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Jan 31 2018

Basic Knowledge of Relay Electronics Tutorial with Video

Warm hints: The word in this article is about 3200 and  reading time is about 20 minutes.


As an electric control device,relay(electornic relay) is a kind of electrical apparatus that causes the controlled variable to have a step change in the electrical output circuit when the change of input volume (excitation volume) meets the specified requirements. It has an interactive relationship between the control system (input circuit) and the controlled system (output circuit). Usually used in automatic control circuits, it is actually a "automatic switch" that uses a small current to control the operation of large current.It plays the role of automatic regulation, safety protection, conversion circuit and so on. In this electronic relay tutorial ,we will learn about what electornic relay is;the working principle of relays; relay types;main relay feature;how to test a relay;electrical symbols of relay;function of relay and so on.

Article core

Comprehensive knowledge about relay


uses a small current to control the operation of a large current.

English name



Electric control device


automatic regulation, conversion circuit,etc.


Electromagnetic relay, solid relay, etc.



I.Relay Definition: What is a Relay?

4. Working principle and characteristics of Optical relay    

VI.How to Test a Relay

II.Working Principle of Relays

V.Mail Relay Features

1. Measuring relay contact resistance

III.Function of Relay

1. Rated operational voltage 

2. Testing a relay coil

IV.Types of Relays

2. DC resistance

3. Measuring the pick-up voltage and current

1. Working principle and characteristics of Electromagnetic Relay

3. pick-up current 

4. Measuring the drop-out voltage and current

2. Working principle and characteristics of Thermal Reed Relay

4. Release current

VII.Electrical Symbols of Relays and its Contact Forms

3.Working principle and characteristics of Solid State Relay (SSR)

5. Contact switching voltage and current

VIII.Relay Development


I.Relay Definition: What is a Relay?

A relay is an electrical device that will switch the controlled output circuit on or off when the input (such as voltage, current, temperature, etc.) reaches a specified value. It can be divided into electrical relays(such as current, voltage, frequency, power,etc.) and non-electrical relays (such as temperature, pressure, speed, etc.) . Both of them have the advantages of fast reaction, stable operation, long service lifespan and small size. They are widely used in electric power protection, automation, motion, remote control, measurement and communication.

relay structure diagram

This is a video about What is a Relay? How does a Relay works!

II.Working Principle of Relays

There are many types of relays, which can be classified according to different principles. The International Electrotechnical Commission (IEC) divides relays into two categories according to the characteristics of their use. In this part, we will divide these relays according to their working principle and structural features. 

Form: Classification according to the working principle and structural features




Electromagnetic relay

A relay that actuates the movable part of a magnetic circuit by the electromagnetic suction generated by the control current passing through the coil and realizes the function of opening, closing or switching contacts.

Electromagnetic relay


DC Electromagnetic relay

Electromagnetic relay with DC current control


AC Electromagnetic relay

Electromagnetic relay with AC current control


Hybrid relay

A relay composed of electronic components and electromagnetic relays.


Solid state relay

The switch control is realized by using the on-off function of the electronic device and the electronic switch with isolated input and output.


High-frequency relay

A relay used to switch AC lines with frequencies greater than 10 kHz.


Coaxial RF relay

A relay with minimum loss for switching between high frequency and radio frequency circuits.

Thermal relay

A relay that operates by taking advantage of a thermal effect.

Thermal relay


Thermostat relay

A relay that acts when the external temperature reaches a predetermined value.


Electrothermal relay

A relay that uses the electric energy in the control circuit to change into thermal energy when it reaches the specified value.

Polarized relay

A relay operated by the integrated action of the polarized magnetic flux generated by the permanent magnet and the control magnetic flux generated by the coil control current . It has a requirement on the polarity of the control signal .

Polarized relay


Two-position polarized relay(magnetic latching relay)

When the relay coil is electrified, the armature is sucked to the left or right according to the current direction of the coil, and the armature does not return after the coil is off.


Two-position bias polarized relay

When the relay coil is off, the armature always rests on one side, and when the coil is electrified, the armature is sucked to the other side.

Delay relay

When the input signal is added or removed, the output solid-state switching circuit or contact group circuit needs to delay or limit the time required to close or disconnect the relays of the controlled line.

Delay relay


Electromagnetic delay relay

After the coil is added with the signal , a delay relay is obtained by slowing down the magnetic field change of the electromagnet .


Electronic delay relay

The delay relay consists of an electronic delay circuit composed of discrete components or a delay circuit of a solid state device.


Hybrid delay relay

A delay relay composed of an electronic or solid state delay circuit and an electromagnetic relay.


Electrothermal delay relay

A relay that uses electrical energy in a control circuit to convert to heat energy and delay action when a predetermined value is reached.


Motor delay relay

A delay relay that are generated by synchronous motors and special electromagnetic transmission mechanisms.

III.Function of Relay

Relay is one of the most important control components in remote control, telemetry, communication, automatic control, electromechanical integration and power electronic equipment, which is one of the most important control components.

Relays generally have inductive mechanisms (input parts) that reflect certain input variables (such as current, voltage, power, impedance, frequency, temperature, pressure, speed, light, etc.); There is also an executive mechanism (output part) that can realize "on" and "off" control to the controlled circuit. Between the input part and the output part of the relay, there is an intermediate mechanism for coupling and isolating the input, handling the function, and driving the output part.


IV.Types of Relays

Here we will mainly introduce four types of these relays.

types of relays

1. Working principle and characteristics of Electromagnetic Relay

Electromagnetic relay is generally composed of iron core, coil, armature, contact reed and so on. As long as a certain voltage is added to the two ends of the coil, a certain current will flow through the coil, thus producing electromagnetic effect. The armature will overcome the pull of the return spring to the core under the action of the electromagnetic force, thus driving the armature's dynamic contact and the static contact ( normally open contact ) to attract. When the coil is cut off, the electromagnetic force also disappears. The armature will return to its original position in the spring reaction force, so that the moving contact and the original static contact (normally closed contact) will be released. In this way, it can be absorbed and released, thus achieving the purpose of conducting and cutting off in the circuit. For the relay "normally open, normally closed" contact, can be divided in this way: relay coil is in the state of disconnection static contact, called "normally open contact"; A static contact that is in a connected state is called a "normally closed contact".

When the input signal x of the relay increases continuously from zero to the action value x x when the armature begins to suck, the output signal of the relay immediately jumps from y=0 to y=ym. Once the contact is closed, the input x continues to increase and the output signal y will no longer change. When the input x drops from a value greater than xx to xf. the relay begins to release and the open contact breaks. This characteristic is called relay characteristics, also known as the input-output characteristics of relays.

The ratio of release value xf to action value xx is called feedback coefficient, that is, Kf = xf / xx.

The ratio of the control power output from the contact Pc to the minimum power absorbed by the coil P0 is called the control coefficient of relay, that is, Kc=PC/P0 .

2. Working principle and characteristics of Thermal Reed Relay

The thermal reed relay is a new type of thermosensitive switch that uses thermosensitive magnetic material to detect and control temperature. It consists of a temperature sensing magnetic ring, a constant magnetic ring, a reed switch, and a heat conduction mounting piece, plastic substrates and other accessories. Thermal reed relays do not require coil excitation. Whether the constant magnetic ring can provide magnetic force to the reed switch is determined by the temperature control characteristics of the temperature sensing magnetic ring.

3.Working principle and characteristics of Solid State Relay (SSR)

Solid state relay is a four-terminal device with two terminals as input and the other two as output. The isolation device that is used in the middle is to realize the electrical isolation of input and output.

Solid state relay can be divided into AC type and DC type according to load power type. Or be divided into normal open type and normal close type according to switch type .According to isolation type, it can be divided into hybrid type transformer isolation type and photoelectric isolation type. Optoelectronic isolation type is the most.

4. Working principle and characteristics of Optical relay 

The relay is a semiconductor relay used for AC/DC . It refers to the integrated device of light emitting device and light receiving device. The input side and output side are electrically insulated, but the signal can be transmitted through optical signal. Its characteristics: semi-permanent life, small current drive signal, high impedance insulation voltage, ultra-small, optical transmission, no contact, and so on.

Mainly used in measuring equipment, communication equipment, security equipment, medical equipment and so on.

V.Mail Relay Features

Here let’s talk some main Technical Parameters of Relay.

relay parameter

1. Rated operational voltage 

It refers to the voltage required by the coil when the relay is working normally, that is to say, the control voltage of the control circuit. According to the type of the relay, it can be AC voltage or DC voltage.

2. DC resistance

The DC resistance of a coil in a relay can be measured by a universal meter.

3. pick-up current 

The minimum current in which the relay can produce the suction action. In normal use, the given current must be slightly larger than the pick-up current so that the relay can work stably. For the coil, the working voltage should not exceed 1.5 times of the rated working voltage, otherwise the coil will be destroyed.

4. Release current

The maximum current in which the relay can produce the release action. When the current of the relay is reduced to a certain extent, the relay will return to the unelectrified release state. The current is much smaller than the current of the suction.

5. Contact switching voltage and current

The voltage and current that the relay is allowed to load. It determines the magnitude of voltage and current that the relay can control. When operating, it can not be used above this value, otherwise it is easy to damage the relay's contact point.

VI.How to Test a Relay

how to test a relay

1. Measuring relay contact resistance

Measuring the resistance of normally closed contact and moving point resistance with the resistance band of a universal meter, and the resistance value should be 0 (the contact resistance value can be measured in a more accurate way within 100 mou); The resistance between the normally opened contact and the moving point is infinite, which can be used to distinguish the normally closed contact and the normally opened contact.

2. Testing a relay coil

The resistance of relay coil can be measured by universal meter R×10Ω, so as to determine whether the coil has an open circuit phenomenon.

3. Measuring the pick-up voltage and current

Find an adjustable power supply and ammeter, input a group of voltages to the relay, and then add an ammeter in the power supply circuit to monitor. Slowly adjust the supply voltage. When hearing the relay sound, write down the pick-up voltage and current. For accuracy, try a few more times to find the average.

4. Measuring the drop-out voltage and current

Also like the above connection test, when the relay is attenuated, then gradually reduce the supply voltage, when hear the relay release sound again, write down the voltage and current at this time. You can also try a few times to get the average drop-up voltage and release current. In general, the relay release voltage is about 10~50% of the pick-up voltage. If the drop-out voltage is too small (less than 1/10 suction voltage), it can not be used normally, which will pose a threat to the stability of the circuit and will not work reliably.

VII.Electrical Symbols of Relays and its Contact Forms

electrical symbol of relays

Relay coils are represented in a circuit by a long box. If the relay has two coils, draw two long parallel boxes. At the same time, the text symbol "J" of the relay is marked in or beside the long box. There are two ways to express the contact point of the relay: one is to draw them directly on the side of the long box, which is more intuitive. The other is to draw each contact into their respective control circuit according to the need of circuit connection, usually in the same relay contacts and coils with the same text symbols. The contact group is numbered to show the difference. There are three basic forms of contact for the relay:

  • Dynamic type: The two contacts are disconnected when the coil is not electrified, and the two contacts are closed when the current is switched on. The two contacts are expressed as "H".

  • Dynamic break type: The two contacts are closed when the coil is not electrified, and the two contacts are disconnected after the current is switched on. The two contacts are expressed as  "D".

  • Conversion type: This is a contact group. There are three contacts in this contact group, that is, there is a moving contact in the middle, one static contact up and down. When the coil is not electrified, the moving contact and one of the static contacts are disconnected and the other is closed, and the coil is electrified. The moving contact moves to make the original open closed, the original closed open state to achieve the purpose of conversion. Such contact group is called the conversion contact. It is expressed as "Z". 


VIII.Relay Development

With the rapid development of microelectronic technology, computer technology, modern communication technology, optoelectronic technology and space technology, new requirements and new techniques have been put forward for relay technology. The development of new technology undoubtedly promotes the development of relay technology.

The rapid development of microelectronic technology and super-large scale IC has also put forward new requirements for relays. The first is miniaturization and laminarization, such as military TO-5 relay (8.5 × 8.5 × 7.0mm) in IC packaging. It has very high vibration resistance, which can make the equipment more reliable; The second is modularization and multi-function, which can be compatible with IC or can be built in amplifier. It requires sensitivity to be raised to micro-watt level; The third is solidification. Solid-state relays have high sensitivity and are resistant to electromagnetic and radio-frequency interference. 

With the popularity of computer technology, the demand for microprocessor-based relays has increased significantly, and relays with microprocessors will develop rapidly. In the early 1980s, digital time relays produced in the United States can be controlled by instructions, and the combination of relays and microprocessors has developed, which can form a small and perfect control system. Industrial robots controlled by computers are now growing at a rate of 3.5% per year, and now. The production system controlled by computer can produce many kinds of low cost relays in one production line, and can finish many kinds of operation and test work automatically.

The development of communication technology is of great significance to the development of relays. On the one hand, the rapid development of communication technology has increased the application of the whole relay. On the other hand, as optical fiber will be the aorta of the future information society, new types of relays such as fiber optic relays and tongue spring fiber switches will appear with the promotion of optical fiber communication, optical sensing, optical computer, optical information processing technology.

Optoelectronic technology will greatly promote relay technology. In order to realize the reliable operation of optical computers, bistable relays have been developed. 

In order to improve the reliability of aeronautical and aerospace relays, it is expected that the failure rate of the relays should be reduced from 0.1 PPM to 0.01 PPM. The manned space station is required to reach 0.001PPM. The temperature tolerance should be more than 200 ℃, and the vibration resistance should be higher than 490 m / s. At the same time, it should be able to withstand the α -ray radiation of 2.32×10(4)C/Kg. In order to meet the requirement of space, it is necessary to strengthen the research on reliability and establish a special high reliability production line.

The development of new special structure materials, new molecular materials, high performance composite materials, optoelectronic materials, oxygen absorbing magnetic materials, temperature-sensing magnetic materials and amorphous soft magnetic materials have contributed to the development of new magnetic retention relays, temperature-control relays and electromagnetic relays are of great significance, and there will be new principles and new effects of relays.

With the development of miniaturization and laminarization technology, the relay will be developed into two dimensional and three dimensional micro and surface mount with only a few millimeters. At present, some international manufacturers produce relays that are only a quarter of a quarter of the size of those produced five to 10 years ago, because when the electronic machine is reducing its volume, smaller relays up to the height of other electronic components are required. Communication equipment manufacturers are more eager for intensive relays. The size of a BA series super dense signal relay produced by Fujitsu Takamisawa Company of Japan is only 14.9(W)×7.4(D)×9.7(H)mm, mainly uses for fax machines and modems. It can withstand 3 kV fluctuating voltage. The volume of AS series surface mounting relays is only 14(W)×9(D)×6.5(H)mm.

In particular, safe and reliable relays are needed in the field of power relays, such as high insulation relays. The JV series of power relays introduced by Fujitsu TaKamisawa of Japan contain five amplifiers. A small section design with high insulation is adopted, with a size of 17.5(W)×10(D)×12.5(H)mm. Due to the use of an enhanced insulation system between the core and the outer edge, the insulation performance of the system is up to 5 kV.The power consumption of MR82 series power relay introduced by Japan NEC is only 200 MW.

Book Recommendation

  • Electric Relays: Principles and Applications (Electrical and Computer Engineering)

Electric relays pervade the electronics that dominate our world. They exist in many forms, fulfill many roles, and each have their own behavioral nuances and peculiarities. To date, there exists no comprehensive reference surveying the broad spectrum of electric relays, save one-Electric Relays: Principles and Applications. This ambitious work is not only unique in its scope, but also in its practical approach that focuses on the operational and functional aspects rather than on theory and mathematics.

--Viadimir Gurevich(Author)

  • The Relay Testing Handbook: Principles and Practice

The Relay Testing Handbook is an indispensable resource that every relay tester should keep at their fingertips. This book was written for relay testers, rather than design engineers, so you no longer have to decipher engineering textbooks when performing relay tests. As modern protective relays become increasingly more powerful and complex, many relay testers continue to use test procedures and philosophies that are based on previous generations of relays and their limitations. Modern relays have very different characteristics that require a different testing philosophy to ensure that they will operate when required.

--Chris Werstiuk  (Author)

  • Protective Relay Principles

This versatile text is ideal for new engineers to use as a tutorial before they open the instruction manuals that accompany multi-function microprocessor-based relays. Guiding readers through the transient loading conditions that can result in relay misoperation, the author elaborates on concepts that are not generally discussed, but can be very helpful in specific applications. Readers will come away with an excellent grasp of important design considerations for working with overcurrent, over- and undervoltage, impedance, distance, and differential type relay functions, either individually or in combination. Also useful for students as a textbook, this book includes practical examples for many applications, and offers guidance for more unusual ones.

--Anthony M. Sleva (Author)

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