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 ?
The transformer is wired as follows:
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.
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.
This is what the OP wants to do:
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.