Wiring doorbell transformers is not too difficult. Make sure the Power is OFF! before you work on it.

Doorbell transformers need to provide the required voltage for the unit in question.The transformer reduces the 120 volt power in your home to the desired voltage for your doorbell. This is usually between 12 and 24 volts.

For related information, see the articles 'How To Fix a Wired Doorbell' ,'Wired Doorbell Buttons' and 'Wired Doorbell Installation'.

You will need to use a multi meter to tell if the transformer is working properly. Measure the voltage between the two terminals on the transformer.

Multi meters are a little tricky to use, take a look at, Using a Multimeter, for more information.

Does it match the secondary voltage that is needed. If not, you will need to replace the transformer. The 'secondary' or 'output'  voltage should be marked on the transformer. It will be on an namplate, possibly embossed on the plastic shield where the low voltage wires are connected.

For the transformer listed in the picture the voltage should read 16 volts.

They are about ten to fifteen dollars. You can get them one from a home supply store. Obtain a new one with the same ratings as the one you have.

Turn off the power and hook up the wiring the same as the old on. Use a voltage tester to make sure the power is off. One side of the transformer has the 120 volt wiring attached to it. This is the side that will shock you if the power is not off. Tag the wires if you need to.

The low voltage side will have two small wires attached to it. The polarity for these two wires does not matter. Remove both sets of wires from the transformer.

After you have removed the wires you can unscrew the transformer and install the new transformer and hook up the wires. The 120 volt wires go to the 'Line' side of the transformer and the low voltage wires connect to the 'Load' side.

Fixing Transformers

Doorbell transformers need to provide voltage for the doorbell unit in question.  You will need to use a multi meter to tell if the transformer is working properly.

There is not much you can fix on a doorbell transformer that is not converting the power correctly.

When this happens you will need to replace the transformer.  They are about ten dollars.  Obtain a new one with the same ratings as the one you have. 

Turn off the power and hook up the wiring the same as the old one.  Tag the wires if you need to.

For related information, see the articles 'How To Fix a Wired Doorbell' ,'Wired Doorbell Buttons' and 'Wired Doorbell Installation'.