Relay

A Relay is an electrically operated switch designed by Joseph Henry in 1835 with the capability of controlling an output circuit of higher power than the input circuit. An electromagnetic relay is simply an adaptation of an electromagnet. Current passing through the relay coil creates a magnetic field which attracts a lever and changes the switch contacts. A relay coil can be energized in following three ways:


1. Normally-open (NO) contacts 

They connect the circuit when the relay is activated and the circuit is disconnected when the relay is inactive. It is also called a Form A contact or "make" contact.      












2.  Normally-closed (NC) contacts

They disconnect the circuit when the relay is activated and the circuit is connected when the relay is inactive. It is also called a Form B contact or "break" contact.














3. Change-over (CO), or double-throw (DT), contacts


 


They control two circuits i.e. one normally-open contact and one normally-closed contact with a common terminal. It is also called a Form C contact or "transfer" contact ("break before make"). Another type of contact that utilizes “make before break" functionality is called a Form D contact.


Application & Selection Criteria