Low-voltage transformers drive many household appliances, from doorbells to air conditioning systems. Low-voltage transformers connect to the home's high-voltage power supply and produce a safe low-voltage power supply.
The technician should use a transformer turns ratio tester to troubleshoot the transformer and find out the cause of the failure before replacing the transformer. Typically, transformers fail only after another part of the circuit has shorted to ground or consumed an unusually high current. In these cases, replacing the transformer without first identifying and correcting the fault will only cause the transformer to fail again.
Use the label as a guide to identify the terminals of the transformer. A transformer contains inputs, called "primary" terminals, and outputs, called "secondary" terminals. The labels of transformers identify the input (high voltage) and output (low voltage) sides, as well as the input and output voltages (measured in voltage alternating current (VAC)) and their corresponding terminals.
Go the multimeter to its VAC function. The multimeter has a variety of settings, including VAC function and resistance function. Each setup measures a specific electrical function.
Test the input voltage of the transformer using a multimeter and the label of the transformer as a terminal guide. Place a multimeter lead on each input voltage terminal and record the voltage.
Check this reading with the input voltage specified by the transformer. If the voltage reading does not match the specified input voltage, troubleshoot the voltage source before continuing to use the transformer.
Test the output voltage of the transformer with a multimeter. Compare the reading with the specified output voltage. If the output voltage reading is correct, the transformer is working properly.
If the input voltage reading is correct, but the output voltage reading is high or low, the secondary winding is faulty. The transformer must be replaced.
If the input voltage reading is correct, but the transformer does not produce any output voltage, check the turn-on of the transformer's secondary windings. Also, check the secondary circuit for short circuits.
Disconnect power from the transformer. Test the voltage at the input. The multimeter reading should be "zero".
Turn the multimeter to the "Resistance, Ohms" function. Contact the leads of the multimeter together. The multimeter should beep to indicate continuity.
Disconnect the wires to the input side of the transformer. Place a lead on each input terminal. The multimeter should show "Continuity". If the multimeter reading is not continuous, the input winding is shorted. Replace the transformer.
If the multimeter reads continuously, reconnect the input lines.
Disconnect the output line of the transformer. Check the turn-through of the secondary windings with a multimeter. If the multimeter shows continuity of the secondary windings, troubleshoot the secondary circuit.
If the multimeter does not show the continuity of the secondary windings, check that the secondary circuit is not shorted to ground – usually bare wire. You will have to replace the transformer.