How to Test a Doorbell Transformer

Doorbell transformers step down household current from 120 volts to around 16 volts. The doorbell transformer might be located anywhere in a home, but electricians typically install them near the home’s main electrical panel or near the heating and cooling system. Once you’ve located the transformer, you can test it with a simple, low-voltage test light, or get an accurate voltage reading with a multimeter.

  1. Locate the transformer box. These are typically only a couple of inches long, wide and deep with two narrow-gauge wires attached with screws. The narrow-gauge wires run to the doorbell buttons. Trace the wires back from those buttons.

  2. Set a multimeter to a low voltage setting -- often 25 AC volts on the meter, with “AC” signifying alternating current. You need not turn off the home's power to check the transformer. Be certain to place the probes on the two narrow-gauge wires leading away from the transformer, not the infeed wires, which carry 120-volt household current.

  3. Touch one probe of the multimeter to each screw head on the transformer, where a thin wire leads toward the doorbell buttons. A reading of 16 volts indicates the transformer works properly. Less than 16 volts indicates the transformer needs replacing.

  4. Touch each probe of a low-voltage test light to each screw head on the transformer if you are not using a multimeter. If the lamp fails to light, the transformer is bad.

  5. Tip

    If the doorbell transformer is mounted on or near a junction box, you can replace the device provided you turn off the electrical power to your home at the main electrical panel. For safety, consult with an electrician rather than tackle the replacement of a transformer mounted on the home’s main electrical panel.


    Don't touch multimeter or test light leads to any screws or exposed wiring on the household current side of the transformer. Doing so could result in electrical shock.