RELAY
In general, a relay is an electrically operated switch based on the magnetic or semiconductor or combination of both. Most of the relays operate the switches mechanically using an electromagnetic energy, while as the semiconductors are used for operations of the in solid-state relays. Relays are the building blocks for access controls and various security products and projects, be it the electric lock, interlocking solutions, delays circuits etc. The main function of relays is to make control of current flow as required for application.
CLASSIFICATION OF RELAYS
The number of poles and throws are the main factors used for the classification of relays. The common terminal for every path in the relays is the pole and each position to which the relay can connect is called as throw. A relay can be of type multi-pole & multi-throws, uni-pole & uni-throw, multi-pole & uni-throw and uni-pole and multi-throws.
| SPST | DPDT | SPDT |
| Single Pole Single Throw | Double Pole Double Throw | Single Pole Double Throw |
| Here it uses one pole | Here it uses two poles, each with | Here it uses one pole |
| two simultaneously controlled throws | and two throws |
Relays are also classified on basis of forms, based on the number of poles & throws as well as the default position of the relay.
FORM A : Normally Open by default, NO (default), SPST type
FORM B : Normally Closed by default, NC (default), SPST type
FORM C : The connection is broken with one throw before making contact with the other. First Break then Make. SPDT type