Question

An enclosed transformer has four wire leads coming from it. How could you determine which wires are paired with which?

Answer 1

There are three ways to connect the four mystery terminals. Out of these only one is correct.

There are many ways to find out the pairs. Two simple ones are described below.

1. We could use an ohmmeter. The two input wires are in no way electrically connected to the two output wires. Therefore, if resistance between any two is small, these two wires form a pair.
2. Connect a DC voltage source to any two wires. Then connect a light bulb of appropriate rating to the remaining two. If the light bulb glows, we have an incorrect wires to form a pair, as DC current is flowing through both coils and the bulb.
   For a correct connection, non varying DC can not induce emf from 'primary' to 'secondary' coil.

We can not identify primary and secondary coils with the help of these methods.