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I have a center tapped 20-0-20 transformer whose wire colors on the secondary are notated as follows:

c || c -------- blue
c || c
c || c
c || c -------- green
c || c -------- blue
c || c
c || c
c || c -------- green

With a volt meter I have identified which wires connect to one of the windings and which connect to the other winding. However, I don't know which pair of wires is the top winding (in the diagram) and which is the bottom. Looking at the diagram, I want to connect the two middle-most wires so that current flows from top to bottom, but I might accidentally connect the two outermost wires so that current flows from middle to bottom then top to middle. Would that actually be any different than connecting the two middle wires?

Of course current will flow in the same direction in both windings.

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    \$\begingroup\$ Connect both middle wires together and measure the voltage on the two outermost wires. If you have the phasing correct, you will measure the sum of the two windings (40 Vac). \$\endgroup\$ Apr 4 at 19:00
  • \$\begingroup\$ That probably means there are two independent coils sharin a core which can be connected together to form a center tap, so you need to know which is which. \$\endgroup\$
    – DKNguyen
    Apr 4 at 19:00
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    \$\begingroup\$ You must connect a green to a blue to create a centre-tap. But you must not connect the blue of one winding to the green of the SAME winding. You may connect the top blue to the bottom green. Or you may connect the middle green to the middle blue - either way, you get a centre-tap. Hey, its AC! \$\endgroup\$
    – glen_geek
    Apr 4 at 19:14
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schematic

simulate this circuit – Schematic created using CircuitLab

Figure 1. As long as the the series connection is green to blue the transformer won't know.

Minor point: what you have is a 0 - 20, 0 - 20 transformer. What you're trying to create is a 20 - 0 - 20 transformer.

If in doubt or if the secondary windings are all the same colour then connect one wire from each secondary, power up (always with a fuse in the primary) and measure the voltage between the other two wires.

  • If you get 40 V then phasing is correct.
  • If you get 0 V then the coils are out of phase (or the transformer isn't outputting anything so check for 20 V on one secondary). Reversing one secondary will fix the problem.
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    \$\begingroup\$ Two 20 V windings in series will be 40 V, not 24 V when correctly connected. \$\endgroup\$ Apr 4 at 19:37
  • \$\begingroup\$ Yup. 20 + 20 = 40 V. Fixed, thanks, @Peter. \$\endgroup\$
    – Transistor
    Apr 4 at 19:45

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