How to Replace a Doorbell Transformer

Standard household voltage is 120 volts. A doorbell system gets power from a standard household circuit but it typically operates on just 10 to 24 volts. The device that makes this possible is the transformer. Electrical transformers use two coils of wire and electromagnetic induction to raise or lower a circuit's voltage. In the case of a doorbell transformer, the 120 volts in the household circuit is reduced to 10 to 24 volts (depending on the transformer voltage rating) for powering the doorbell chime unit and button(s).

The transformer typically mounts to the outside of an electrical junction box and includes wire leads that connect to the circuit wires inside the box. These are known as the line-voltage wires, and they carry 120 volts. On the front of the transformer, there are two screw terminals for connecting the two low-voltage wires for the doorbell system. Because these terminals carry only low voltage, they can be exposed without risk of serious electrical shock.

To locate your doorbell's transformer, start at your home's electrical service panel, or breaker box. There may be a metal junction box (with transformer attached) mounted to the panel box itself or on a wall near the panel. If not, look for a transformer junction box in your garage, attic, basement or crawlspace, as applicable. In addition to the transformer box, keep your eye out for two thin low-voltage wires—if you find these wires, following them can lead you back to the transformer found at one end of the wires. Transformer junction boxes may be mounted to the sides of wall studs, ceiling or floor joists or attic rafters.

Once you find the transformer, make sure it's the right transformer—your house may have several. Most houses have a transformer for the thermostat that controls the heating and cooling (HVAC) system. An HVAC transformer usually is located on or near the furnace (or other heating equipment) or between the furnace and the thermostat; it's not likely to be in the attic, for example. Check the rating printed on the outside of the transformer: doorbell transformers typically are 24 volts or less. Anything higher indicates an HVAC transformer, although these can also be 24 volts or less. Another way to check is to look at the low-voltage wires connected to the transformer. If these don't match the wires coming into the doorbell's chime unit or the doorbell button (you have to remove the chime unit cover or the button to check), you have the wrong transformer.

To test your doorbell transformer for failure, set a multimeter to test for AC voltage (VAC) up to at least 24 volts. Touch the tester probes to the low-voltage screw terminals on the face of the transformer, touching one probe to each terminal. If the tested voltage differs from the transformer's rated voltage by more than 2 volts, the transformer has failed. If the test voltage is within 2 volts of the rating, the doorbell has a problem somewhere else in the circuit; check the doorbell button, chime unit and low-voltage wiring.

Although the doorbell itself operates at low volatagge that is unlikely to shock you, replacing a transformer involves working with line-voltage circuit wires. Make sure you understand how to shut off power to the circuit controlling the tansformer before attempting this repair. If you are unsure of your skills, call an elecrician.