Installing an electrical transformer isn't as simple as installing a circuit breaker. There are many types of electrical transformers for a wide range of needs. One common type is used in doorbells. Doorbells don't need the full 120 volts from the standard house circuit to operate; installing a doorbell requires a transformer to "step down" the voltage to 24 volts or less depending on the doorbell. A single-button doorbell has three connections. Starting with the power source, the first connection goes from the circuit to the transformer, then from the transformer to the bell, and a third from the bell to the button.
Pushing the button connects the circuit and activates the bell. Without the transformer, there would be far too much voltage and the circuit breaker would trip to prevent damage from an overload. This is one of the most simple electrical transformer installations, depending on your knowledge of electronics you may be capable of more complex installations. If you aren't sure of your abilities, consult a professional instead.