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Things you need to know about relays

Most electronic and mechanical appliances require relays to convert small electrical input into a high-current output they receive. Traditionally, relays were used in long distance telegraph circuits as signal repeaters. That is, the signal coming in from one circuit is refreshed by transmitting it on another circuit. Relays were extensively used in telephone exchanges and early computers to perform logical operations.


There are many types of relays out there to meet the demands of various applications. Today you’ll get to know the definition, functions, applications, selection considerations, components, diagram, types, and working of a relay. You’ll also get exposed to their advantages and disadvantages.

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What is a relay?

A relay is an electric switch that works with electromagnetism to convert small electrical stimuli to larger currents. This conversion takes place when an electrical input activates electromagnets to form or break existing circuits.

Leveraging weak inputs to power stronger current, relays can effectively act as a switch or an amplifier for the electric current. These depend on the desired applications.

Relays are also said to be a magnetically operated switch that activates and deactivates when an electromagnet is energized. The voltage applied to the relay input terminals energizes the electromagnet.

The relay was invented by the US scientist Joseph Henry in 1835.

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Functions of Relays

Below are the functions of relays in their various applications:

  • The primary function of a relay is to serve as a switch where circuits need to be controlled.
  • Some relay types use an electromagnet to close and open contacts,
  • It protects electrical circuits from overload or faults, serving as protective relays.
  • Relays functions also allow a system to run only for a set period of time or to start only after a set period of time, thus known as time-delay relays.
  • Another purpose of relays is for switching electric motors and lighting loads.
  • A single relay can serve as a connector of multiple contacts, so they can all move together when the relay coil energized or de-energized. If one of the contacts in the relay stops moving, the rest contact won’t be able to move. Relays with this effect are also known as safety relays.
  • Some types of relays have great functions where radio transmitters and receivers share one antenna. The relay serves as transmit-receive, which switches the antenna from the receiver to the transmitter.

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Applications of relays

Below are applications of relays:

  • Relay circuits are used to realize loci functions, playing important role in offering safety critical logic.
  • Just as earlier mentioned, relays provide time delay functions, as they time the delay open and delay close of contacts.
  • Relays are used to control high voltage circuits with a low voltage signal. They also use low current signals to control high current circuits.
  • Relays serve as protection to appliances, as they detect reception and get them isolated during transmission.

An overload relay is an electro-mechanical device that is used to secure motors from power failure or overloads. They are often used in motors to protect the motor from sudden current spikes that may cause damage.

An overload relay switch working is like the current overtime but different from circuit breakers and fuses, where a sudden trip will turn off the motor. Thermal overload relay is the most used type, where a bimetallic strip is used to turn off the motor. This strip makes contact with a contactor by bending itself with rising temperatures due to excess current flow.

The contact between the strip and contactor causes the contactor to de-energize and restrain power from the motor, thus turn the system off.

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Selection considerations of a relay

Below are factors to be considered while selecting a relay for a system:

  • Protection – While selecting a relay for a specific project, one must consider how the relay will protect the system from overloads or sudden power spike. Some other protection such as contact protection and coil protection must be considered. Contact protection will help to reduce arcing in circuits using inductors. While coil protection helps in reducing surge voltage produced during switching.
  • Standard relays with all regulatory approvals should be put into consideration.
  • High-speed switching relays are vital for the switching time, you might need one.
  • Relays current and voltage ratings should be put into consideration. The current ratings vary from few amperes to about 300 amperes, whereas the voltage ratings vary from 300 volts AC to 600-volt AC. Some high voltage relays of about 15,000 volts are also available.
  • Isolation between a coil circuit and contacts should also be considered.
  • Know the types of contact it carries, whether it’s NC or NO or closed contact.
  • Know whether “Make Before Break” or “Break Before Make” contact is the best option for your system.

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Components of a relay system

Below are components of the various types of relay system and their functions:

Frame – is a container or heavy-duty frame that contains and supports the various parts of the relay.

Coil – is a wire wounded around a metal core. It’s the part that causes an electromagnetic field

Armature – is a moving part that opens and closes the contacts. There is an attached spring that returns the armature to its original position.

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Contacts – it’s the conducting part that causes the relay to make (closes) or breaks (opens) a circuit.

Relays have two circuits; energizing circuit and contact circuit. The energizing side has the coil while the relay contacts have the contact side. A relay coil is energized when the current flow through the coil and creates a magnetic field. In an AC unit, the polarity changes 120 times per second, polarity is also fixed in a DC system.

A magnetic coil attracts a ferrous plate, a part of the armature. One part of this armature is attached to the metal frame, which is formed so that the armature can pivot. The other end opens and closes the contacts which come in different configurations.

These configurations depend on the number of the relay’s breaks, poles, and throws. i.e. a relay might be called single-pole, single-throw (SPST), or Double-pole, Single-throw (DPST).

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A break is the number of separate places or contacts that a relay uses to open or close a single electrical circuit. These contacts are either single or double break; single break contact (SB) breaks an electrical circuit in one place. while a double break contact (DB) breaks it in two places.

A single break contact is normally used when switching lower power devices like indicating lights. Whereas, a double break contact is used when switching high-power devices like solenoids.


Pole is a number of an isolated circuit that a relay can pass through a switch. A single-pole contact (SP) can carry current through only one circuit at a time. While a double-pole contact (DP) can take current through two isolated circuits simultaneously. Well, the maximum number of poles a relay can carry is 12, depending on its design.


A throw is the number of closed contact positions per pole that are available on a switch. A single throw switch can control only one circuit, while a double-throw can control two.

In brief, an electromagnetic relay consists of a coil of wire wrapped around a softcore (a solenoid), an iron yoke that provides a low reluctance path for magnetic flux, a movable iron “armature,” and one or more sets of contacts. All these are explained above, I hope you grabbed it.

 Diagram of a relay:

diagram of a relay

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Types of relays

Below are the various types of relays used and suitable for different applications:

Latching relays:

Latching types of relays maintain their state after being actuated. This is why they are also called impulse relay, keep relays, or stay relays. It’s used in most applications to limit power consumption and dissipation.

Latching relay types consists of internal magnets so that when current is supplied to the coil, the internal magnet holds the contact position. With this, the system requires no power to maintain its position. This is why after being actuated, it manages to maintain the last contact position even if the current is removed from the coil.

Solid-state relays (SSRs)

Solid-state types of relays use components such as BJTs, thyristors, IGBTs, MOSFETs, and TRIACs. These components carry out switching operations. Comparing to electromechanical relays, power obtained in solid-state relays is much higher because the power needed to control the circuit is much lower. These relays can work for both AC and DC supply.

Solid-state types of relays have high switching speeds since there are no mechanical contacts. There is a sensor in a solid-state relay which is also an electronic device. This sensor helps to switch on or off the power to load after responding to a control signal.

Reed relays:

Just like electromechanical types of relays, reed relays also work with mechanical actuation of physical contacts to open or close a circuit path. However, the reed relays have low mass and much smaller contacts compared to the electromechanical types.

Reed switch is wounded as it acts as an armature. It’s a glass tube or capsule filled with an inert gas contained in two overlapping reeds or ferromagnetic blades, which is hermetically sealed.

Differential relays:

Differential types of relays start working when the phasor difference of two or more similar electrical quantities exceeds a predetermined value. Current differential relays operate when the system experience comparison between the magnitude and phase difference of the currents entering and leaving the system to be protected.

if the system is working at normal operating conditions, currents entering and leaving are equal in magnitude and phase. This causes the relay to be inactive. But if a fault occurs in the system, the currents are no longer equal in magnitude and phase.

Polarized relay:

Just as it’s named, the polarized types of relays are very sensitive to the direction of a current by which it’s energized. It’s a DC electromagnetic relay provided with an additional source of a permanent magnetic field to move the armature in the relay.

In polarized relays, magnetic circuits are built with permanent magnets, electromagnets, and an armature. Instead of spring force, these types of relays use magnetic force to attract or repel the armature. This armature is a permanent magnet, positioned between the pole faces formed by an electromagnet.

Buchholz relays:

The Buchholz types of relays are gas-operated or actuated relays. They are widely used to detect incipient faults or internal faults which are minor initially but could cause major faults with time. These relays are mostly used for transformer protection and they are mounted in a chamber between the transformer tank and conservator.

These relay types are only used for an oil-immersed relay that is specifically utilized for power transmission and distribution systems. The figure below shows the working of a Buchholz relay.

Inverse definite minimum time relays (IDMT Relays):

Inverse definite minimum time relays are types of relay that offer definite-time current characteristics of a fault current at a higher value. And also, an inverse time-current characteristic of a fault current at a lower value.

These IDMT relays are widely used for protecting distribution lines and they help to set limits for current and time settings. In these types of relays, their operating time is approximately inversely proportional to the fault current near the pickup value.

Overload protection relays:

Overload protection types of relays are purposely designed to offer overcurrent protection to electrical motors and circuits. These overload relays are of different types such as fixed bimetallic strip type, and electronic or interchangeable heater bimetallic, etc.

Whenever an electric motor is overloaded, such a motor will require these relay types to protect the system from overcurrent. For this reason, overload sensing equipment such as heat operated relay must be employed. This heat operated relay contains a coil that heats up a bimetallic strip or solder pot, which then melts.

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Working principle

The working principle of a relay depends on its type and what it’s designed to do. However, a simple electromagnetic relay consists of a coil of wire wrapped around a soft iron core (a soft iron core). It also contains an iron yoke that provides a low reluctance path for magnetic flux, a movable iron armature, and one or more sets of contacts.

This armature is hinged to the yoke and mechanically linked to one or more sets of moving contacts. A spring helps to hold the armature in place so that when the relay is de-energized there is an air gap in the magnetic circuit. In some type of relay, one of two sets of contact closed, and the other set is open.

Some relays may have more or fewer sets of contacts depending on their purpose of use. There is a wire connecting the armature to the yoke, which ensures continuity of the circuit between the moving contacts on the armature. when an electric current passed through the coil it generates a magnetic field that activates the armature, and the consequent movement of the movable contacts either makes or breaks a connection with a fixed contact.

If the set of contacts was closed when relays are de-energized, then the movement opens the contacts and breaks the connection, and vice versa if the contacts were open. When the current to the coil is not energized, the armature is returned by a force, approximately half as strong as the magnetic force, to its relaxed position. The force is usually provided by a spring, gravity is also used in industrial motor starters.

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Watch the video to learn more about the working of relays:

Advantages and disadvantages of a relays


Below are the advantages of various types of relays:

  • It allows a remote device to be controlled.
  • Contacts are changing easily.
  • Isolates activating a part of an actuating part.
  • It works well at high temperatures.
  • It can be activated with a low current and can activate large machines of great power.
  • A single signal can be used to control several contacts at once.
  • Direct current or alternating current can be a switch.


Despite the good benefits of relays some limitations still occur. below are the disadvantages of relays in their various applications:

  • Contacts in the system damage over time. It often experiences wear, oxidation, etc.
  • Switching time is high
  • Sounds of activation and deactivation of contacts can be disturbing.


Relays are great components serving varieties of purposes on appliances depending on a relay effect needed. In this in-depth article, we’ve covered the definition, functions, applications, selection consideration, types, and working of relays. We’ve also seen their advantages and disadvantages.

I hope you enjoyed the reading, if so, kindly comment on your favorite section of this post. And please don’t forget to share with other technical students. Thanks!

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