The Relay

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At A, pictorial diagram of a simple relay. At B, the schematic symbol for the same relay.

A relay makes use of a solenoid to allow remote-control switching of high-current circuits. A diagram of a relay is shown in above figure. The movable lever, called the armature, is held to one side by a spring when there is no current flowing through the electromagnet. Under these conditions, terminal X is connected to Y, but not to Z. When a sufficient current is applied, the armature is pulled over to the other side. This disconnects terminal X from terminal Y, and connects X to Z.

There are numerous types of relays. Some are meant for use with dc, and others are for ac; a few will work with either dc or ac. A normally closed relay completes the circuit when there is no current flowing in its electromagnet coil, and breaks the circuit when current flows through the coil. A normally open relay is just the opposite, completing the circuit when current flows through the electromagnet coil, and opening the circuit when current ceases to flow through the coil. Normal, in this context, refers to the condition of no current applied to the electromagnet.

The relay shown in above figure can be used as either a normally open or normally closed relay, depending on which contacts are selected. It can also be used to switch a line between two different circuits.

Some relays have several sets of contacts. Some relays are meant to remain in one state (either with current or without) for a long time, while others are meant to switch several times per second. The fastest relays can operate several dozen times per second. In recent years, relays have been largely supplanted by switching transistors and diodes, except in applications where extremely high current or high voltage is involved.