True electrical transformers are AC-to-AC (alternating current) converters that provide electrical isolation and proportional voltage and current multiplication between input (primary) and output (secondary) coiled wire windings that may have an iron core coupling them. Transformers work by converting an alternating current flowing through the primary winding into an alternating magnetic field in the core, which then induces the proportional alternating electrical current in the secondary. A regular ohmmeter or volt-ohm meter may be used to check the correct operation of a transformer.
Completely disconnect the transformer from its power source. If the output of the transformer goes to a DC power supply with an output capacitor, make sure the capacitor is completely discharged by carefully shorting its leads with an insulated screwdriver; capacitors larger than a C size battery should be discharged with a power resistor. Once discharged, keep the capacitor shorted with a test lead with clips.
Disconnect the transformer's secondary from the application. If soldered in, you may need to clip its leads, and resolder when testing is complete.
Check the transformer's isolation of primary from secondary. Set the ohmmeter to 1000 ohms. Touch the two lead clips together and the ohmmeter will go from high or infinite resistance down to zero ohms. Clip one lead right to one of the primary winding tabs. Touch the other lead to one of the secondary output leads. It should read infinity, which means the input and output are completely isolated. Touch the second lead to all of the secondary tabs on multiple voltage transformers. Touch the iron core and check it.
Repeat the process with the other primary tab connected. All readings should be infinity. Any other reading means the transformer may have a short that could be hazardous in that input line power may be able to make its way to the output and pose a shock hazard. The transformer should be discarded.
Test the primary windings. A simple 110-volt AC input transformer will have just two tabs. A 220-volt AC transformer may have three tabs, which means it is just two 110-volt windings in series. The input resistance may vary between 2 and 100 ohms, depending on the amount of current the transformer puts through the primary. If the transformer has three tabs, most likely the two outside tabs will read about twice the resistance of either outside tab with the middle tab.
Test the secondary windings with the ohmmeter for an open circuit reading -- infinity, or for a direct short, zero ohms. They will most likely be neither.
Test the transformer's correct operation. Carefully power the primary with its specified AC voltage. Switch the ohmmeter to AC voltage mode and set the range for 300 volts. Note the reading and compare to the transformer's labelling. It should be within 5 per cent.
Check an erratic transformer that stops working only when it's warm by heating it for two hours in a 225-degree oven, then carefully perform the Step 4 and 5 continuity checks on both primary and secondary windings. If there is an open circuit indicated when the unit is hot, a winding is separating internally, and the transformer will have to be replaced.
Wear safety eyewear when working with electrical equipment. Always use insulated wire leads on test equipment.
Always wear insulated footwear when working with electrical equipment.