding not be used to drive a circuit with a rectifying circuit? There would still be a potential difference between the transformer terminals and the center tap. Could one not use at least one of the secondary windings? I am very confused regarding this and would appreciate your help in understanding this.

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Sure, no matter how these two windings are connected, you can use one or both windings simultaneously as usual.

They just have one common terminal.

The two windings just happen to have common terminal selected so that both free ends are in same phase and voltage.

In theory you could just connect the loose ends together and get double the secondary current, in practice, this should not be done unless the manufacturer says the transformer can be used with secondaries in parallel.

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  • \$\begingroup\$ Thank you for your reply @Justme but if you connect the two loose terminals (top and bottom) how could there be a current if both terminals are at the same potential? Or do you mean to connect both loose terminals to the the center tap? \$\endgroup\$ Commented Nov 28, 2023 at 20:07
  • \$\begingroup\$ @BaldovínCadenaMejía I don't mean using the top and bottom terminals alone as they have no voltage and they are useless. Connecting one or either of them to center tap will cause a short circuit, maybe a molten transformer or blown fuse. What you can do, is use center tap as first terminal, and then you can use bottom or top or both of them as second terminal. \$\endgroup\$
    – Justme
    Commented Nov 28, 2023 at 20:10
  • \$\begingroup\$ In that case @Justme it would be as I postulated in my question description, but how would you get double the current? The voltage would be halved as either top/bottom terminal would be connected to the center tap, making the winding with the unused terminal totally useless \$\endgroup\$ Commented Nov 28, 2023 at 20:18
  • \$\begingroup\$ @BaldovínCadenaMejía You have two individual windings with voltage V and amperage A. Their dotted terminals are already together. If you connect the two non-dotted terminals together, you have two identical windings in parallel, with voltage V and twice the amperage A. \$\endgroup\$
    – Justme
    Commented Nov 28, 2023 at 20:24
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    \$\begingroup\$ Now I understand! In a transformer with this arrangement, either the secondary windings the top and bottom leads can be connected together so that both windings are parallel to each other, or you only use one of the secondary windings to drive the circuit (top or bottom lead AND center tap, leaving the other terminal loose). \$\endgroup\$ Commented Nov 29, 2023 at 15:18

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