A relay is one of the simplest electrical switching device. It is could be used as an input device or as an output device. Its contact current rating varies from a fraction of an ampere to a double digit values. Switching speed varies inversely to the contact current carrying capacity of the relay. Coil voltage rating can be AC or DC. A relay is commonly used when a load requires higher current than the driving circuit output current rating or when the load requires a different voltage level than the driving circuit output voltage level.
There are three (3) forms of relay contacts for switching circuit:
- Open (NO)
- Normally-open (NO) contacts connect the circuit when the relay is activated. The circuit is disconnected when the relay is inactive or when the power to the relay coil is turned off. It is also called Form A contact or "make" contact. Form A contact is ideal for applications that require to switch a high-current power source from a remote device.
- Normally Closed (NC)
- Normally-closed contacts disconnect the circuit when the relay is activated. The circuit is connected when the relay is inactive or when the power to the relay coil is turned off. It is also called Form B contact or "break" contact. Form B contact is ideal for applications that require the circuit to remain closed until the relay is activated.
- Change-over contacts control two circuits, one normally-open contact and one normally-closed contact with a common terminal. It is also called Form C contact.