Relay:

In some of the electronic devices, it is not convenient to place a switch accurately where it must be. For illustration, you may want to switch a communications line from one branch to the other from a long distance away. In wireless transmitters, a few of the wiring carries high frequency alternating currents which should be kept in certain parts of the circuit and not routed out to the front panel for switching. A relay makes use of a solenoid to permit remote-control switching.

A diagram and drawing of a relay are shown in figure below. The movable lever, termed as the armature, is held to one side by a spring whenever there is no current flowing via the electromagnet. Under these circumstances, terminal X is connected to terminal Y however not to terminal Z. Whenever a sufficient current is applied, then the armature is pulled over to the other side. This dis-connect terminal X from terminal Y and connects X to Z.

Figure: (a) Pictorial drawing of a simple relay. (b) Schematic symbol for the similar relay.

There are several types of relays, each used for a various purpose. A few are meant for use with dc, and the others are for ac; some will work with either ac or dc. A generally closed relay entire a circuit whenever there is no current flowing in its electromagnet and breaks the circuit whenever current flows. A generally open relay is just the reverse. (Common in this sense means "no current in the coil.") The relay shown in figure below can be used either as a generally open or normally closed relay depending on which contacts are selected. It can also be use to switch a line among two various circuits.

Nowadays, relays are used only in circuits and systems carrying extreme voltages or currents. In most ordinary applications, electronic semiconductor switches, that have no moving sections and can last far longer than relays, are favored.