Relays Overview

What Is a Relay?

A relay is a type of switch that is controlled by an electrical circuit rather than by a human operator. These devices are among the most common of all electrical components and they come in a wide variety of designs.

Relays first came about as the result of telegraph technology. They continued to be used in communications, being employed in telephone exchanges when voice telephones became the norm.

Relays can even be used for simple computer logic. The first mechanical computers employed these devices, though they have long since been replaced by semiconductor devices that allow logic gates to trip based on electrical input alone.

Relays can be used as safety devices, cutting power under dangerous conditions. They come in all sizes, with very large relays sometimes being used to control the function of very large machinery and high-powered electrical circuits. They are inexpensive, very durable and can provide a wide range of functions within a device.

How Does a Relay Work?

The actual working parts of a relay are very simple electronic components. The most basic type of relay contains contacts for the electrical circuit to which it is attached, an armature that is linked to the contacts, a spring and an iron core with a coil.

When the coil receives electrical energy, it creates a magnetic field. This field is what powers the armature. The contacts either open or close because of this armature. There are relays available that are normally open and normally closed.

When power is removed from the device, the relay goes into the opposite condition that it was in when it was powered up. The powered state relies on the magnetic field to move the armature. In the depowered state, a spring is usually employed to return the armature to its other position.

Other components may be utilized in a relay. For example, a diode is often employed in DC relays to prevent any voltage spikes from occurring due to the change in the relay’s state. Arcing and spiking can be problems with resistors that are solved through various means.

What Type of Applications Need Relays?

Relays are used in an incredible number of different applications. They are used for similar reasons almost across the board, however.

Relays are used in alarm systems extensively. A normally open relay may be part of a door alarm, for instance. When the circuit’s condition is changed due to the door being opened, the relay may close, completing the alarm circuit and triggering the alarm to sound.

In some cases, relays are used to protect electrical components, devices and human beings. These are called protection relays. They are used on circuits ranging from very low-power designs all the way up to electrical distribution grids.

Relays are often used as repeaters for signals and, in fact, that was one of their first applications. On early telegraph systems, a repeater could receive a weak signal and repeat it down the line, allowing the intended recipient—or the next station—to receive the message clearly.

In some vehicles, there may be more than one battery present. This is particularly true in vehicles that are designed for rugged usage, such as 4x4s, where a dead battery may spell disaster. A relay can be used to draw power from one battery and, once that battery has expired, to automatically switch over to the other battery as a power source.

Though they have been displaced by semiconductor devices over the years, there are still applications where relays are used for logic functions. Under some conditions, semiconductors do not hold up well. This is true in situations where radiation is a hazard. Mechanical relays are not susceptible—or as susceptible—to being damaged by radiation, which makes them ideal solutions for such situations.

A relay can also be used to switch machinery over to emergency power in the event of an outage. This is a common usage for the device and one that provides a margin of safety for the machinery to which it is attached. This can allow electronics to keep functioning off of stored power in the event of an outage, or make an indispensable facility’s power stay on if there is an outage and backup source of power, such as a generator, has to be tapped.

What Are the Different Types of Relays?

Relays are simple, but there are several different types that have some significant differences between them.

Latching relays are common devices that can be normally open or normally closed. Whichever state the relay was in when it was last powered up is the state it will remain in until it is powered up again. This operation requires that two coils be present in the device rather than the one that operates a simple relay.

Latching relays come in several variations, including stepping relays and circuit breakers designed to provide earth leakage protection. These were among the types of relays used in telephone exchanges.

A reed relay is similar to a reed switch with the addition of a solenoid. The switch operates on magnetism, with the solenoid providing the relay function. These devices are typically encased in glass, protecting the magnetic components from the outside atmosphere.

Overload protection relays are commonly used on electric motors. These devices are designed to sense an overload. When under overload conditions, the relay is opened up, preventing power from flowing to the motor and preventing further damage.

A solid-state contactor relay is a large type of relay, often referred to as a solid-state contactor. These devices are designed to be used with devices that draw a large amount of current, such as heaters. They are solid-state devices, so that means that there are no moving parts involved and the devices are incredibly durable and reliable. They are sometimes enhanced with microprocessor controls to make them better suited for precision applications.

Vacuum relays are contained in glass, from which all or most of the air has been removed. This allows them to handle very large frequencies. The absence of air reduces the chance of arcing, which makes them suitable for purposes where controlling that phenomena is vital to the machinery or other system protected by the relay.

Forced guided contacts relays are designed so that the contacts move with the coil. This prevents one set of contacts from moving if another is damaged or disabled, providing an additional lawyer of safety for the user and the system.

What Are Auxiliary Contact Blocks?

An auxiliary contact block is a group of contactor relays mounted together into one unit. They provide an easy way to group control circuits and auxiliary circuits together and are usually modular components.

These come in various mounting styles, including front, snap on and others. They are differentiated further by their current ratings, their normal state configurations and their pole and throw configuration. They have various terminal types that allow them to be used in different circuit designs.

Other types of relays include coaxial, which are used with radio technology; ratchets, which can operate in the absence of continuous current; and machine tool relays, which are used in industrial processes and which have contacts which can have their normal state changed easily.

What Are Busbars?

A busbar is a device that is used to conduct electricity throughout a switchboard, a distribution board or another device. This is not a mechanical component and is generally just a strip of conductive material, such as copper, that is used to provide electricity to a host of different devices.

This is sometimes spelled bussbar, which is incorrect. The name is a shortened version of the word omnibus. That term is Latin and translates to “for all”, which is exactly what a busbar does; provide power for all of the components to which it is attached.

Busbars come in all sizes. There are small ones used in household circuit breaker boards, in some cases, and there are huge ones used in electrical substations, providing power for some of the largest electrical components available.

What Are Mount Adaptors and How Do They Work?

A mount adapter is a device that allows a relay to be mounted to a DIN rail without having to modify the device at all. It is usually a plastic component that snaps into place. There’s no need to use a socket with these devices, making them among the most convenient ways to mount adaptors on DIN boards. They are differentiated by size, making them suitable for use on different DIN systems.

What Are Relay Covers?

A relay cover is a safety device that is mounted over the top of the relay. It is used to protect operators from accidently touching any exposed parts and to protect the device itself from contact with the environment. These devices are sometimes easy to label, which makes it more convenient to organize electrical circuits on a control board or distribution board.

What Type of Relay Interfaces Exist and What Are They Used For?

Relay interfaces are available in many different designs, allowing them to be used with a variety of different systems.

DIN rail interfaces make it easier to hook relays up to a DIN rail and eliminate the need to perform any custom modifications to any of the hardware involved. They also make it easier to change out relays, if needed.

Panel mount relay interfaces are designed to provide a fast way to mount these devices to a control panel.

A screw mount interface comes with non-powered screw terminals that allow the relay to be mounted to any surface where it is feasible.

A socket mount relay interface plugs into a socketed fixture. These make it very easy to remove and replace relays whenever needed.

A PCB relay interface is used to attach relays to printed circuit boards. They are compact and designed to insert into the board easily, making construction, prototyping and maintenance much easier.

What Are Relay Labels?

Relay labels are simply designed to make it easier for technicians to determine the relay’s place in an electrical circuit. These are no different than the labels used on any other electrical component. They commonly come in strips that make it convenient to ensure that all the labels are of uniform size and design, aiding in standardization.

What Are Relay Mounting Kits?

A relay mounting kit comes with the materials needed to install a relay in a specific configuration. For example, there are mounting kits available for surface mounts and for Moeller mounts, which come with the appropriate hardware to get the job done correctly.

These mounting kits are often used in the design of new circuits and when a relay needs to be added to an existing surface. Purchasing the materials in a kit can be more economical than purchasing them alone.

What Are Relay Sockets?

A relay socket provides one of the simplest and easiest ways to mount a relay to a circuit and to remove and replace it when needed. They function like any other type of electrical socket, being designed to work with a specific type of connector and preventing other types of connectors from being inserted. These are commonly used on time delay relays and in other applications.

Relay sockets have to be matched to the components being used with them.

What Are Retaining Clips and What Are They Used For?

Retaining clips simply provide a means of physically securing the relay in place. They may be a simple plastic device that the relay snaps into or they may be devices that need to be fastened down manually when the relay is put into place and unfastened when it is removed.

These are usually designed to work with a specific shape of relay, which means that they have to be matched to the component before being selected. There are many different manufacturers of these devices. Some of them are clips, some are clamps, but they all provide a great deal of physical stability to a relay, particularly when it is large and needs to have its weight supported for safety and connectivity purposes.

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