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Basic Design and Operation of Relays

Author: Tanssion Date: 2023-08-10 Hits: 0

Ⅰ. Basic design and operation of relay
Ⅱ. History of Relays
Ⅲ. Test Types of Relays
Ⅳ. Electrical symbols and contact forms of relays
Ⅴ. How to install and connect the relay? What precautions?
Ⅵ. Selection of relays
Ⅶ. How to choose the relay type suitable for a specific application?
Ⅷ. The difference between high-frequency relays and ordinary relays
Ⅸ. The role of the electrical isolation of the relay

Ⅰ. Basic design and operation of relay

A simple electromagnetic relay consists of a coil wound around a soft iron core (solenoid), an iron yoke that provides a low reluctance path for magnetic flux, a movable iron armature, and one or more sets of contacts (with contacts in the relay in both diagrams).

Basic Design and Operation of Relays

The armature is hinged to the yoke and mechanically connected to one or more sets of moving contacts. The armature is held in place by a spring, so when the relay is de-energized, there is an air gap in the magnetic circuit. In this case, one of the two sets of contacts of the relay in the diagram is closed and the other is open. Other relays may have more or fewer contact sets depending on their function.

When current is passed through the coil, it creates a magnetic field that activates the armature, and the resulting movement of the movable contact makes or breaks (depending on construction) the connection with the fixed contact.

If the contact set is closed when the relay is de-energized, the movement opens the contacts and breaks the connection, and vice versa if the contacts are open. When the current to the coil is cut off, the armature returns to its relaxed position with about half the magnetic force. Usually this force is provided by a spring, but gravity is also commonly used in industrial motor starters. Most relays can run quickly. In low voltage applications, this reduces noise; in high voltage or current applications, it reduces arcing.

When the coil is powered with DC, a flyback diode or snubber resistor is usually placed across the coil to dissipate energy from the collapsed magnetic field (back EMF) when deactivated, which would otherwise generate voltages harmful to semiconductor circuit components peak.

Such diodes were not widely used before transistors were used as relay drivers, but soon became ubiquitous, as early germanium transistors were easily destroyed by such surges. Some automotive relays contain a diode inside the relay box. Resistors, while more durable than diodes, are less effective at smoothing out voltage spikes generated by relays and are therefore not commonly used.

Ⅱ. History of Relays

In the 18th century, scientists believed that electricity and magnetism were two incompatible physical phenomena. After the Danish physicist Oersted discovered the magnetic effect of current in 1820, the British physicist Faraday discovered the phenomenon of electromagnetic induction in 1831. These discoveries confirmed that electric energy and magnetic energy can be transformed into each other, which also laid the foundation for the birth of electric motors and generators; human beings entered the electrical age because of these inventions.

In the 1830s, American physicist Joseph Henry invented the relay by using the phenomenon of electromagnetic induction when studying circuit control. The earliest relay is an electromagnetic relay, which uses the phenomenon that the magnetic force of the electromagnet is generated and disappeared under power-on and power-off to control the opening and closing of another circuit with high voltage and high current. Its appearance makes the remote control and protection of the circuit work be carried out smoothly.

The relay is a great invention in the history of human science and technology. It is not only the basis of electrical engineering, but also an important foundation of electronic technology and microelectronic technology.

Ⅲ. Test Types of Relays

1. Electrical parameter test: These tests include the measurement of the relay's rated current, rated voltage, contact resistance, contact voltage drop and other parameters. These parameters determine the performance of the relay under a specific load.

2. Communication interface test: For some relays with communication interfaces, it is necessary to test the communication function between them and other devices and systems.

3. Operating characteristics test: Test the operating characteristics of the relay, such as pull-in time, release time, excitation voltage and excitation current, etc. These tests evaluate the relay's switching speed and stability.

4. Anti-electromagnetic interference test: Evaluate the anti-electromagnetic interference and electrical interference ability of the relay to ensure its reliability in the electromagnetic noise environment.

5. Load capacity test: Test the performance of the relay under different load conditions, including rated load, overload and short circuit conditions. This helps to understand the reliability of the relay under various load conditions.

6. Relay life test: By simulating the switching operation of the relay in actual use, test its mechanical life and electrical life. This can help determine the useful life of the relay.

7. External interference test: Test whether the relay has sufficient resistance to external physical interference (such as vibration and shock).

In the past two years, the State Grid has gradually standardized the technical requirements for electric meters and formulated relevant industry standards and technical specifications. This has raised some technical problems for relay parameter testing, such as relay load on-off capability and switching characteristic testing. Therefore, it is urgent to study a device to realize the comprehensive detection of relay performance parameters.

According to the test requirements of relay performance parameters, the test items can be divided into two categories, one is the test items without load current, such as operating value, contact resistance, mechanical life; the other is the test items with load current, such as contact Contact voltage, electrical life, overload capacity.

Ⅳ. Electrical symbols and contact forms of relays

1. Common abbreviations are:

NO (Normally Open) means normally open contact (commonly known as A contact). It is usually open circuit (open circuit), and it becomes closed circuit (connected with the common contact point COM) after the coil is energized.

Basic Design and Operation of Relays

NC (Normally Closed) means normally closed contact (commonly known as B contact). It is usually in a closed circuit (connected to the common contact COM), and it becomes an open circuit (open circuit) after the coil is energized.

COM (Common) means common contact.

2. Switch contact type

The relay coil is represented by a rectangular box symbol in the circuit. If the relay has two coils, draw two parallel rectangular boxes. At the same time, mark the text symbol "J" of the relay in the rectangular box or beside the rectangular box.

There are two ways to represent the contacts of the relay: one is to draw them directly on the side of the rectangular box, which is more intuitive. The other is to draw each contact into its own control circuit according to the needs of circuit connection. Usually, the same text symbols are marked on the contacts and coils of the same relay, and the contact groups are numbered. To show the difference.

There are three basic forms of relay contacts:

Dynamic break type (D type, normally closed type, B type contact) the two contacts are closed when the coil is not energized, and the two contacts are disconnected after energization. It is represented by the word-breaking pinyin prefix "D".

The two contacts of the moving type (H type, normally open type, A type contact) coil are disconnected when the coil is not energized, and the two contacts are closed after the power is applied. It is represented by the pinyin prefix "H" of the ligature.

Conversion type (Z type) This is the contact group type. This contact group has three contacts in total, that is, a moving contact in the middle, and a static contact at the top and bottom. When the coil is not energized, the movable contact and one of the static contacts are disconnected and the other is closed. After the coil is energized, the movable contact moves, making the original disconnected become closed, and the originally closed become open, reaching the switching point. Purpose. Such a set of contacts is called a changeover contact. It is represented by the pinyin prefix "z" of the word "turn".

If it is disconnected first and then contacted with another contact, this method is called Form C contact. If it is in contact with another contact first, and then disconnects the original contact, this method is called a D-type contact.

Ⅴ. How to install and connect the relay? What precautions?

1. Installation steps

Selecting an appropriate location: First determine where the relay will be installed. Considering the working environment of the relay and the surrounding equipment, choose a location that is dry, ventilated and not susceptible to external physical damage.

Fix Relay: Fix the relay at the selected position. The relay can usually be mounted to a bracket, panel or other suitable location using screws, bolts or other suitable fixtures.

Wire management: For the input and output wires of the relay, good wire management should be carried out to ensure that the wires are not squeezed, stretched or twisted, so as not to affect the normal operation of the relay.

2. Connection steps

Cut off power: Before connecting the relay, always cut off the power related to the relay to ensure safe operation.

Check the connection: Depending on the type and application of the relay, verify how the relay is connected. Typically relays have inputs and outputs that need to be connected according to the circuit diagram or manufacturer's instructions.

Connecting wires: Connect the input signal wires to the input terminals of the relay, and the output signal wires to the output terminals. Make sure the connection is tight so that it does not come loose.

Insulation treatment: Insulate the wires with insulating sleeves or insulating tapes to prevent short circuits or external interference.

Double-check: After the connections are made, double-check all connections to make sure there are no loose, misaligned, or poor connections.

3. Precautions

Follow electrical safety regulations: When installing and connecting the relay, be sure to follow local electrical safety regulations and standards to ensure safe operation.
Cooling and Ventilation: Under high load conditions, the relay may generate some heat. Make sure that the installation location has adequate ventilation and heat dissipation.

Correct connection polarity: For relays, there is usually a polarity requirement. Make sure to connect the positive and negative polarity of the relay correctly to avoid wrong connections.
Proper Relay Type: Make sure to select the proper relay type and size to meet the requirements of the application, such as current, voltage, load type, etc.

Avoid arcing: When connecting the relay, especially when disconnecting the power supply, care should be taken to prevent arcing. Devices such as arc arresters can be used to reduce the effects of arcing.

Avoid Overloading: Make sure that the relay's current and voltage ratings do not exceed its rated range to prevent relay overloading.

Ⅵ. Selection of relays

1. Understand the necessary conditions first

The power supply voltage of the control circuit, the maximum current that can be provided; the voltage and current in the controlled circuit; how many sets and what forms of contacts are required for the controlled circuit.

When selecting a relay, the power supply voltage of the general control circuit can be used as the basis for selection. The control circuit should be able to provide enough working current to the relay, otherwise the relay pickup will be unstable.

2. Model and specification number

After checking the relevant information to determine the conditions of use, you can look up the relevant information to find out the model and specification number of the relay you need. If you already have a relay at hand, you can check whether it can be used according to the data. Finally, consider whether the size is appropriate.

3. Pay attention to the volume of the appliance

If it is used for general electrical appliances, in addition to considering the volume of the case, the small relay mainly considers the layout of the circuit board. For small electrical appliances, such as toys and remote control devices, ultra-small relay products should be used.

Ⅶ. How to choose the relay type suitable for a specific application?

1. Understand the load requirements: First determine the type of load you want to control, such as motors, lights, heaters, etc. According to the current, voltage and power requirements of the load, select the rated parameters of the relay.

2. Determine the working environment: Consider the environment in which the relay will work, including temperature, humidity, vibration and other conditions. Select durable and adaptable relays based on environmental conditions.

3. Select the relay type: Select the appropriate relay type according to the application requirements, such as signal relay, power relay, high frequency relay, etc. Different relay types are suitable for different application scenarios.

4. Electrical parameter matching: Make sure that the rated current and voltage range of the selected relay is suitable for the needs of the application. The rated parameters of the relay should be greater than the electrical parameters of the load to ensure stable operation.

5. Consider the switching characteristics: According to the requirements of the relay switching speed, pull-in time and release time in the application, select a relay with appropriate switching characteristics.

6. Anti-electromagnetic interference: If there is electromagnetic interference in the working environment, choose a relay with good anti-interference ability to ensure stable operation.

7. Safety requirements: If the application requires a high degree of electrical isolation and safety, safety relays or solid state relays can be considered.

Ⅷ. The difference between high-frequency relays and ordinary relays

1. The difference in scope of work:

High Frequency Relays: High frequency relays are specifically designed to handle high frequency and radio frequency (RF) signals, typically operating in the frequency range of a few hundred megahertz (MHz) to several gigahertz (GHz).

Ordinary relays: Ordinary relays are usually designed to handle power frequency signals (such as 50Hz or 60Hz AC), and their operating frequency range is low, which is suitable for power control and general low frequency applications.

2. The difference between packaging and design:

High-frequency relay: The packaging and design of high-frequency relays need to consider the electromagnetic characteristics of the signal to reduce signal reflection and interference, and usually have a small and specially designed housing.

Ordinary Relays: Ordinary relays are usually designed with more emphasis on mechanical strength and stability, since they are primarily used to switch electrical loads.

3. Differences in electrical parameters and performance:

High-frequency relays: High-frequency relays usually feature low insertion loss, high isolation, and low intermodulation distortion to ensure performance stability under high-frequency signals.

General Relays: The electrical parameters and performance of general relays are usually optimized for handling low frequency signals and larger current/oltage loads.

4. Differences in application fields:

High-frequency relays: High-frequency relays are mainly used in applications that need to process high-frequency signals, such as wireless communications, radar systems, microwave equipment, and antenna switching.

Ordinary relays: Ordinary relays are widely used in various fields, including household appliances, industrial control, automation systems, etc.

5. Special requirements:

High-frequency relay: Due to the special electromagnetic characteristics of high-frequency signals, the design and manufacture of high-frequency relays require special technologies and materials to ensure performance stability.

Generic Relays: Generic relays are relatively more conventional in design, often using a wider range of technologies and materials.

Ⅸ. The role of the electrical isolation of the relay

1. Isolation of high-voltage and low-voltage circuits: In some applications, it may be necessary to separate high-voltage and low-voltage circuits for safety. With electrical isolation, there is no direct connection between the high-voltage and low-voltage circuits, reducing the risk of accidental contact with high-voltage parts.

2. Prevent signal interference: In a complex circuit system, there may be different electrical characteristics and levels between different parts, and the interference of the input signal may be transmitted to the output signal, resulting in unstable system operation. Through electrical isolation, the propagation of these interfering signals can be prevented to ensure the accuracy and stability of the output signal.

3. Satisfy safety standards: In some applications, safety is of paramount importance. Galvanic isolation of relays can help meet safety standards and certification requirements, ensuring that electrical systems do not pose a hazard to personnel and equipment.

Basic Design and Operation of Relays

4. Protection control system: In the control system, relays are often used to connect control equipment and actuators. Electrical isolation can prevent external factors (such as voltage peaks, peak currents, etc.) from interfering with the control system, thereby protecting the stability and safety of the control system.

5. Avoid electrical crosstalk: When the relay controls multiple loads, the electrical isolation between input and output can prevent the problem of one load from affecting the normal operation of other loads.

6. Reduce noise: Electrical isolation can reduce the transmission of electromagnetic interference and electrical noise, thereby improving the signal quality of the system.


Frequently Asked Questions

1、What is the difference between relay and switch?
A switch is an electrical device that is used to turn on or off the flow of electricity in a circuit, while a relay provides control over the same flow.
2、What can cause a relay to fail?
Springs will loose resiliency with time. Relays can also fail due to poor contact alignment and open coils. Selection of the proper relay type for a given application is the most significant factor affecting relay reliability. Many poor design practices are used when designing them into circuits.
3、Which relay is used for AC and DC?
Solid state relays can be designed to switch both AC and DC using the output of an SCR, TRIAC, or switching transistor instead of the usual mechanical normally open (NO) contacts.
4、What is the function of no volt relay?
Electricity Power Feed to a NVR Panel (no Volt Relay) which will turn itself OFF if there is a power failure and invertor will not restart until power restore again. Also it is protect to single phase, over voltage and under voltage.

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