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I have a transformer with two secondaries, one is 24 V / 500 mA and the other is 12 V / 200 mA. I want to wire them in series to make a center-tap transformer:

schematic

simulate this circuit – Schematic created using CircuitLab

I know the voltage is going to be sum of two winding.

How do I calculate the current?

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    \$\begingroup\$ Now you have a 36V/200mA output. The lower one determines the upper limit because the load current will flow through both of them. You shouldn't draw 500mA from a 200mA-rated winding. \$\endgroup\$ – Rohat Kılıç Mar 12 '20 at 14:56
  • \$\begingroup\$ That's not a centre tap. \$\endgroup\$ – vu2nan Aug 20 '20 at 10:39
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    \$\begingroup\$ @vu2nan Look at the right side of the diagram, what is it? \$\endgroup\$ – ElectronSurf Aug 20 '20 at 12:34
  • \$\begingroup\$ That would be a centre tap only if two identical transformer secondaries were to be connected in series. \$\endgroup\$ – vu2nan Aug 20 '20 at 13:27
  • \$\begingroup\$ We can perhaps call it a "between tap" :) \$\endgroup\$ – Vladimir Cravero Aug 21 '20 at 7:33
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How do I calculate the current?

Those currents aren't currents that will flow, they are the winding's maximum rating. That means the current should not exceed that value or you'll overload the transformer.

The maximum current when using 36 V will be 200 mA as that is limited by the 200 mA rated 12 V winding.

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If you look closely, you will see the primaries are wired out of phase. Hence the output voltage would be 12VAC only.

The primaries would need to be connected this way to get 36VAC. The load current would be required to be limited to 200mA.

enter image description here

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Bear in mind that if you are sticking a bridge rectifier/smoothing capacitor on the output of that transformer then the output voltage will be about 1.414*36V minus a couple of volts drop across the bridge rectifier (also allow for ripple) and the maximum dc load current which can be drawn will be about 200mA/1.7.

200mA is the secondry RMS rating.

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