A relay is nothing more than an electrical switch. However, while a simple switch, like that used in a household light, is operated by hand, in a relay a second electrical circuit is used to open or close the circuit.
The advantage of using a relay is that a low power electrical circuit can control one carrying much more power. There are several reasons for wanting to do this. First, it keeps humans away from potentially lethal current and voltage, and second, it avoids the need to bring thick cables into the switches and control panels that a human operator would work with. A third reason is to provide electrical isolation between two circuits. This is achieved because no current flows between the low power and high power circuits.
The relay is an essential component of many electrical and electronic systems. Relays are found in household appliances such as washing machines and dishwashers as well as automobiles and in industrial applications. A common use is to control motor circuits. When first started, an electric motor draws a high current and so needs a thick cable to supply this power. By using a relay a low power circuit – often 24 volts or less and using less than one amp – can energize the motor circuit.
The heart of a relay is a coil of wire. When electricity flows through the wire it creates a magnetic field strong enough to attract or repel a nearby magnet. (Remember that opposites attract while like poles repel.) But when the current is turned off this force disappears.
In a relay the magnet is a rod or “armature” positioned along the center of the coil. When a current flows through the coil a magnetic field is created that pushes the armature in one direction. If the current flow was reversed an opposite magnetic field would be created and the armature would move in the other direction. In practice it's difficult to reverse the direction of the current in a circuit, so relays use a spring to return the armature to it's at-rest position when the current is turned off.
The armature operates a pair of contacts that, when closed, complete the high power circuit. Relays can be designed so that completing the low power circuit either opens or closes the contacts. If it is the type that closes the contacts when current flows on the low power side it is termed “normally open” (designated NO.) This can be thought of as a fail-safe design since, if the low power circuit is broken current also stops flowing in the high power circuit. Alternatively, if current flows in the high power circuit until the low power circuit is energized the relay is termed “normally closed” (NC.)
Relays have two weaknesses: for some applications they may react too slowly to the opening and closing of the low power control circuit, (this is a function of the way in which the magnetic field forms and dissipates,) and the high power circuit tends to suffer from arcing across the contacts. Both of these can be minimized by careful attention to the design, but a relatively new alternative is the solid state relay.
A solid state relay is a semiconductor device that uses transistors to switch the high power circuit. These offer higher reliability than the traditional contact-type relays but tend are relatively expensive and can be burnt out by higher than expected voltages. Should this happen, replacement is the only option.
Relays can be found in many electrical and electronic systems. Solid state versions have been developed but for cost and simplicity it's hard to beat using an armature inside a coil to open and close a pair of contacts.