I have a center tap transformer that does 110 or 220VAC stepdown to two 6V rails (with opposing phases). On the 110/220 side there are what appear to be two separate rails, and the wiring diagram is a bit confusing to me:

Two black/red rails on left, 6/0/6 center-tap rails on right

In order to use this on a 110VAC source and get the proper 6VAC output, how do I wire up the mains side? Per the diagram it seems like I would wire the mains to a single black/red set (with the other set tied off and insulated), or possibly tie them in parallel (black to black, red to red). Is the idea that for 220VAC input I'd tie the center black and red together and wire the 220 mains to the outer pair of wires?


Your understanding is correct.


simulate this circuit – Schematic created using CircuitLab

Figure 1. Primary and secondary windings.

Note that the voltage between the two blue terminals will be 12 V AC.

You could use only one primary for 110 V operation but the maximum power you can draw from the transformer would be halved.

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  • \$\begingroup\$ Thanks! Your answer came in slightly later but it also had useful diagrams, so I’ve accepted this one. \$\endgroup\$ – fluffy Dec 24 '17 at 11:54
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    \$\begingroup\$ Thanks. I usually advise that that you wait for the planet to turn at least once on its axis to give the whole of humanity a chance to answer your question. Even if mine answers it satisfactorily you can receive some other insights and someone may have some relevant points that will help your understanding. \$\endgroup\$ – Transistor Dec 24 '17 at 11:56
  • \$\begingroup\$ Sure. In that unlikely event (given the simplicity of the question) I could change the accepted answer. :) Mostly I just needed confirmation that my recollection of how transformers work was correct (it’s been a while). \$\endgroup\$ – fluffy Dec 24 '17 at 12:05
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    \$\begingroup\$ You could use only one primary for 110 V operation but the maximum power you can draw from the transformer would be halved... AND you'd have to insulate the other primary wires as there's 110v on them by transformer action \$\endgroup\$ – Neil_UK Dec 24 '17 at 12:32
  • \$\begingroup\$ If you're interested, this is the project I asked this question for. I linked to this answer in the build instructions since it was helpful to me, and might be for someone else too! thingiverse.com/thing:2740698 \$\endgroup\$ – fluffy Jan 1 '18 at 2:59

Parallel black to black. Parallel red to red. Apply 110 Vac between black and red. Take out 6 Vac between blue and yellow.

Yes, the idea for 230 V operation is to tie them in series. Very common arrangement to be able to have the same transformer for the world market. Dual 6 V secondaries is most probably to be able to get symmetrical + and - voltage DC around center ground on the secondary.

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    \$\begingroup\$ Cannot do blue to blue, the phasing is not marked, but that is a centre tapped winding... Connecting blue to blue will let the smoke out. \$\endgroup\$ – Dan Mills Dec 24 '17 at 14:51
  • \$\begingroup\$ @DanMills Oh! Didn't see that. I'll edit. \$\endgroup\$ – winny Dec 24 '17 at 14:58

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