How to Replace a Doorbell Transformer

finger pressing doorbell in bright hallway
  • 1-2 hours
  • Intermediate
  • 50

Replacing a doorbell system or its components is less demanding than it might sound. The first thing to realize is that when replacing components, all the wiring is already in place, meaning that there’s a junction box nearby with a 120 volt feed from the breaker panel or from another fixture or an outlet box. If one of the push buttons, or the doorbell itself, needs replacing, all there is to do is unscrew and pull it out, then remove the wires and hook up the new component the same way as the old, and return it to its place.

Unlike the push button or the doorbell that reside in full view, the transformer is concealed. It will likely be in the immediate vicinity, though—in the attic right above, in the basement, or in a nearby closet. A look around the doorbell should give a pretty good idea where it might be.

Once it is located, there should be a junction box near it, where the primary windings of the transformer are connected to a 120 volts power feed. The following steps make up a methodical and secure procedure to replace the component.

Step 1 - Shut off the Power

The first thing to do is to secure the site from possible electrical injuries by turning OFF the breaker assigned to that circuit at the main breaker panel.

Step 2 - Check Power at Junction Box

Removing the cover from the junction box exposes all the wires and wire nuts. Going back to the junction box, the multimeter should be used to determine if the power is really off, by adjusting to AC Volts and inserting the two probes inside the wire nuts on the black and the white wires. Any voltage reading would mean that the wrong breaker was turned off and to keep looking to shut off the right one.

Step 3 - Remove the Wires

After getting the power off, the wire nuts can be removed and the wires pulled out of the box. After this is done, the type and size of the transformer are to be determined.

Most transformers for this purpose have a male threaded fitting with a lock nut on one side with the three wires from the primary winding coming out of it as in figure 1. That fitting is inserted into a knockout plug in the junction box where it is to be secured, leaving the wires inside the box.

wires in a doorbell transformer

If the opposite side of the transformer has two or three terminal screws, the output for a two-terminal transformer is either 16 volts or 24 volts, and it should be determined which the doorbell requires.

If the transformer has three terminal screws, it is a tri-voltage transformer putting out 8 volts, 16 volts, and 24 volts, depending on which two terminals are tapped upon (Figure 2).

Once this is determined, a new transformer can be purchased. If unsure, it might be better to get a tri-voltage.

doorbell transformers

The required voltage might also be determined by unclipping the cover from the doorbell or even removing the doorbell from the box to read the electrical specifications on it (Figure 3).

doorbell transformer with highlighted area

Also, it should be noted that many doorbells now come with built-in transformers and work wirelessly. To keep things simple, it can be connected to the same place as the old transformer was. The push-buttons can then be screwed to the door frames and the job is done!

Step 4 - Disconnect Old Transformer

With a new transformer of the proper voltage in hand, the defective one is removed by disconnecting the two wires at the output of the transformer, and next the two black and one green wire must be disconnected.

Step 5 - Remove Old Transformer

With that done, the large lock nut (Fig.1) can be removed to release and remove the transformer from its knockout opening of the junction box.

Step 6 - Insert New Transformer

The new transformer can be put in place in the same knockout opening and secure with the lock nut.

Step 7a - Connect New (Single Voltage) Transformer

With a single-voltage transformer matching the voltage of the doorbell, the two wires in the cable leading to the doorbell are to be attached to the two terminal screws on the transformer’s secondary winding. It doesn’t matter which wire goes to which terminal screw.

Step 7b - Connect New (Tri-Voltage) Transformer

If the transformer at hand is a Tri-Voltage, the following drawing in figure 4 shows how it should be hooked up to get the proper voltage to avoid damaging the coil activating the doorbell.

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doorbell transformer diagram

Step 8 - Check for Wire Mix-ups

In the event that during the entire process so far, some of the wirings got pulled out and mixed-up and the whole bunch of wires becomes a confusing web-like mess, it shouldn’t become a reason to panic. When the front cover is removed from the doorbell, a wiring diagram along with other specifications will usually be found inside as in figure 5. But even if there isn’t one, the wiring can be done as in this diagram.

doorbell transformer with wiring diagram

Step 9 - Attach Buttons

With this wiring done so far, the two push-buttons are hooked up to the doorbell, which in turn is wired up to the output of the transformer. It’s time to add power to the mix by connecting the green wire from the transformer to the ground screw or ground wire in the junction box. The two black wires can now be connected to the black and the white wires coming from the breaker panel. Here again, it doesn’t matter which goes where. All the connections must be capped with wire nuts.

Step 10 - Turn Power Back On

The circuit breaker dedicated to the circuit can now be turned back on, and the new transformer can be tested by pressing the push-buttons for the front and the back doors.

If the test fails, the doorbell can be quickly checked by verifying first that one wire from each of the push-buttons and the output of the transformer should have a common connection together as the three white wires in figure 6.

doorbell transformer with cover on and off

By shorting out the middle terminal with either outside terminal on the doorbell, they should both ring, in which case it can be assumed that there is a problem in the switch or the wiring to the switch.

If by shorting the terminals neither side of the bell rings, there is a problem with the supply voltage (transformer) or the cable from the transformer up to the doorbell’s terminal board. They can be checked with the multimeter set on AC Volts. The live wire feeding the transformer should also test at around 120 Volts.

Once everything checks out and all is working, the covers to the junction box and for the doorbell can be returned in place as in figure 7 to have everything back to normal.