How to identify the matching transformer terminal by means of terminal resistance? and which terminals is for input or output?
Doing a resistance test on a transformer will not give the information you require as you will read very little, if any resistance. What you need to do is an impedance check and that gets involved. If it is a normally wound transformer, not an auto transformer, you can isolate which is the primary and the secondary windings. Unless the transformer is a multi voltage input unit, the primary side will have continuity through only two terminals. The secondary could be more than two if it is a multi output voltage one. If the transformer is a step down, the primary windings will be the thinner wired windings and vice-versa. To actually test the transformer, get hold of a transformer with about a 20 vac output and try it across the two terminals you have identified as the primary windings, you should ge a proportional voltage of the actual output. eg. with 20 vac input and a 2 vac reading at the secondary, the actual output would be 11 vac if your mains voltage is 110 volts. Take care that you are using the transformer at the voltage it is designed for, otherwise it will overheat and burn out.