An SPST relay is a switch that switches a current across its contacts. The contacts of a relay are normally open (NO) when they are unpowered and closed when they are powered. The NC contacts are also known as "break" or Form A contacts. The S-contact resistance of an SPST relay is 0 or 1 when the relay is open.

An SPST relay is composed of five pins: two on the input side and three on the output side. The input section consists of a coil connected to one or both terminals. The second section is composed of contactors, with three changeover contacts on each set. The common contact is connected to NC when there is no supply, while the remaining two contacts are open when power is applied.

Electric relays are electrical switches that work by using an electromagnet. A small electrical signal activates an electromagnet, which then opens and closes a switch based on the direction of the current. SPST relays are useful because they can control several circuits with a single trigger. They are also useful because they have different configurations.

The SPST relay is one of the most popular types of electromechanical relays. This type of switch has a wide range of applications, including air conditioning, refrigeration, and heating.