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i bought this transformer to use in a power supply http://www.dhwelectronics.com/Safety_Isolating_019000600020/P028_1A_Secondary_30VA_Transformer_With_15015_Vac_Ouputs/ i want to have two outputs but as there is only three wires was wondering do i have to cut into the transformer to add a fourth wire or can i solder that wire onto the strip board and then branch a wire off from there ?

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    \$\begingroup\$ What do you want your outputs to be? 30VAC and 15VAC? \$\endgroup\$ – Mark B Apr 26 '13 at 19:52
  • \$\begingroup\$ well i'm assuming that if i have two outputs that they would only be 15vac each. its just the fact that there is only three wires on the output that is confusing me \$\endgroup\$ – sean Apr 26 '13 at 20:34
  • \$\begingroup\$ Yes, but Sean mentions using a potiometer, probably to adjust the outputs. Pots should not be used in the path of lots of current (more than tens of milliamps). If he wants to control AC output voltage, that's one thing but if he wants to have variable DC outputs (like split rails, + and -) then I suggest he ought to use the pots on adjustable regulators (e.g. LM317/337). \$\endgroup\$ – gman Apr 29 '13 at 18:07
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The transformer is wired as follows:

schematic

simulate this circuit – Schematic created using CircuitLab

Notice that the two 15V wires are 180° out of phase, resulting in a 30V(AC) between the outer two windings and 15V relative to the middle wire.

You mustn't cut the transformer's insulation as that is a safety hazard. The two 15V wires share a common ground (0V) wire, you cannot change that. You do have several choices how to use them, but to answer that you have to elaborate in your question on what your goal is.

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  • \$\begingroup\$ thanks. i just want a power supply so i can power other small projects, i have the two circuits built and a potentiometer on each to adjust the voltage i just need to connect the transformer. \$\endgroup\$ – sean Apr 26 '13 at 21:47
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The transformer has one secondary which is center tapped. That is why there are only 3 wires. The voltage across the entire secondary is 30 VAC and the voltage from the center tap to each of the other wires is 15 VAC. You can get two 15 VAC outputs but they will have a common connection, the center tap wire.

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  • \$\begingroup\$ so should i just connect another wire to the center tap so that i can have two separate circuits ? or could i just connect the center tap to one circuit and then branch off a wire to the second circuit? \$\endgroup\$ – sean Apr 26 '13 at 21:30
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    \$\begingroup\$ @Sean Read Mark B's comment - let me re-ask - what do you want your output voltages to be OR what are you trying to feed power to? \$\endgroup\$ – Andy aka Apr 26 '13 at 21:32
  • \$\begingroup\$ i want two outputs therefore would be 15vac each \$\endgroup\$ – sean Apr 26 '13 at 21:35
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This is what the OP wants to do:

schematic

simulate this circuit – Schematic created using CircuitLab

So, yes, you add another wire to the center tap, to be used on both supplies. You must realize that the two supplies share a common connection. Whether that will be a problem depends on what you do with it. If you treat each of the common wires as a "ground", you will be OK.

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