Door chimes typically operate with a power source between 8 and 16 volts. A door chime's transformer, called a step-down transformer, creates a low-voltage power source from a standard 120-volt electrical circuit. Older door chimes often use an external transformer mounted to the electrical panel or to the wall near the chime. Newer chimes usually use self-contained transformers that mount inside the chime's housing. Both types of transformers operate exactly the same. Technicians test a transformer's input and output voltages before replacing them.
Things You'll Need
- Screwdriver set
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Access the transformer. If the door chime uses a self-contained transformer, open the chime's lid. If the door chime uses an external transformer that does not mount to the wall above the chime, unscrew the circuit breaker box's cover with a flat-head screwdriver.
Look at the transformer's identification label. It will state the transformer's primary and secondary voltages. The label often reads "PRIM 120VAC" and "SEC 16VAC." The secondary voltage will match the chime's operating voltage.
Turn a voltmeter to its "VAC" setting. If the voltmeter has multiple VAC settings, choose the low-voltage setting.
Place a voltmeter lead on each of the transformer's low-voltage terminal screws. A door-chime transformer uses two screws, located on the top portion of the transformer, to connect the low-voltage wires that feed the door chime and its switches.
Read the voltmeter. If the meter reads the same voltage as the transformer's stated secondary voltage, the transformer works. If the meter reads below the stated secondary voltage, make note of the voltage.
Turn off the circuit breaker to the door chime. Usually the door chime connects to the same circuit breaker as the lighting or the circuit breaker that controls the room that the chime hangs in.
Access the door chime's two high-voltage wires. Unscrew door chimes with self-contained transformers from the wall with the correct screwdriver. If the door chime's transformer mounts to the wall above the chime, unscrew the wall plate the transformer mounts to. If the transformer mounts to the circuit breaker box, either unscrew the junction box lid that the transformer mounts to or follow the wires that exit the rear of the transformer to the wire nuts.
Unscrew the wire nuts that hold the transformer's high-voltage wires to the circuit that feeds the transformer. Position the bare wires so they do not touch anything.
Turn the circuit breaker to the door chime's transformer on.
Change the voltmeter's setting, if needed, to a high-voltage setting above 120 volts.
Place a voltmeter lead on each high-voltage wire. Use care to avoid touching the wires, or voltmeter leads, together.
Read the voltmeter. If the voltmeter reads between 110 and 120 volts and the secondary voltage reads below the stated voltage, replace the transformer. If the voltmeter reads between zero and 110 volts, check the circuit breaker and the wires between the transformer and the circuit breaker for the problem.