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Jan 17 2019

The Role of the Relay and Its Working Principle

Warm hints: This article contains about 4000 words and reading time is about 18 mins.

Introduction

A relay is an electronic control device that has a control system (also called an input loop) and a controlled system (also called an output loop). It is usually used in an automatic control circuit. It actually uses a small current to control the larger. An "automatic switch" of current. Therefore, it plays the role of automatic adjustment, safety protection and conversion circuit in the circuit.

Catalog

Introduction

Ⅰ What is a Relay

1.1 Relay Definition

1.2 Relay Symbol

1.3 Working Principle of Relay

Ⅱ What is the Purpose of a Relay

2.1 Relay Function Overview

2.2 The Role of the Intermediate Relay

Ⅲ Relay Types

Ⅳ Relay Detection

4.1 Test Instruction

4.2 Testing Process

4.3 Relay Test Precautions


Ⅰ What is a Relay

1.1 Relay Definition

A relay is an automatic control device that changes the output when the input quantity (electricity, magnetism, sound, light, heat) reaches a certain value.

A relay is an electronic control device that has a control system (also called an input loop) and a controlled system (also called an output loop). It is usually used in an automatic control circuit. It actually uses a small current to control the larger. An "automatic switch" of current. Therefore, it plays the role of automatic adjustment, safety protection and conversion circuit in the circuit.

A relay is an electronic control device that has a control system (also called an input loop) and a controlled system (also called an output loop). It is usually used in an automatic control circuit. It actually uses a small current to control the larger. An "automatic switch" of current. Therefore, it plays the role of automatic adjustment, safety protection and conversion circuit in the circuit.

 

1.2 Relay Symbol

Electrical Relay Symbols

Since the relay is composed of two parts: the coil and the contact group, the graphical symbol of the relay in the circuit diagram also includes two parts: one long square indicates the coil; and one set of contact symbols indicates the contact combination. When the contactless circuit is relatively simple, the contact group is often drawn directly on one side of the coil frame. This drawing is called a centralized representation.

 

1.3 Working Principle of Relay

Why and How to use Relay | Relay Working Principle

Relays generally refer to electromagnetic relays, which are mechanical actions. The essence of the relay is to use a loop (usually a small current) to control the on and off of another loop (generally a large current), and in this control process, the two loops are generally isolated, and its basic principle is to utilize The electromagnetic effect is used to control the mechanical contact to achieve the purpose of switching, and the core coil is energized - the coil current generates a magnetic field - the magnetic field absorbs the armature action switching contact, and the whole process is "small current - magneto-mechanical - large current" a process.

 Working Principle of Relay

The figure above is a dynamic diagram of a relay control bulb. The relay has a normally open contact and a normally closed contact. The movable contact is a common end. This is a DC relay, that is, when the relay coil passes DC power (the battery is used on the figure). Power supply), the coil with iron core will output the corresponding magnetic field, the armature will be attracted, and the moving contact will run from the normally closed contact side to the normally open contact side, which is equivalent to the normally open contact. It is. From the figure, the start/stop button, the battery, and the relay coil form a control loop. As long as this loop is engaged, the coil will have a current through it and a magnetic field will be generated.

 

Working Principle of Relay

The normally open contact, the bulb, and the control power supply of another bulb (the other battery on the figure) form a loop. When the normally open contact is closed, the loop is closed and the current will be from the control power supply. The positive end, flowing through the bulb, passes through the closed normally open contact and then back to the negative pole so that the bulb will illuminate.

 

Working Principle of Relay

When the start/stop button is disconnected, the coil will lose current, so that the armature has no magnetic attraction and will be reset by the spring, so that the other end of the movable contact will return from the normally open contact side to the normally closed contact. Here, the energized circuit of the bulb is forcibly disconnected, and the bulb has no current, and naturally it will be dark.

Working Principle of Relay

Relay Structure

Therefore, the relay is also called "magnetism" by some old electricians. It uses the function of the electromagnet to control the pull-in or disconnection of another circuit. Inside the electromagnetic relay, coils, iron cores and springs are needed. It is composed of key accessories such as contacts. The contacts generally have normally open contacts and normally closed contacts. The two often have a common end. When the coil is not energized, the normally closed contact and the common end are shorted, and the normally open contact and the common end are open. After the coil is energized, the normally open contact and the common end are shorted, and the normally closed contact and the common end are open, just reversed, so that the voltage (current) of the coil can be controlled, and the circuit of the contact series can be controlled. 

Working Principle of Relay

When designing, choose the appropriate contact capacity, the coil voltage (AC DC), so that the isolation control of the two circuits can be realized. For example, the button that can be designed to be in contact with humans is 12 volts, and the coil of 12 volts is selected. It is safer, people just touch the voltage of the coil, and they will not be able to electricity themselves. On the contact side, it is possible to control a voltage of 220 volts or higher to directly drive the start and stop of a device such as a motor, or other load with a relatively large current, so that the control function of "four or two pounds" can be realized.

Working Principle of Relay

The relay was invented by American scientists around 1831. The unit of the inductor was named after him. The electromagnetic effect was discovered earlier than Faraday, but it was not patented. After more than 100 years of development, the relay has formed various forms, such as Time relays, temperature relays, reed relays, thermal relays, differential relays, optical relays, acoustic relays, Hall relays, and now solid state relays, from mechanical to electronic, in various forms.

Ⅱ What is the Purpose of a Relay

2.1 Relay Function Overview

a. Expand the control range: For example, when the multi-contact relay control signal reaches a certain value, it can switch, disconnect, and turn on multiple circuits at the same time according to different forms of the contact group.

b. amplification: For example, sensitive relays, intermediate relays, etc., with a very small amount of control, can control a very high power circuit.

c. Integrated signal: For example, when multiple control signals are input into the multi-winding relay in a prescribed form, after a comprehensive synthesis, a predetermined control effect is achieved.

d. automatic, remote control, monitoring: For example, the relay on the automatic device together with other electrical appliances, can form a program control circuit, thus achieving automatic operation.

 

2.2 The Role of the Intermediate Relay

2.2.1 Intermediate Relay

The general circuit is often divided into two parts: the main circuit and the control circuit. The relay is mainly used for the control circuit. The contactor is mainly used for the main circuit. The relay can realize the function of controlling one or several signals with one control signal to complete the start and stop. Control, linkage and other controls, the main control object is the contactor; the contacts of the contactor are relatively large, and the bearing capacity is strong, through which the control of weak electricity to strong electricity is realized, and the control object is an electric appliance.

The Role of the Intermediate Relay

2.2.2 Use of Intermediate Relay

a. Instead of Small Contactors

The contacts of the intermediate relay have a certain load capacity. When the load capacity is small, it can be used to replace the use of small contactors, such as electric shutters and some small appliances. This has the advantage that it can not only serve the purpose of control, but also save space and make the control part of the appliance more delicate.

b. Increase the Number of Contacts

In a circuit control system, the contact of a contactor needs to control multiple contactors or other components. It should not be plugged into other forms, because it is not conducive to maintenance, but an intermediate relay is added to the line, which will not change the control form. And easy to repair.

c. Increase the Contact Capacity

Although the contact capacity of the intermediate relay is not very large, it also has a certain load capacity, and the current required for its driving is small, so the intermediate relay can be used to expand the contact capacity.

d. Conversion Contact Type

In industrial control lines, such a situation often occurs. The control requires the use of a normally closed contact of the contactor to achieve the control purpose, but the normally closed contact of the contactor itself has been used up and the control task cannot be completed. At this time, an intermediate relay can be connected in parallel with the original contactor coil, and the normally closed contact of the intermediate relay can be used to control the corresponding components, and the type of the contact is switched to achieve the required control purpose.

e. Conversion Contact Type

In some control circuits, the switching of some electrical components often uses intermediate relays, which are controlled by the opening and closing of their contacts. For example, the automatic degaussing circuit commonly used in color TVs or displays, the triodes control the on and off of the intermediate relays, thereby achieving control of the degaussing coils. The role of continuity.

f. The Conversion Voltage

The voltage in the industrial control line control line is 24 volts DC. The contactor KM2 needs to control the on and off of the solenoid valve KT, and the coil voltage of the solenoid valve is AC 220 volts. Connecting the coil of the solenoid valve directly to the contact of the contactor is not in principle, but it takes into account maintenance habits and safety issues. An intermediate relay should be installed in another location to control the solenoid valve through an intermediate relay. This can separate DC from AC, high voltage and low voltage. It is convenient for future maintenance and is conducive to safe use.

g. Eliminate Interference In the Circuit

In industrial control or computer control lines, although there are various interference suppression measures, the interference phenomenon is more or less present. The general induced current will not cause the action of the intermediate relay. Only when the button in the original line is operated will the intermediate relay be activated to give the PLC a normal input signal, thus achieving the purpose of eliminating interference.

 

Ⅲ Relay Types

a. According to the Working Principle or Structural Characteristics of the Relay
1) Electromagnetic relay: An electrical relay that works by the suction force generated between the electromagnet core and the armature by the circuit inside the input circuit.

2) Solid state relay: A type of relay in which the electronic component performs its function without mechanical moving parts, and the input and output are isolated.

3) Temperature relay: A relay that operates when the outside temperature reaches a given value.

4) Reed relay: a relay that opens, closes or switches the line by using a reed action that is sealed in the tube and has the dual action of the electric spring and the armature magnetic circuit.

5) Time relay: When adding or removing the input signal, the output part needs to delay or limit the time to close or open its controlled line relay until the specified time.

6) High-frequency relay: A relay used to switch high-frequency, RF lines with minimal loss.

7) Polarized relay: A relay that has a polarized magnetic field and a control action that operates in conjunction with a magnetic field generated by a control coil. The direction of operation of the relay depends on the direction of the current flowing through the control coil.

8) Other types of relays: such as optical relays, acoustic relays, thermal relays, instrumentation relays, Hall effect relays, differential relays, etc.

b. According to the Size of the Relay
1) Micro relay
2) Ultra-small miniature relay
3) Small miniature relay

Note: For sealed or enclosed relays, the dimensions are the maximum dimensions of the relay body in three mutually perpendicular directions, excluding the dimensions of the mounting, extraction, rib, crimp, flange and seal welds.

c. According to the Load Classification of the Relay
1) Micro power relay
2) weak power relay
3) Medium power relay
4) High power relay

d. According to the Protective Characteristics of the Relay
1) Sealed relay
2) Closed relay
3) Open relay

e. According to the Principle of Relay Action
1) Electromagnetic type
2) Inductive type
3) Rectified type
4) Electronic type
5) Digital type, etc.

f. According to the Physical Quantities of Reactions
1) Current relay
2) Voltage relay
3) Power direction relay
4) Impedance relay
5) Frequency relay
6) Gas (gas) relay

g. According to the Role of Relay in Protection Circuit
1) Start relay
2) Measurement relay
3) Time relay
4) Intermediate relay
5) Signal relay
6) Exit relay

 

Ⅳ Relay Detection

4.1 Test Instruction

a. Measure the working voltage range of the relay (including the lowest closing voltage and the highest breaking voltage).
b. Measure the power consumption (rated current) and internal resistance of the relay.
c. The relay's long-term working conditions, withstand voltage.
d. The icon description:

DC source            Ammeter           Voltmeter                    Resistance measurement                  Buzzer

DC Source,  Ammeter,  Voltmeter,  Resistance Measurement,  Buzzer

 

4.2 Testing Process

a.  Measuring internal resistance and rated current
1) internal resistance test: test the resistance between the relay 1 and 8 feet, as shown below

Testing Process

2) Rated current test: DC 24V for relays 1 and 8 and 30 seconds for reading ammeter data

Note: For current test, insert the multimeter meter into the current input port and adjust the range position (mA) corresponding to the current file.

Testing Process

b. Measuring the working voltage range of the relay

measuring the working voltage range of the relay

1) Minimum closing voltage test: The DC power supply starts from 0V, and the voltage is gradually increased until the buzzer alarms, recording the current voltage value U1. (Keep the current supply value of DC voltage)

Note: The voltmeter and buzzer files in the figure are all realized by a multimeter.

2) The highest disconnection voltage test: the DC power supply starts from U1, and the voltage is gradually lowered until the buzzer stops alarming, and the current voltage value U2 is recorded.

 

c. Measure normally open normally closed withstand voltage and coil and contact withstand voltage

1) Preparation before the test: Turn the “leakage currentknob of the withstand voltage tester to 0.5mA, timing

Knob hits "60" s, "Voltage Range" knob hits "5" KV, "Voltage Adjustment" knob hits 0V, "power" knob hits "OFF", and two output lines are connected to high voltage output "_DC" ", a ground.

2) Measure the normally open normally closed withstand voltage test: “power”—> ON, voltage regulation”—> increase to the withstand voltage tester trip alarm voltage, read the voltage at this time, as shown below:

withstand voltage tester

3) Coil and contact withstand voltage: "power" -> "ON", "voltage adjustment" -> 5KV or more, withstand voltage tester trip does not alarm, coil and contact withstand voltage greater than or equal to 5KV, as shown below:

withstand voltage tester

 

4.3 Relay Test Precautions

a. When testing the rated current, the coil in the relay will generate electromagnetic induction when the voltage is suddenly applied. The current will become smaller and smaller. After the voltage is stabilized, the electromagnetic induction disappears and the current is stable within a range. Just like OMRON's G5RL-14-E, the power-on current is around 16mA-17mA, and the stable voltage is around 14mA-15mA after 4-5 minutes. But our test is to read the voltage just after 30 seconds of power-on.

b. In the normally closed normally open withstand voltage value, the relay will generate electromagnetic induction after the first trip. The disappearance of electromagnetic induction takes time, and the second trip voltage will be much smaller. But we test the voltage of the first reading.

c. If you read the stable rated current value, you should read the second normally closed normally open withstand voltage value. If you are reading the rated current value for 30 seconds, you should read the normally closed normally open withstand voltage value of the first action.

 

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