Different Types of Relays and Their Working Principles

The development of relays was initiated in the period 1809. As a part of the invention of the electrochemical telegraph, the electrolytic relay was found by Samuel in the year 1809. Thereafter, this invention was asserted by the scientist Henry in 1835 so as to make an improvised version of the telegraph and later developed this in the year 1831. Whereas in 1835, Davy absolutely discovered the relay, but the original patent rights were given by Samuel in the year 1840 for the initial invention of the electric relay. The approach of this device appeared the same as a digital amplifier thus replicating the telegraph signal and allowing for longer distance propagation. And this article gives a clear explanation of knowing what a relay is, different types of relays, working, and many other related concepts.

What is Relay?

Relays are generally employed where it is required to regulate a circuit through an individual minimal power signal or used where multiple circuits need to be regulated through a single signal. The initial utilization of relays was in the extended length of telegraph circuits like signal repeaters as they invigorate the wave that is received and transmits to other circuits. The major implementation of relays was in telephone exchanges and the initial version of computers.

Relays are the primary protection as well as switching devices in most of the control processes or equipment. All the relays respond to one or more electrical quantities like voltage or current such that they open or close the contacts or circuits. A relay is a switching device as it works to isolate or change the state of an electric circuit from one state to another.

As the relay makes sure the protection of the circuit to not let any damage to take place. Every relay comprises of three crucial components and those are calculated, comparing, and controlling components. The calculated component knows the variation in the actual measurement and the comparing component evaluates the actual amount with that of a prechosen relay. And the controlling component handles quick variation in the measured capacity like the closing of the current functional circuit.

Reclosing relays are used to connect various components and devices within the system network, such as synchronizing process, and to restore the various devices soon after any electrical fault vanishes, and then to connect transformers and feeders to line network. Regulating relays are the switches that contacts such that voltage boosts up as in the case of tap changing transformers. Auxiliary contacts are used in circuit breakers and other protective equipment for contact multiplication. Monitoring relays monitor the system conditions such as the direction of power and accordingly generates the alarm. These are also called directional relays.

The general kind of a relay makes use of electromagnet so as to perform opening and closing of contacts, whereas in the other types of approaches like in the solid-state type of relays they use semiconductor properties for controlling purposes without depending on the movable components. Relays those have calibrated properties and, in some cases, various functioning coils are employed to safeguard electric circuit systems from overload currents. In the current day power systems, these operations are accomplished by digital devices where those are called protective types of relays.

Different Types of Relays

Depending on the operating principle and structural features relays are of different types such as electromagnetic relays, thermal relays, power varied relays, multi-dimensional relays, and so on, with varied ratings, sizes, and applications. Classification or the types of relays depend on the function for which they are used.

Some of the categories include protective, reclosing, regulating, auxiliary and monitoring relays. Protective relays continuously monitor these parameters: voltage, current, and power; and if these parameters violate set limits they generate an alarm or isolate that particular circuit. These types of relays are used to protect equipment like motors, generators, and transformers, and so on.

In general, relays classification is dependent on the electric capacity which is activated by current, power, voltage, and many other quantities. Classification is based on the mechanical capacity activated by the speed of gas or liquid outflow, pressure. Whereas based on thermal capacity activated by heating power, and the other quantities are acoustical, optical, and others.

Different Types of Relays in Electromagnetic Types

These relays are constructed with electrical, mechanical, and magnetic components, and have operating coil and mechanical contacts. Therefore, when the coil gets activated by a  supply system, these mechanical contacts get opened or closed. The type of supply can be AC or DC. These electromagnetic relays are further classified as

• DC vs AC relays
• Attraction type
• Induction type

DC vs AC Relays

Both AC and DC relays work on the same principle as electromagnetic induction, but the construction is somewhat differentiated and also depends on the application for which these relays are selected. DC relays are employed with a freewheeling diode to de-energize the coil, and the AC relays use laminated cores to prevent eddy current losses.

The very interesting aspect of an AC is that for every half cycle, the direction of the current supply changes; therefore, for every cycle, the coil loses its magnetism since the zero current in every half-cycle makes the relay continuously make and break the circuit. So, to prevent this – additionally, one shaded coil or another electronic circuit is placed in the AC relay to provide magnetism in the zero current position.

Attraction Type Electromagnetic Relays

These relays can work with both AC and DC supply and attract a metal bar or a piece of metal when power is supplied to the coil. This can be a plunger being drawn towards the solenoid or an armature being attracted to the poles of an electromagnet as shown in the figure. These relays don’t have any time delays so these are used for the instantaneous operation. There exist more variations in the attraction type of electromagnetic relay and those are:

• Balanced ream – Here, two measurable quantities are related due to the generated electromagnetic pressure varies double to the number of ampere-turns. The proportion of functional current for this kind of relays is very minimal. The relay holds a tendency to overreach when the device is set to function in fast operation.
• Hinged armature – Here the relay’s sensitivity can be enhanced for DC functionality by inserting the permanent magnet. This is also termed as polarised movement relay.

These are the different types of electromagnetic relays.

Induction Type Relays

These are used as protective relays in AC systems alone and are usable with DC systems. The actuating force for contact movement is developed by a moving conductor that may be a disc or a cup, through the interaction of electromagnetic fluxes due to fault currents.

These are of several types like a shaded pole, watt-hour, and induction cup structures and are mostly used as directional relays in power-system protection and also for high-speed switching operation applications. Based on the structure, induction relays are classified as:

• Shaded Pole – Structured pole is generally activated by the flow of current in a single coil which is wounded on a magnetic structure that has an air gap. The air gap instabilities developed by the adjusting current are split up as two flux displace by a shaded pole and in time-space. This shaded ring is constructed with copper material that surrounds each section of the pole.
• Double Winding also Called as Watt/hr Meter – This type of relay is included with an E and U-shaped electromagnet having a disc-free to revolve in between the electromagnets. The phase shift which is in between the fluxes generated by the electromagnet is achieved by the developed flux of the two electromagnets which have various resistance inductance values for both the circuit systems.
• Induction Cup – This is based on the theory of electromagnetic induction and so-termed as induction cup relay. The device consists of either two or more electromagnets where those are activated by the coil present in the relay. The coil that surrounds the electromagnet creates the revolving magnetic field, Due to this revolving magnetic field, there will be an induction of current in the cup and so the cup gets to rotate. The current rotation direction is similar to that of the cup rotation direction.

Magnetic Latching Relays

These relays use a permanent magnet or parts with a high remittance to remain the armature at the same point as the coil is electrified when the coil power source is taken away. A latching relay consists of a minimal metal strip where it turns in between the two edges.

The switch is either attached or magnetized on one end of the small magnet. The other side is attached to a small-sized wire which is termed as solenoids. The switch is included with a single input and two output sections at the edges. This can be utilized for switching the circuit to ON and OFF positions. The latching relay symbol is shown as follows:

Solid State Relays

Solid State uses solid-state components to perform the switching operation without moving any parts. Since the control energy required is much lower compared with the output power to be controlled by this relay that results in the power gain higher when compared to the electromagnetic relays. These are of different types: transformer-coupled SSR, photo-coupled SSR, and so on.

The above figure shows a photo coupled SSR where the control signal is applied by LED and it is detected by a photosensitive semiconductor device. The output from this photodetector is used to trigger the gate of TRIAC or SCR that switches the load.

In the transformer-coupled type of solid-state relay, a minimal amount of DC current is provided to the primary winding of the transformer using a converter of type DC to AC. The supplied current is then transformed to AC type and stepped up to make the SSR to function along with the triggering circuit. The amount of isolation between the output and input sections is based on the transformer design.

Whereas in the scenario of photo coupled solid-state device, a photosensitive SC device is employed for the switching functionality to take place. A regulated signal is provided to the LED and this makes the photosensitive component move into conduction mode through the detection of light that is radiated from the LED. The isolation that is generated from the SSR is comparatively more when compared with that of the transformer-coupled type because of photodetection theory.

Mostly, SSR’s have quicker switching speeds than that of electromechanical type of relays. Also as there are no movable components, its life period is more and they generate minimal noise too.

Hybrid Relay

These relays are composed of electromagnetic relays and electronic components. Usually, the input part contains the electronic circuitry that performs rectification and the other control functions, and the output part includes an electromagnetic relay.

It was known that in the solid-state type of relays, more power is wasted as heat foem, an electromagnetic relay has the issue of contact arching. In order to get rid of these drawbacks in solid-state and electromagnetic relays, a hybrid relay is used. In a hybrid relay, both the EMR and SST relays are parallelly operated.

The solid-state device takes in the load current where it removes the arching issue. Then the controlling system activates the coil in EMR and the contact gets closed. When the contact in the electromagnetic relay is settled, then the regulating input of solid-state is taken out. This relay also reduces the problem of heat.

Thermal Relay

These relays are based on the effects of heat, which means – the rise in the ambient temperature from the limit, directs the contacts to switch from one position to another. These are mainly used in motor protection and consist of bimetallic elements like temperature sensors as well as control elements. Thermal overload relays are the best examples of these relays.

Reed Relay

Reed Relays consist of a pair of magnetic strips (also called reed) that is sealed within a glass tube. This reed acts as both an armature and a contact blade.  The magnetic field applied to the coil is wrapped around this tube that makes these reeds move so that switching operation is performed.

Based on dimensions, relays are differentiated as microminiature, subminiature and miniature relays. Also, based on the construction, these relays are classified as hermetic, sealed, and open type relays. Furthermore, depending on the load operating range, relays are of micro, low, intermediate, and high power types.

Relays are also available with different pin configurations like 3 pins, 4 pins, and 5 pin relays. The ways in which these relays are operated is shown in the below figure. Switching contacts can be SPST, SPDT, DPST, and DPDT types. Some of the relays are normally open (NO) type and the other is normally closed (NC) types.

Differential Relay

These relays function when the phasor variation between the two or more same kind of electrical quantities is more than a specified range. In the case of the current differential relay, it functions when there is an output relation between the magnitude and phase variation of currents receiving and exiting from the system which needs to be safeguarded.

In the general functional conditions, the currents that are receiving and exiting from the system will possess the same amount of phase and magnitude so that the relay is out of action. Whereas when an issue takes place in the system, these currents will have no similar magnitude and phase values.

This relay will have a connection in the way that the variation between the currents those are getting in and exiting flows across the functional coil of the relay. Hence the coil in the relay is activated in the issue condition because of the variation in the amount of current. So the relay functions and circuit breaker gets opened and thus tripping happens.

In a differential relay, one CT has a connection with the transformer’s primary winding and the other CT with the transformer’s secondary winding. The relay relates the current values on both sides and when there is any destabilization in the value, then the relay will have functioned.

There will be current, voltage, and biased types of differential relays.

Different Types of Relays in the Automotive Industry

These are the general kind of electrochemical relays utilized in various automobiles such as cars, vans, trailers, and trucks. They allow a minimal amount of current flow for regulation and function more amount of current circuit in vehicular appliances. These are available in many types and sizes, a few of those are:

Change Over Relays

This is the most implement automotive relay and it has five pins which have wiring connection as follows:

• Normally Open through 30 and 87 pins
• Normally Closed through 30 and 87a pins
• Change over wired through 30 and (87 and 87a)

When the relay functions in Change Over mode, it will be switched from one circuit to another and gets back to the original state based on the coil condition (OFF or ON).

Normally Open Relays

As a change over relay can have wiring connection as Normally Open, whereas, in this type, it has only four pins that permit to have wiring connection only in a single way that is normally open.

Flasher Relays

Any general type of relay has either 4 or 5 pins, but in this flasher relay, there will be 2 or 3 pins.

In a two-pin flasher relay, one pin has a connection with the light circuit and the other with power. While in a three-pin flasher relay, two pins are connected to power and light and the third one has a connection with an LED indicator which indicates that the flasher is in ON condition. Even though the name indicates that this is a type of relay, few of them behave like a circuit breaker.

Electro-Mechanical Flasher

This type of automotive relay contains a circuit board that is included with a capacitor, pair of diodes, and one coil to generate a flash shape the same as a standard flasher. These relays hold the ability to manage increased loads delivering enhanced performance than that of thermal flashers. Even though more lights are connected in this type, it shows minimal impact on the result.

Thermal Flashers

Most of the flasher relays are thermally regulated such as circuit breakers. The flow of current across the flasher coil generates heat, when there is a required amount of heat production, it caused deflection of contacts thus triggering open contacts and interrupts the current flow. When there is a required amount of heat dissipation, then the contacts deflection changes to the original state and there will be again current flow.

This process of continual contact breaking and making generates the flash pattern of the signals. The total number of lights that have a connection with the thermal flasher shows an impact on the output.

LED Flashers

These are entirely electronic in regulation and functionality. These are managed by minimal solid-state IC boards. The total number of lights that have a connection with the LED flasher does not show an impact on the output. These relays are mainly intended to operate on minimal current using LED without imposing any kind of issues.

In addition to these, there are even more many different types of automotive relays and those are:

• Potted
• Wig-Wag
• Skirted
• Time delay
• Dual open contact

Mercury Wetted Relay

This comes under the classification of reed relay which uses a mercury switch and the contacts in this relay are moistened using mercury. This metal decreases the value of contact resistance and alleviates the corresponding voltage drop. Damage to the shell might lessen the conductivity performance for minimal current value signals.

Whereas for increased speed of applications, mercury removes the feature of contact rebounding and offers almost quick circuit closure. These relays are completely susceptible to position and need to be fitted as per the designer’s requirement. But with the properties of noxiousness and price of liquid mercury, the mercury wetted relays are minimally used in the applications.

The increased speed of the switching functionality in these relays is an added benefit. The mercury drops that are present on each edge combine and current value increment across the edges are normally taken into consideration as picoseconds. But in the practical circuits, it might be regulated through wiring and contacts inductance.

Overload Protection Relay

Electric Motors are widely implemented in multiple applications such as in motors having revolving tools. As motors are a bit expensive,  it is more crucial to observe that motors should not undergo damage.

In order to prevent damages, there has to implementation of overload protection relays. Overload protection relays preclude motor destruction by observing the current value in the motor and thus breaks the circuit when there happens electrical overload or any phase damage is found. As relays are not costly than motors, they offer an inexpensive approach to safeguarding motors.

There exist various kinds of overload protection relays and few types are electromechanical relays, electronic relays, fuses, and thermal relays. Fuses are extensively implementing for safeguarding minimal current devices such as in household applications. Whereas electronic, thermal, and electromechanical relays are utilized in safeguarding increased current values in devices such as engineering motors. The crucial advantages of using overload protection relay are:

• Simple operation
• Application corresponding mountain kits will be in accessibility for multiple kinds of overload protection relays
• Exact synchronization with contractors
• Reliable protection

Static Relays

Relays which do not have any movable components are termed as static relays. In these static relays, the result is attained by the static parts such as electronic and magnetic circuits and other static devices. The relay which is included in the electromagnetic and static relay is even termed as static relay due to the reason that static sections receive the feedback whereas electromagnetic relay is employed for switching purposes. Few of the benefits behind static relays are

• Minimal reset time
• Utilizes minimal power where this decreases the load on measuring devices and so the preciseness enhances
• Provides quick output, extended life period, enhanced reliability, and high preciseness
• Unnecessary tripping is minimal and because of this efficiency will be enhanced
• These relays will not have come across any thermal storage issues
• Input signal amplification is done in the relay itself and this enhances the sensitivity
• These devices can function in earthquake-prone locations also which shows that these are shock resistance also.

There exist different types of static relays. A few of those are:

Electronic Static Relay

These electronic static relays were the initial ones to be known in the classification of static relays. A scientist named Fitzgerald showed a carrier current test which conveys the safeguard of transmission lines in the year 1928. In a consequence of this, a sequence of electronic systems for the majority of the general kinds of safeguarding gear relays was discovered. The devices that are utilized for measuring purposes are electronic valves.

Transductor Static Relays

This device basically consists of a magnetic core that comprises two sections of windings commonly termed as functional and regulation windings. Every section might consist of one winding or else when there more than one winding then there will be a magnetic linkage of all the similar types of windings. When there exist windings of various groups, then these will not be linked in a magnetic way.

While the regulation windings get activated using DC and the functional windings are energized through AC. This relay functions in order to represent changing values of impedance to currents that flow across the functional windings.

Rectifier Bridge Static Relays

The relays hold enhanced popularity due to the enhancement of semiconductor diodes. It is included with two rectifier bridges, and a movable coil or else polarized movable iron type of relay. Then the general type is relay comparators that are dependent on the rectifier bridges where these might be arranged in the form of phase or amplitude comparators.

Transistor Relays

These are the generally used type of static relays. The transistor that functions in the way of triode might overwhelm most of the drawbacks those are created by electronic valves and so these are the most developed type of electronic relays so-called static relays.

The reality that the transistor might be utilized both as an amplifying instrument and also as a switching instrument which allows it to be appropriate for accomplishing any type of operational feature. The transistor circuits will not only carry out the important purposes of a relay (like that of comparing inputs, calculation, and assimilating them) even they also offer essential elasticity to match with the multiple relay necessities.

In addition to these the other types of static relays are:

• Hall effect relays
• Inverse time overcurrent relay
• Directional static overcurrent relay
• Static differential relay
• Static distance relay

Applications Of Different Types of Relays

As there are multiple kinds of relays, these devices will have applications in various industries across electrical, aeronautical, medical, space, and others. The applications are:

• Used for the regulation of various circuits
• Protects devices from overload voltage and current values and decreases the impact of electrical damage to the circuits
• Implemented as automatic change over
• Utilized for the isolation of minimal level voltage circuit
• Automatic stabilizers are one of its implementations where a relay is implemented. When the level of the supply voltage is not the same as that of the rated voltage, then an array of relays analyze the voltage modifications and regulates the load circuit by integrating circuit breakers.
• Utilized to regulate the electric motor switches. To turn ON an electric motor we generally require a 230V AC supply but in a few situations/applications, there might be the case to switch ON the motor using a DC supply voltage. In this kind of situation cases, a relay can be employed.

These are some of the different types of relays that are employed in most of the electronic as well as electrical circuits. The information about the different types of relays serves readers’ purposes and we hope that they will find this basic information very useful. Considering the huge significance of relays with zvs in circuits, this particular article on them deserves its readers’ feedback, queries, suggestions, and comments. It is even more important to also know about other topics related to relays like relay vs contactor, relay and switch, and many more.