Doorbell Transformers and Why Video Doorbells Fail

Why this matters

A wired video doorbell that reboots, drops video at night, goes offline after a ring, or makes the chime buzz is usually not a bad doorbell. It is a doorbell asking a circuit built for a plain push button to run a small computer with a camera, an infrared illuminator and a radio, all day, through a chime coil. The fix that clears most of these calls is a transformer with more capacity, and the reason it works is worth understanding, because a tech who only knows the fix will also install it on the one house where the problem was the chime or the wire.

Safety first: the transformer has a 120 V side

The bell transformer's primary is wired to a 120 V branch circuit, usually mounted on a junction box near the electrical panel, on the side of the furnace or air handler, in the attic, or (in older houses) inside the chime housing itself.

• Work on the secondary screws and the bell wire is power-limited work. Turning off the breaker anyway costs nothing and keeps a slipped screwdriver from shorting the secondary and opening the transformer's internal protection.
• Replacing the transformer is line-voltage work. The breaker is off, locked and tagged, and the primary leads are proved dead on a known live source before and after, per NFPA 70E-2021, 120.5, with work practices at 29 CFR 1910.333(b)(2). Those rules are written for qualified persons; if you are not qualified for line-voltage work, the transformer swap goes to an electrician and your job ends at the secondary. Who may do low-voltage work at all is set by your state.
• And if the transformer is not on a box (leads spliced in an open joist bay, or a transformer screwed to a joist with its primary hanging), you do not swap it as found. It goes to an electrician to be put on a box.

The circuit you are actually working on

A plain doorbell circuit is one series loop. The transformer's two secondary terminals feed it. One side goes to the chime's TRANS terminal, through the chime's solenoid coil, out of the chime's FRONT terminal, down to the button and back to the transformer. Press the button and the loop closes, current flows through the coil, the plunger strikes the tone bar.

A wired video doorbell replaces the button in that same loop:

 line voltage            16 V Class 2 loop
 (inside a box)
 +-------------+  sec 1
 | transformer |----------------------+
 | 16 V  30 VA |                      |
 +-------------+                 chime TRANS
        | sec 2                  [ chime coil ]
        |                        chime FRONT
        |                             |
        +------ video doorbell -------+
               (where the button was)

Everything the doorbell gets passes through the chime coil. At idle the doorbell draws a small current continuously, and that current flows through the coil. The doorbell is designed so this current is too small to pull the plunger. To ring a mechanical chime, the doorbell briefly shorts its own terminals the way a button would, the coil gets the full current and strikes, and the doorbell runs for that moment on internal storage.

That series arrangement explains the three classic chime symptoms:

• The chime buzzes or hums all the time. The doorbell's idle current is high enough to vibrate the coil. Most doorbell makers supply a small module ("power kit" or "chime bypass") that mounts across the chime terminals and carries the idle current around the coil. Install the one the doorbell's maker specifies; a generic substitute can stop the chime ringing.
• The chime does not ring at all, or rings weakly. Often the chime is electronic (it plays a tune) rather than mechanical, and many video doorbells cannot drive one directly. The doorbell's sheet lists compatible chime types.
• The coil burns out. A doorbell holding the loop closed, or a stuck button, keeps full current on a coil rated only for a momentary press. The coil discolors and smells, and the chime stops.

What the transformer label tells you

A bell transformer is labeled with its secondary voltage and its rating in volt-amperes (VA). Voltage times current equals VA, so dividing the rating by the voltage gives the current it can deliver continuously:

10 VA at 16 V is the long-standing builder default, sized for a button that is pressed for a second at a time. It was never meant to carry a continuous load. Many older houses have 10 V transformers, which are lower still.

Most wired video doorbells ask for 16 to 24 V AC, and many recommend a transformer of around 30 VA. Read the doorbell's own sheet for both numbers, and for whether its voltage range is measured at the transformer or at the doorbell, because the two differ by the drop across the wire and the chime.

Do not substitute the furnace transformer. It is 24 V and sized for the heating and cooling equipment, and tying the doorbell to it adds an unplanned load to the furnace's control circuit and can over-volt a 16 V chime coil.

Why voltage sags, and why it sags at the worst moment

A transformer's secondary voltage is not fixed. With no load it reads above its label (a 16 V unit reading 17 to 18 V unloaded is normal), and as current rises the voltage falls, because of the resistance of the transformer's own windings. The closer a transformer runs to its rating, the further it sags. A 10 VA unit carrying a video doorbell's idle load is already well into its rating before anyone rings the bell.

Three moments stack load:

1. Night. The doorbell's infrared illuminator switches on and its draw rises.
2. A ring. The doorbell shorts the loop to strike the chime, and the coil draws far more than the doorbell ever does.
3. A video upload or live view right after the ring, when the doorbell's radio and processor are at full load and its internal storage is depleted.

So the failure lands exactly when the customer is watching: someone rings at night, the voltage sags under the chime's current, the doorbell's storage cannot ride it out, and it reboots while the visitor is on the step.

The wire adds its own drop. Bell wire is commonly 18 or 20 AWG. Copper at 20 AWG is about 10.2 ohms per 1,000 ft. A loop of 140 ft of conductor (transformer to chime to doorbell and back) is 140 x 10.2 / 1,000 = 1.43 ohms. At a 1.1 A chime current that loop alone drops 1.1 x 1.43 = 1.6 V, and at a 0.45 A idle current, 0.45 x 1.43 = 0.64 V. Long runs on thin, old wire are a real part of the budget.

Worked example: the doorbell that reboots when the bell rings at night

The complaint. A video doorbell installed by the homeowner two months ago reboots most evenings when someone rings it, and twice it has gone offline overnight. Daytime rings work. The chime is mechanical. The transformer, on a junction box beside the electrical panel in the basement, is labeled 16 V, 10 VA, Class 2. The doorbell's sheet asks for 16 to 24 V AC at the transformer and recommends 30 VA.

Measurements, with the breaker on, metering the secondary screws. A clamp meter on one bell wire at the transformer, an AC voltmeter across the two secondary screws:

• No load (doorbell wires lifted): 17.6 V.
• Doorbell idle, daytime: 0.30 A, 15.6 V. That is 0.30 x 15.6 = 4.7 VA, already 47 percent of the 10 VA rating, and the voltage is just under the 16 V floor on the sheet.
• Doorbell idle, night (illuminator forced on from the app): 0.45 A, 14.8 V. That is 0.45 x 14.8 = 6.7 VA, 67 percent of the rating.
• During a ring: 1.1 A, 10.9 V. The transformer is now delivering 1.1 / 0.625 = 1.76 times its rated current, and the voltage has fallen 17.6 - 10.9 = 6.7 V from its no-load value.

Reading it. Even at idle, at night, the transformer is below the doorbell's stated floor before a ring happens. A ring pushes it deep into overload. The doorbell's internal storage was drained by the night load and could not ride through 10.9 V.

Ruling out the wire and the chime first. With the breaker off and both ends lifted, the loop resistance read 1.4 ohms, which matches about 140 ft of 20 AWG and is in line with the route the homeowner described. The chime coil was not discolored and the chime did not hum at idle, so the bypass module the doorbell shipped with was doing its job. Neither explains a 6.7 V sag at the transformer itself; that drop is the transformer's own.

The fix. An electrician on the same visit replaced the transformer with a 16 V, 30 VA Class 2 unit on the same box, with the primary proved dead before and after per the safety section. Restoring the breaker is the step that re-energizes the 120 V side, so the box cover went back on first and the breaker was closed with the tech standing to the side of the panel, not in front of it. Re-measured:

• No load: 17.2 V.
• Idle, night: 0.45 A, 16.6 V. That is 0.45 x 16.6 = 7.5 VA, 25 percent of the 30 VA rating, and inside the 16 to 24 V range.
• During a ring: 1.1 A, 15.8 V. That is 1.1 / 1.875 = 59 percent of rated current, and the sag from no load is 17.2 - 15.8 = 1.4 V rather than 6.7 V.

Two weeks of evening rings with no reboots confirmed it.

What would have changed the call. If the loop resistance had come back at 5 or 6 ohms (a corroded splice, or a run four times longer than expected), the wire would have been taking a large part of the drop and a bigger transformer would only have partly helped; the run gets traced or replaced. See related: Tracing a Low-Voltage Run in a Finished House. If the chime had been humming with the bypass module installed, the module was wrong for that chime or wired incorrectly, and no transformer would have quieted it. And if the measured voltage had been inside the range at every moment, including the ring, the doorbell was rebooting for another reason, and the next look is at its Wi-Fi signal at the front door, which belongs to a sibling card.

How to verify you got it right

• Secondary voltage measured three ways and written on the ticket: no load, idle at night, and during a ring. All three are inside the doorbell's stated range, measured where its sheet says to measure.
• Transformer current during a ring is under its rated continuous current, or the overage is momentary and small.
• The chime is silent at idle and rings cleanly on a press.
• The transformer is on a box, labeled Class 2, and its rating is in the device record for the next tech.

References

• Doorbell and chime manufacturer documentation for voltage range, VA recommendation, compatible chime types and the specified bypass module
• Published copper conductor resistance tables (ohms per 1,000 ft by AWG)
• NFPA 70E-2021, 120.5, and 29 CFR 1910.333(b)(2) for work on the transformer primary
• See related: Low-Voltage Wiring Basics for Non-Electricians; Measuring Wi-Fi Signal Where the Equipment Lives; Leaving a Device Record the Next Tech Can Use