What is Relay? How does Relay work?

What is Relay? Relay works? Types?

Relay is an electrically controllable switching element. Although the throne of the relay that works with electromagnetism has been shaken by transistors and MOSFETs with the development of semiconductor technology, it is still widely used. There are different types according to AC and DC voltage types.

The requirement to transmit electric current over long distances was one of the biggest problems of the 19th century. Because with the introduction of the telegraph, communication has been started over long distances. It is obvious that as the transmission distance increases in electric current, losses in power occur. Famous for discovering new inventions in extraordinary situations, mankind found the relay in search of a remedy for the weakening of telegraph signals.

The inventor of the relay is known as Joseph Henry. Henry invented the relay in 1835. People started to use the relay in wider areas over time. So much so that the relay was developed and became the centerpiece of electricity and formed the basis of electronic technology.

What is Relay?

The main reason for using relays operating with an electromagnetic system in electrical and electronic circuits is that low currents and high currents can be controlled. The relay is a passive electrical element. Therefore, it is not active when there is no load on it. Another important feature of the relays is that they can work with both AC and DC voltage types. Relays have more than one contact structure. Therefore, it can control the opening and closing of more than one element simultaneously. Since the relay contacts can be in both open and closed positions at the same time, contact connections are expressed as normally open (NO) or normally closed (NC).

Structure of the Relay

Relays consist of three parts as a structure. These; coil, spring, and contact.

Coil: It is one of the most basic parts of the relay. The role of the coil in the relay creates a magnetic field inside, and it changes the position of the moving contact inside. It should be used according to the properties of the coil in the relay. One of these features is to know whether the coil operates with DC voltage or AC voltage. It is also necessary to know the operating voltage of the coil. The coil in the relay is usually operated with voltages such as 5V, 12V, 24V, 120V, 220V. Voltage specifications are printed on the outer casing of the relay.

Contact: There is more than one contact in the relay. Contacts are generally referred to as single contact or double contact. Single contact has one normally open end and one normally closed-end, while there are two normally open and normally closed ends in a double contact structure. The contacts in the relay can withstand very high voltages and currents. Which of the contacts are open and closed is usually indicated at the bottom where the contacts are located. In addition, the maximum current value it can withstand is specified on the top of the sheath. Electronic relay contacts are generally manufactured to withstand 10A or slightly more current, while relay contacts used in electrical systems can withstand higher current values. The biggest problem of contacts is that the moving contact sticks to a fixed contact over time.

Spring: The duty of the spring in the relay is to make the moving contact return to its normally closed position after the electrical energy is cut off. As can be understood from the purpose of the use of the publication, the relay has a mechanical feature rather than an electrical one.

Relay: Usage Areas

Because relays can switch high currents, they are widely used in high current applications. Since their switching frequency is low and they cannot switch quickly, they are mostly used in slow switching applications.

Relay Types

Relays can be used to operate on both AC and DC. Generally; relays are classified as switches and measuring relays. Also; relays are also divided into groups according to their operation, the purpose of use, and connection to the circuit. Relays are named according to the places they are used and their working methods. There are many more types of relays such as current, power, starting, thermal, magnetic reverse current, locking.

Relay types should be selected according to the circuit they are used in. For example, if switching will be made by magnetic effect, tongue contact can be selected, and over current protection, relays can be selected to limit high current.

One of the most useful tools of modern automation sciences is the relay. For example, telephone systems are made according to the railway signal systems relay layout. The operation of automatic machines is provided by relays. Semiconductor relays have begun to replace electromechanical relays.

SEE ALSO: What is a Reed Relay?

Magnetic Relay

The current flowing through a conductor wire covered with an insulating surface wrapped around an iron rod creates a magnetic field and attracts nearby metals. By using this feature of the current, magnetic relays are produced.

Core: It is the metal part where the copper wires are wrapped and the reel is passed. It is made from one piece of soft iron or siliceous sheet according to the size of the relay. The reason for using soft iron is that it loses its magnetism immediately.

Coil: It consists of conductors wound in the form of a reel on an insulating material. The diameter of the conductor and the number of spirals change according to the size of the relay and the current it will draw. When voltage is applied to the coil ends, the coil becomes electromagnet with the core. Thus, it pulls the palette opposite and provides the contact or separation of the contacts.

Contacts: It is the part that can be opened and closed while it is in contact with each other. During the closing of the contacts, there may be deterioration that we call “arc” due to electric splashes. To prevent this, the contacts are made of very good (non-rusting) conductors. Contacts can be normally open and closed according to their construction.

Body: It is the part on which relay parts are mounted. Electrically isolated.

Relay Works

When energy is applied to the relay coil, the magnetic field formed in the coil magnetizes the core and attracts the metal pallet opposite. The contacts connected to the pallet ends open or close according to the contact status. In other words, the normally open contact closes and acts as a switch to transfer energy to the circuit it is connected to.

The coil consists of a plurality of conductive wires (denoted 3) wrapped in a plastic sheath threaded over an iron core (designated 4) produced in a rectangular prism or cylindrical shape. The pallet (indicated by 5) is a soft piece of iron that gets activated as a result of being affected by the magnetic field of the coil. The reason for choosing soft iron as the core is that it easily loses this feature while gaining an easy magnetic field. At the point where the pallet touches the contacts, there is an insulating material called fiber (indicated by 7). When there is no magnetic field, it is held in tension by a spring. Contacts (indicated by 1 and 2) are good conductive materials that allow current to flow when they touch each other. A relay has an insulating ground substance (indicated by 6) that prevents unintentional contact of other contacts.

Core and coil work like electromagnets. When tension is applied to the coil ends, it creates a magnetic field and pulls the pallet towards itself (the pallet is free in the yellow ring and the pallet is pulled towards the core in the green ring). This causes the contacts of the pallet to change. As a result of this movement of the pallet, normally closed contacts (shown with red rings in the left part of the figure) are opened.

If a load is connected to the ends of these contacts, it will stop working. Again, normally open contacts (shown with red rings in the right part of the figure) will close and the load connected to these contact terminals will operate. Attention should also be paid to the change of the current path indicated by G (current input) and Ç (current output) in the figure.

It can be grouped according to the number and type of contacts on magnetic relays. There is one switch on the single-contact and single-position relay and there is no other switch that works opposite to this switch. It only helps to switch a load on and off. On the other hand, single contact and double position relays operate one load as a result of the displacement of the switch on it and stop the other. Similarly, multi-contact relays can be created by increasing the number of parallel working contacts in the relay. The relay whose internal structure is illustrated in the above figure is a double-positioned relay with two contacts.

Magnetic relay types

Magnetic relays are divided into types according to the current and voltage conditions that the contacts can withstand, the number and type of the contacts, their operating voltage, and the purpose of use.

Usage areas of the magnetic relay:

Relays are used in all kinds of circuits we want to switch. The uses for energizing single and three-phase motors in TV receivers, PLC circuits, ladder automatics can be given as examples.

Contact Relays

The relay made by placing metal contacts easily affected by the magnetic field into the ventilated glass tube is called tongue contact, if the number of contacts is high, it is called tongue contact relay. Usage areas of tongue contact relays are warning lights of automobiles, devices operating underwater, liquid level controls, remote control switches, etc. used in places.

In tongue contact relays, it is sufficient to bring a piece of magnet closer to close the contact or to pass current through the coil wound on the glass cover. Tongue contacts are magnetic field controlled switches. Switching the switch on and off depends on the magnetic field change.

Thermal relay

It is a type of relay that opens or closes its contacts depending on the temperature change in the environment. Thermal relays are often used to stop motors that overheat during operation or to prevent further heating at the appropriate temperature level in heat-generating devices such as hairdryers and irons.

With a special metal compound called bi-metal in thermal relays, when the ambient temperature rises above the specified temperature value, it cuts the current going to the load. Bi-metal consists of two metals that expand under heat. One of the metals expands rapidly and excessively. The other makes the bi-metal compound bend in one direction with little expansion, converting heat energy into motion energy. This motion energy is used to close the contacts. Thermal relays come in many different structures. While there is a chrome-nickel heater inside the thermal relays used in the motor control circuits, the thermal relays used in the devices such as irons use the heat inside the device and there are only bi-metal and contacts inside.

The internal structure of these two different thermal relays is as in the figure above. In the first relay, bimetal moves the contacts by leaning on the pallet with the effect of heat. In the second relay, with the effect of the weight of the bi-metal loosened by the heat effect, it falls on the contact and causes the contact to open.

Over Current Protection Relays

Fuses placed at the beginning of the line in a load circuit protect the line, not the load, due to their operating qualities. Different relays are used to protect the loads before malfunctions. Here, the overcurrent relay is used to prevent the damage that excessive currents can cause to the motor windings. An overcurrent protection relay is made to protect motors from overcurrent.

Overcurrent relays connected to only one conductor in single-phase alternating current circuits and to each 3-phase conductor in 3-phase circuits control the normally closed contact connected in series to the circuit in the control circuit. The current setting in overcurrent relays is made with the adjustment screw on the relay. Current adjustment is made at certain limits according to the rated currents of the motors.

Overcurrent relays are connected in series to the load circuit and load current passes through them. Windings are not damaged due to a temporary failure of the load or short-term overcurrents drawn during the start. In this case, the relay is requested not to open the circuit. In this, the opening of the relay is prevented by a delay system.

If the current value is within normal limits, the output relay is in the pulled position. If the current exceeds the adjusted value, the output relay deactivates the motor or system to which it is connected by leaving the contact at the end of the delay time.

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