Twenty-four-volt transformers are typically used as control devices. These low-voltage devices are used as step-down transformer in which a higher voltage, generally 120 VAC, is made into a lower working voltage for switches or relays. This basic procedure is an efficient way to test a low-voltage transformer for proper operation.
Remove all electrical power from the electrical appliance or device. Never perform this test with any outside source of electrical power going to the device. Use the screwdriver to remove any electrical covers that may conceal the transformer. This may entail using a manufacturer's set of instructions to correctly access this location.
Understand that there are two sides to the 24-volt transformer. The high side is the line-voltage of the transformer. This is the electrical connection to the feeding voltage, generally a 120-VAC power. The secondary side, or low-voltage, side is the power that is transformed to 24 volts. There is no direct electrical connection between the high side and the low side areas of a transformer used for a 24-volt application.
Switch the volt ohmmeter to "ohms" and insert the test leads into the meter. The red lead goes into the slot that reads "ohms" and the black lead goes into the common opening. Touch the two leads together and the ohmmeter should be reading "zero ohms," or a closed circuit.
Place the leads of the ohmmeter to the leads on the high side of the transformer. The meter should be identifying a resistance reading. This reading will vary with the type of transformer used in the circuit. If the meter indicates an open circuit or infinite resistance, the transformer is bad on the high side and must be replaced.
Follow the same procedure from Step 4 on the low side connections. The meter should give the same resistance reading, results in ohms for the low side. If the meter shows an infinite reading or wide open, the low side is broken internally and the transformer must be replaced.
If the transformer tests are positive (i.e., a good reading), check for blown fuses in the circuitry. The electrical schematic for the circuit should be consulted. Follow all local and state electrical codes when servicing or replacing any electrical device.