If you need to step down 120 volts AC to 24 or 12 volts AC, the best way to do this involves a voltage transformer. However, there are some differences in single phase transformers to be aware of. For example, some are designed for isolation transformer applications, and here is more information about them.
What does it mean for a transformer to isolate something? It means the voltage is not directly connected to the power source. This could apply to just about any kind of voltage transformer, but single phase isolation transformers are different. An isolation transformer has both primary and secondary coils separated from each other. There are no electrical connections as with the case of autotransformers which share similar connections.
An isolation transformer takes in voltage from the primary coil and supplies voltage (either stepped up or down) to a circuit, without being part of the circuit. In other words, the power source of the transformer is never connected to the circuit getting power from the secondary coil.
Here is an example of an isolation transformer at work. A computer laptop may run on a 19-volt battery and power source. The voltage going to the laptop is about 120 volts. Single phase isolation transformers keep 120 volts separate from 19 volts. If not, the laptop could receive extensive damage from high voltage AC current.
One of the most important benefits of isolation transformers is safety. For instance, a typical industrial motor control circuit may run on 120 volts AC single phase power. However, the power source going to the circuit may be 277 volts AC three phase electricity. It is very important to keep the 120 volt circuit separate from the 277 volt power source, and an isolation transformer does the job. It also helps to eliminate possible electrical shock hazards.