Relays are electromechanical or electronic switches that open and shut circuits. Relays operate one electrical circuit by opening and shutting contacts in another. When a relay contact is usually closed (NC), the contact is closed even when the relay is not powered. When a relay contact is usually opened (NO), the contact is opened even when the relay is not powered.
Contacts of relays may be made from metal or solid plastic. Metal contacts suffer very little erosion due to use over long periods because they conduct electricity so well. However, they can short circuit if not properly handled or stored. Plastic contacts tend to break down more quickly because they cannot conduct as much current. But since they cost less than metal, they are used instead of metal where durability is important.
Relays were originally used in radio receivers to select inputs for tuning radios. This is still done today with complex radios such as those found in phones and cars. A single receiver might have 20 different buttons that must all be able to turn on the radio without interfering with each other. Each button would open its own separate circuit through a relay. When the operator wants to tune the radio to a specific channel, he or she would close the circuit for that channel's button.
Buttons on equipment such as record players and televisions work on a similar principle. A single knob or switch controls several functions by using multiple contacts inside a casing.
When a relay contact is usually open (NO), there is an open contact when the relay is not activated, as seen in relay schematics. When the relay contact is normally closed (NC), there is a closed contact when the relay is not activated.
A relay diagram shows which terminals of the relay are NC and which are NO. It does this by showing black circles on the diagram for NC terminals and white circles for NO terminals. A line drawn from a NO terminal to any other terminal or back to the NO terminal represents an open circuit. This means that if current is trying to flow through this connection, it can't because the two terminals are not connected.
To use a relay in a circuit, connect its coil to a power source, then connect either NC or NO terminals to some part of the circuit being controlled by the relay. When the coil is energized, the relay will close either because it has been wired that way or because magnetic force pulls the armature down into contact with the next set of contacts.
Coils of most relays have either single or double windings. A single winding coil only requires connecting it to a power source in order to magnetize it so that it will remain magnetized until it is removed from the source.
A relay is essentially a device with contacts that opens and shuts a switch in response to an input signal (voltage or current) delivered to a coil. Relays can be divided into two types: mechanical relays and solid-state relays. Mechanical relays include magnetic, electric, thermal, and fluid-operated varieties. Solid-state relays are used instead.
All electrical circuits have some form of open circuit or short circuit when they are operating correctly. A mechanical relay operates on the principle of electromagnetism: when an electric current is passed through a coil, a magnetic field is created which causes a metal armature attached to the mechanical relay to move and make or break electrical contact. This allows electricity to flow or not flow through it depending on whether the contact it controls is making or breaking the circuit.
Solid-state relays use semiconductor components such as transistors or triacs instead. They are more reliable because they cannot burn out like their mechanical counterparts would. However, they are also less responsive than mechanical relays. For this reason, they are usually used in applications where speed or reliability matter more than control flexibility. Examples include power supplies for computers or telecommunications equipment where changing the state of the supply too quickly could cause problems for the hardware affected by its output.