Electromagnetic Relay SPDT

Electromagnetic relay is an electrically operated switch, which operates on the principle of electromagnetic induction. The current flowing in one circuit causes opening and closing of another circuit. A typical relay consists of a coil, sometimes called solenoid and three terminals, common pole (COM), normally open (NO), normally closed (NC). When no current is flowing in the coil, there is no electromagnetic induction and common terminal is internally connected with NC terminal. This is the normal or non-triggering state of relay. When current flows through the coil, electromagnetic induction occurs and COM terminal gets connected with NO terminal due to magnetic force. This is called operated or triggered state of the relay. Such type of relay is called single pole double through (SPDT) relay and can be used for make and break of circuit. The current through coil is controlled by external circuit and a dc voltage source of usually 12 to 24V. We are using 12V dc SPDT relay. A typical SPDT electromagnetic relay is shown in figure 1.