Why is a photocoupler required?

1. To Connect Circuits with Different GND Potentials: When exchanging signals between a circuit running on an AC line power source and a telephone circuit, for example, you cannot connect through common GND because their GND potentials are different. As they take GND differently, a modem connected to a PC uses photocoupler, for example.
2. To Prevent an Effect of Noise: When you use a microcomputer, which is operated by microsignals, to control big loads such as a motor, solenoid, etc., for example, high noise produced by the load may have a bad effect on the microcomputer via a GND line. If this is the case, it is convenient to use the photocoupler which provides electrical insulation.
3. For Your Safety: A home appliance takes in power from an AC line, but you must ensure that the human body does not come into direct contact with this AC line for safety reason. This provision has been provided by the safety standards in each country, and the relevant product has to comply with it. Intended to electrically insulate the parts where you may come into contact with AC line, the photocoupler is available as a device compliant with the safety standards.