1. Connecting circuits with different ground potentials
For example, when signals are exchanged between a circuit running with an AC line power supply and a telephone circuit, since their earth ground potentials are different, they cannot be connected to a common ground. Photocouplers are used for signal transmission to circuits with different ground potentials as these.
2. Preventing influence of common-mode noise
For example, when it is desired to control a large load, such as a motor and a solenoid, using a microprocessor operated based on weak signals, large common-mode noise produced by such a load may badly affect the operation of the microprocessor via the ground line. In such a case, by using a photocoupler capable of electrical separation, malfunction can be prevented.
3. Safety of the human body
Home electronics products are powered from the AC power line, and safety must be secured to prevent the human body from directly contacting this AC power line. Safety standards are defined in each country and it is compulsory for products to comply with such standards. For the purpose of electrical isolation such as electrical shock prevention, a photocoupler can be used as a safety standards-compliant component.