I previously asked about controlling a large voltage with a small voltage, and it has got me thinking about transformers. When I browse around Digi-Key's site, looking at transformers, I understand most of the selections but I'm confused as to what the difference between a primary and secondary voltage are as well as dual or single primary and secondary windings.
I thought that transformers are bi-directional, so why are there primary and secondary sides?
While transformers can be used in either direction the primary voltage specificiation is especially important for AC line power applications. In this application the transformer must withstand considerable winding-to-core voltage. In the USA this is usually 1500 VAC for one minute. These transformers must also meet creepage and clearance specifications, use reinforced or double insulation on the primary side, not exceed a specified winding temperature, and use materials rated for that temperature. The reason for these requirements is to have a safe product that will not shock a person or cause a fire.
Dual secondary windings are often identical and can be connected in series or parallel.
davecuthbert is right as usual, but also note that "dual primary" means there are 2 primary windings rated for 115vac each. These can be connected in series for 230vac use, but also in parallel for 115vac use. Care must be taken in both cases to get the phases right.