Relays are switches that are turned on and off by a small electrical current.
Inside a relay is an electro-magnet. When a small current energizes this electro-magnet, it attracts an armature blade and closes contact points. Current that the relay is designed to switch on or off can then flow across the points.
As long as the small switching current flows to the relay, the much larger current will flow through its contact points.
Solid-state relays act like a mechanical relay, but do not have any moving parts.
Relays can be used to control the high levels of current in a circuit with a low-current signal, such as in a starter motor solenoid. Relays can also be protective switches, breaking circuits when faults are detected.
