A relay is an electromagnetic switch operated by a relatively small electric current that can turn on or off a much larger electric current. It consists of a set of input terminals for a single or multiple control signals, and a set of operating contact terminals. Because of unique characteristics, it is widely used in many fields. How does a relay work? What the functions of relays, let’s check the following details.

1 Working Principle

The relay is generally composed of an iron core, coil, armature, contact reed and so on. As long as both ends of the coil having a voltage, a certain current will flow through the coil, which will produce electromagnetic effects. Under the action of electromagnetic force, the armature will overcome the pull force of the return spring and attract to the core, thereby the movable contact and the static contact (normally opened terminal) are in the state of pull-in.
When the coil is power-off, the electromagnetic attraction will disappear. The armature will move back to its original position under the reaction force of the spring, making the movable contact and the static contact (normally closed terminal) contract together. Under the actions of pull-in and release, achieve the purpose of conducting and cutting off in the circuit.

1.1 Operation Example: Relay as a Switch

The following figure is the circuit diagram of the relay controlling the light. The relay has normally opened contacts and normally closed contacts. The movable contact is a common terminal. This is a DC relay powered by a battery. When the coil of the relay is powered by a DC power supply, the coil with the iron core will generate the corresponding magnetic field to adsorb the armature, and the movable contact will move from the normally closed contact side to the normally open contact side, which is equal to the normally open contact being pulled in. We can see that, the start/stop button, battery, and relay coil form a control loop. As long as this loop is closed, current will flow through the coil and a magnetic field will be generated.
The normally open contact, the lamp, and the control power supply (the other battery in the picture) form a loop. When the normally opened contact is closed, the loop is closed and the current will flow from the positive of control power supply to the bulb, passing through the closed normally opened contact to the negative pole, so that the light will on.
When the start/stop button disconnects, the coil has no current. So that the armature will not be attracted by the magnetic force, and will be reset by the spring. So that the other end of the moving contact will go from the normally opened contact to the normally closed contact. The circuit of the bulb is forcibly disconnected, and does not turn on.

1.2 Working Principle of Different Relays

1) Electromagnetic relay
It works by using the suction force generated by the circuit in the input circuit between the electromagnet core and the armature.
2) Solid state relay
Electronic components have their functions without mechanical moving parts, and the input and output are isolated.
3) Temperature relay
It will act when the outside temperature reaches a given value.
4) Reed relay
Using the reed action sealed in the tube, open, close or switch the circuit with the function of the electric contact reed and the armature magnetic circuit.
5) Time relay
When the input signal is added or removed, the output part needs to be delayed or time-limited before closing or opening its controlled circuit until the specified time.
6) High-frequency relay
It is used to switch high-frequency and radio frequency lines with minimal loss.
7) Polarized relay
A polarized magnetic field and a control current are combined to act by the magnetic field generated by the control coil.

2 What is the Function of a Relay?

2.1 Summary

Relay is an automatic switching element with isolation function, which is widely used for remote control, telemetry, communication, automatic control, electronic equipment, etc. It is the most important control element in the circuits.
Relays generally have a sensing mechanism (input part) that can reflect certain input variables (such as current, voltage, power, impedance, frequency, temperature, pressure, speed, light, etc.); there is a mechanism (input part) that can realize "switch on" and "switch off" to the controlled circuit. Between the input end and the output end of the relay, there is also an intermediate mechanism for coupling isolation of the input quantity, functional processing and driving the output part (driving part).
As a control element, relays have the following functions:
1) Expand the control range
For example, when the control signal of a multi-contact relay reaches a certain value, multiple circuits can be switched, disconnected, and connected at the same time from different forms of contact groups.
2) Amplification
For example, using a very small control quantity can control a large power circuit, such as sensitive relays, intermediate relays and so on.
3) Integrated signal
For example, when multiple control signals are input to a multi-winding relay in a prescribed form, they will be relatively integrated to achieve a predetermined control effect.
4) Automatic, remote control, and monitoring
For example, the relay on the automatic device and other electrical appliances can form a program control circuit to realize automatic operation.

2.2 Types of Relay

Intermediate Relay
It is the function of converting and transmitting the control signal. That is, its input signal is the power off/on signal of the coil, and the output signal is the contact action of the intermediate relay. In essence, it belongs to one of the voltage relays and has the characteristics of multi-contacts (six pairs or even more). The contact can withstand a large current (rated current is 5A~10A), and its action is more sensitive (response time less than 0.05s).

Voltage Relay
Its main principle uses the voltage signal, and determines the action of the contact according to the coil voltage, in addition, the coil needs to be connected in parallel with the load during circuit design. The voltage relay can be divided into AC and DC type according to the coil voltage, and can be divided into overvoltage and undervoltage according to the operating voltage. Therefore, their functions are also different. As for the overvoltage relay, when the coil voltage is within the rated value range, the armature will not make any pull-in action. On the contrary, the action will execute if the coil voltage is exceeded. The AC overvoltage relay plays the role of overvoltage protection in the circuit. When the coil voltage reaches or exceeds the rated value of the coil, the armature will make a pull-in action, and the coil voltage will be lower than the rated value. The undervoltage relay mainly plays a role of undervoltage protection in the circuit when the armature is released immediately.

Current Relay
It works according to the current signal, and determines the contact action according to the current of the coil. The current relay coil needs to be connected in series with the load when having installation. According to the coil current, it can be divided into two types AC and DC. According to the action current, it can be divided into overcurrent and undercurrent type.
Since the load current will pass through the coil when having overcurrent, the coil rated current (that is, the setting current) is usually chosen to be equal to the maximum load current. When the load current is not higher than the setting value, the armature will not act. On the contrary, if it exceeds, a pull-in action will occur. The main function of the overcurrent relay is to play overcurrent protection in the circuit, especially in some occasions where impulsive over-current occurs. Because it has a good protective effect. The principle of the undercurrent relay is that when the current in the coil reaches or exceeds the operating current value, the armature will perform a pull-in action. On the contrary, the armature will be released immediately when the coil current is less than the operating current value. In a normal state, if the load current exceeds the working current of coil, the armature will also perform pull-in. When the load current drops below the coil current, the armature will be released.

Time Relay
It belongs to a relay that starts with the input signal (that is, when the coil is powered on or off) and will output the signal (contact closed or disconnected) after a preset delay in advance. Time relays are generally used in relatively low voltage or current circuits to turn on and off higher voltage or current, just as an electric switch device in the circuit used for automatic control.

3 Applications

Relays are employed in a wide range of fields, and their environmental conditions and technical requirements vary greatly. What’s more, in the same application field there are different requirements. Here are some examples and a brief description.

3.1 Automotive Field

The automotive industry is increasingly using relays. The more common relays are: starting relays to start motors, horn relays, open circuit relay of motor or generator, regulating relays for charging voltage and current, flashing relays, control relays fro light brightness,control relays for air conditioning, and so on. The power supply in the car now mostly uses 12V, and the coil voltage is mostly set to be 12V. Due to the battery power supply, the voltage is unstable. The environmental conditions are not good, for example, the suction voltage is less than 60VH (rated working voltage), and the overvoltage of the coil is required to 1.5VH. What’s more, the power consumption of the coil is relatively large, generally 1.6~2W, and the temperature rise is relatively high. Their environmental requirements are also quite harsh: the ambient temperature range is -40℃~100℃; the relay used in the engine box must be able to withstand the damage of sand, dust, water, salt, and oil; vibration and shock are undoubtedly affecting normal operation.

3.2 Household Alliance

1) Air conditioning relays are mainly used to control compressor motors, fan motors and cooling pump motors to have control functions. Owing to the moment when the load starts, a large inrush current appears, which is about 6 times of the full-load operating current. It takes a long time for the compressor motor to reach full speed (the power of the home appliance compressor motor is generally 1 to 3 horsepower, where the fan motor and cooling pump motor are 1/4 to 2 horsepower.), which is a serious threat to the relay contacts to eliminate as much as possible the contact bounce when the relay is sucked. Because the relay is required to release fast, minimize contact bounce as much as possible. The safety requirements are also strict and must be recognized by a safety certification agency. For example, as for product environmental conditions, the ambient temperature requires -40 to 55℃, relative humidity up to 40%, 90RH, and have rainwater infiltration. Because weight and size are not important indicators, the relay is required to be robust and impact resistant.
2) Relay used in washing machines, microwave ovens, electric heaters, etc. Relay contact load: the large load can reach 220V, 5000W heater (or 1 horsepower motor), and the small load can be as small as driving solenoids load, other relay coils load, indicator light load, etc. The expected life span of the relay is required to reach 5 to 10 years. That is to say, the electrical life of the relay is required to reach 105 times to 2×105 times. Ambient temperature: -40 to 55°C (85°C for microwave ovens and electric heaters); relative humidity 20 to 95%/RH.

3.3 Industrial Relays

In industrial control, the main control function is completed by the universal AC relay. The relay is usually driven by a button or limit switch. It is also used in traffic signal controllers, temperature controllers, etc. The contacts of the relay can control solenoid valves, larger start motors, and indicator lights. What’s more, the field of digital control has expanded the application of relays. Copy milling and coordinate boring are operated by data programming, and the signals are sent to the machine tool controller, memory unit and other logic elements to control 2 to 5 axes of the coordinate servo motor. With this mechanical control method, it is easy to control drilling machines, hexagonal lathes, ordinary lathes and automatic profiling machines.
The digital control system requires the relay to have the ability to adapt to low-level signals, medium sensitivity, fast action and high switching reliability. The environmental conditions for the installation of industrial machinery must be considered. For instance, operating industrial machinery and surrounding equipment always transmit some shocks and vibrations to the control cabinet, and there also has the influence of splashing cutting coolant. So that these unfavorable environmental conditions must still be considered when selecting and designing relays. With strict safety requirements, high requirements are needed for electrical insulation, voltage resistance, and flame retardants.

3.4 Example Analysis: JYB-714 Liquid Level Relay

Liquid level relay is a kind of relay that uses liquid level to control circuit. Be specific, this is a relay with electronic circuits inside. Based on the conductivity of the liquid, when the liquid level reaches a certain height, the relay will act to cut off the power; when the liquid level is lower than a certain position, turn on the power to make the pump work. To achieve the role of automated control, this control is composed of sensors and control actuators. According to the conductivity of water, but it is poor and cannot directly drive the relay. Therefore, there must be an electronic circuit to amplify the current to drive the relay to work. So the sensor of the liquid level controller is generally a wire. The line is divided into three types, high and low, and the middle line. The high is the water level overflow point to control the water level freely, in addition, the water will stop to fill in automatically. At low water level, the low point is the automatic water filling point. Where the middle is a constant contact.

JYB-714 Liquid Level Relay
1) Structure
①, ⑧ are the working power connecting terminals of the relay. ① is connected to L1, ⑧ is connected to N.
②, ③, and ④ output the automatic control signal, and the working voltage of the output terminal is AC220V. ③ is the output signal common end, the level control signal of the water supply pump is output between ② and ③, and the drainage pump level control signal is output between ③ and ④.
⑤, ⑥, ⑦ are the wiring terminals corresponding to the liquid level electrodes A, B, C in the pool. ⑤ is connected to the high water level electrode A, ⑥ is connected to the low water level electrode B, and ⑦ connect to the lowest common electrode C. Note that in the experiment, the water inlet electrode uses a copper hard insulated wire of 1 to 1.5mm2, and the water inlet end is stripped of 5mm insulation. In addition, the safety voltage between the liquid level electrode terminals is DC24V.

2) Drainage type liquid level relay instructions
"High" is the upper limit liquid level control point of the pool. When the water level rises to the high level, the water contacts the probe (electrode), and the controller automatically turns on the pump and starts to drain.
"Middle" is the lower limit liquid level control point of the pool. When the water level drops below the midpoint level, the water and the probe (electrode) are out of contact, and the controller automatically turns off the pump and stops draining.
"Low" is the ground line of the pool, the lowest point of the pool.

3) The difference between water-supply type liquid level relay and drainage-type liquid level relay:
Water-supply type liquid level relay works in water shortage and stops when the water is full.
The drainage-type relay works when water is full, and stops in water shortage.

4 Relay Selection Rules

To use the relay well, the correct selection is very important. First of all, you must get a thorough understanding of characteristics and requirements of the controlled object, and have careful consideration. The principle, purpose, technical parameters, structural characteristics, specifications and models of the selected relays should be analyzed. On this basis, the relay should be correctly selected according to the actual situation and specific conditions of the project.
1. The necessary conditions
① The power supply voltage of the control circuit, the maximum current that can be provided.
② Voltage and current in the controlled circuit.
③ Contact: When selecting a relay, on the one hand, you should consider whether the control circuit can provide enough working current, otherwise the pull-in of the relay is unstable. When the pull-in and release time of the relay cannot meet the requirements, the time constant of the coil loop can be changed to solve the problem. On the other hand, there is the elimination of electric sparks. Due to the small on-off current of the relay contacts, there will be no arc between the contacts, but "spark discharge" will occur. This is due to the presence of inductance in the contact circuit, and an overvoltage will appear on the inductance when it is disconnected. Together with power supply voltage on the contact gap, so that the contact gap will break down and discharge. Because of energy limitation, only spark discharge will generate. The alternating energy conversion between the capacitance and inductance existing between the contacts makes the spark looming and becoming a high-frequency signal. In addition, spark discharge will cause damage to the contacts, resulting in short service life.

2. After consulting the relevant materials to determine the conditions of use, you can find the relevant materials to find out the specific relay. If you already have a relay on hand, you can check whether it can be used according to the datasheet, and finally consider whether the size is appropriate.

3. Pay attention to the size of the appliance. If it is employed to general electrical appliances, in addition to the cabinet volume, small relays mainly consider the circuit board installation layout. For small electrical appliances, such as toys and remote control devices, ultra-small relay products should be used.

4. Rated load and service life are a reference value, which will vary greatly according to different environmental factors, load properties and types. So it is better to confirm in actual or simulate actual use.

5. Try to use rectangular wave control for DC relays, and use sine wave control for AC relays.

6. In order to maintain the performance of the relay, please be careful not to drop the relay or subject it to strong shocks.

7. Do not use the relay in an environment with much dust and harmful gas. Harmful gases include gas containing sulfur, silicon, nitrogen oxides, etc.

8. As for the magnetic latching relay, it should be placed in the action or reset position as needed before use.

9. For polarized relays, please pay attention to the polarity of the coil voltage.

10.The relay is a heat-resistant component. High temperature can speed up the aging of the internal plastic and insulating materials of the relay. Contacts are oxidized and corroded, making it difficult to extinguish the arc. The technical parameters of the electrical components decay and the reliability reduces. So that good ventilation conditions should be maintained.
And meanwhile, low temperature cannot be ignored. Low temperature can aggravate the cold adhesion of the contacts and expose the contact surface. Many manufacturers’ relays indicate that the minimum temperature is -25°C, but high-voltage switches are also used in extreme cold. So it is recommended to leave room when selecting the model to avoid the relay being unreliable due to low temperature. If circumstances permit, add heaters in the high cold area to ensure that the relay operates reliably and ensure the stability of the entire system.

11. Under the condition of low air pressure, the heat dissipation condition of the relay goes bad, and the temperature of the coil rises, which changes the given pull-in and release parameters of the relay, affecting the normal operation of the relay. The low air pressure can also reduce the insulation resistance of the relay. It is difficult to extinguish the arc, and is easy to melt the contacts and affect the reliability of the relay. It can be used normally at an altitude of less than 2000 meters, and it needs capacitance derated used at an altitude of more than 2000 meters.

12. Reduce the impact of mechanical stress on the relay. Mechanical force mainly refers to the stress such as vibration, impact, and collision on the control system. The self-vibration of the circuit breaker in the high-voltage switch and the vibration caused by the opening and closing operations has a greater impact on the relay. An intermediate relay with a balanced armature mechanism should be selected. Electromagnetic relays have cantilevered beam structure, the natural frequency is low, oscillation and impact will cause resonance, resulting in the relay contact pressure to drop and contact instant disconnection or contact vibration, which will affect the reliability of the relay. It suggests that vibration measures should be taken to prevent resonance.
Andrewen 7 august 2020, 3:53