What is a doorbell transformer?

It bolts to a knockout on a junction box, which puts it in unglamorous places: the side of the electrical panel, a basement or crawlspace joist bay, the ceiling of a coat closet or pantry, the attic near the chime, or tucked directly behind the chime cover itself. Look for a metal cube roughly 2 by 2 inches with two small screws and thin bell wire running off it. The reliable way to find yours is to pull the chime cover and follow the low-voltage wire back, or simply check the chime housing first, since a surprising number hide right there.

The transformer is energized 24 hours a day, drawing a couple of watts even when nobody rings, and its real job is holding voltage under load the instant the button closes the circuit. Two failure modes show up. An open winding gives you nothing: the chime is silent, the video doorbell is dead, and a multimeter across the two screws reads 0 volts AC. The subtler failure is an undersized or tired unit that still reads fine unloaded but sags once a camera draws from it, producing a doorbell that reboots during calls, an LED ring that flickers, or a mechanical chime that hums instead of striking. A healthy 16V transformer measures 16–24V AC unloaded; under 10V means replace it. The circuit feeding it is often an unlabeled lighting circuit, so identify the right breaker before touching anything.

Nothing about it is scheduled, which is exactly why it gets forgotten. Press the button twice a year during the same walkthrough where you test smoke and carbon monoxide alarms, and confirm the chime actually strikes rather than buzzes. Before installing any video doorbell, meter the terminals and read the VA stamp: replacing a 10VA transformer costs $15 and forty minutes, while chasing intermittent camera dropouts for six months costs your weekends. Add the doorbell test to your home maintenance checklist so it rides along with work you already do.