Ⅰ. Introduction and function of relay
A relay is an electronic or electrical device used to control the switching operation of one circuit in another circuit, and is usually used to convert a low-power signal or control signal into a high-power signal for remote control or automated operation.
When the input quantity (excitation quantity) changes to meet the specified requirements, an electrical appliance that causes the controlled quantity to undergo a predetermined step change in the electrical output circuit. It has an interactive relationship between the control system (also known as the input loop) and the controlled system (also known as the output loop). Usually used in automatic control circuits, it is actually an "automatic switch" that uses a small current to control the operation of a large current.
A relay is a type of electrical switch. It consists of a set of input terminals for single or multiple control signals and a set of operating contact terminals. A switch may have any number of contacts in various contact forms, such as make contacts, open contacts, or a combination thereof.
Relays are used where circuits need to be controlled by independent low power signals, or where multiple circuits must be controlled by one signal. Relays were originally used as signal repeaters in long-distance telegraph circuits: they refreshed a signal from one circuit by transmitting it on another. Relays were widely used in telephone exchanges and early computers to perform logic operations.
Traditional forms of relays use electromagnets to close or open contacts, but relays have been invented that use other operating principles, such as solid-state relays, which use semiconductor properties for control rather than relying on moving parts. A relay with calibrated operating characteristics, sometimes using multiple operating coils, to protect a circuit from overload or fault; in modern power systems these functions are performed by digital instruments still called protective relays.
Latching relays require only one pulse of control power to operate the switch continuously. Another pulse applied to the second set of control terminals, or a pulse with the opposite polarity, resets the switch, while repeated pulses of the same type have no effect. Latching relays are useful in applications when loss of power should not affect the circuit controlled by the relay.
The role of the relay:
Relay is an automatic switching element with isolation function. It is widely used in remote control, telemetry, communication, automatic control, mechatronics and power electronic equipment. It is one of the most important control elements.
1. Expand the control range: For example, when the control signal of the multi-contact relay reaches a certain value, it can switch, break, and connect multiple circuits at the same time according to the different forms of the contact group.
2. Circuit isolation: The relay can isolate the control circuit from the controlled circuit, prevent the control signal from interfering or damaging the controlled equipment, and improve the stability and reliability of the system at the same time.
3. Signal amplification: Relays can amplify low-power signals or weak signals to a high enough power level to drive devices that require greater current or voltage.
4. Integrated signal: For example, when multiple control signals are input into the multi-winding relay in the prescribed form, they will be compared and integrated to achieve the predetermined control effect.
5. Timing control: By adding a timer element to the relay circuit, the delayed start or delayed disconnection of the circuit can be realized, thereby realizing the timing control task.
6. Fault detection: Relays can be used to detect faults in the circuit, once a fault occurs, the relay can trigger an alarm or take other measures.
7. Logic control: Relays can be used to build complex logic control circuits, and achieve specific logic functions by combining the states of multiple relays.
8. Remote control: The relay can remotely control the electrical signal to realize the start-stop and state switching of the equipment within a relatively long distance, which is convenient for personnel operation or automatic control.
9. Protection: The relay can be used as a circuit protection device. When the parameters such as current, voltage or temperature exceed the set value, the relay will trigger, thereby cutting off the circuit to prevent equipment damage or danger.
Ⅱ. Working principle of relay
The working principle of the relay is based on electromagnetic induction, which is mainly realized by controlling the flow of current in the relay coil.
Basic working principle steps:
1. Energizing the coil: When current is passed through the coil of the relay (also known as the excitation coil), an electromagnetic field is generated within the coil. The strength of this electromagnetic field depends on the magnitude of the current passing through the coil.
2. Electromagnetic attraction: The electromagnetic field generated affects the iron core (also known as the active chip) located near the coil. A core is usually a movable piece of metal that can be attracted or repelled by an electromagnetic field.
3. Change of contact state: After the iron core is attracted by the electromagnetic field, the state of the mechanical contacts on the iron core will change. These contacts are usually mechanical switches connected to coils that close or break an electrical circuit.
4. Changes in the state of the control circuit: Changes in the state of the mechanical contacts will affect the control circuit of the relay. If the relay is of the "normally open" type, when the coil is energized, the contacts change from an open state to a closed state, making the control circuit complete. If the relay is of the "normally closed" type, the contacts change from closed to open when the coil is energized, cutting off the control circuit.
5. State change of the controlled circuit: The state change of the control circuit will affect the state of the controlled circuit. For example, if a relay is used to control a light bulb, when the coil is energized, the contacts close and current can flow through the contacts, causing the light bulb to light up.
Ⅲ. The difference between relays and transistors
1. The difference in working principle:
Relay: Works on the principle of electromagnetic induction, and controls the state of mechanical contacts through the electromagnetic field generated by the coil, thereby switching signals between different circuits.
Transistor: Based on the electronic characteristics of semiconductor materials, the output signal is adjusted by controlling the input signal (such as voltage or current), and functions such as signal amplification, switching, and amplification are realized.
2. Differences in application fields:
Relays: Mainly used in high-power or high-voltage circuits that require isolation or slow response, such as industrial control and power systems.
Transistors: Mainly used in low power and high frequency applications such as amplifiers, logic circuits, integrated circuits, etc.
3. The difference in speed:
Relays: Movement of mechanical components causes relays to respond slowly, usually on the order of milliseconds.
Transistors: Semiconductor devices operate very fast, typically on the nanosecond scale, and are therefore advantageous in high-frequency applications.
4. Differences in size and volume:
Relays: Relays are relatively large and bulky due to the presence of mechanical components.
Transistors: Transistors made of semiconductor materials can be very small, making them suitable for small and integrated circuits.
5. The difference between durability and life:
Relays: due to the mechanical components involved, have a relatively short lifespan, typically in the millions of operations range.
Transistor: Since there are no mechanical parts, the lifespan is long and can be in the billions of operations range.
6. The difference in power consumption:
Relays: Relays typically have higher power consumption due to the need to control the current in the coil.
Transistors: Transistors consume less power during operation, especially in switching applications.
7. The difference in noise:
Relay: The opening and closing of mechanical contacts will generate mechanical vibration and electromagnetic interference, which may cause some noise.
Transistors: Semiconductor-based operations generally do not produce noise.
Ⅳ. Types of relays
A solid state relay is an electronic device similar to a traditional electromagnetic relay, but its working principle is based on semiconductor technology and has no mechanical contacts. It uses semiconductor devices (usually optocouplers, transistors, triacs, etc.) to realize the switch control of the circuit, so as to realize the function of the relay. Solid state relays have no mechanical contacts, thus avoiding mechanical wear and contact resistance issues, thereby increasing reliability and longevity.
Compared with electromagnetic relays, solid state relays do not generate mechanical vibration and electromagnetic interference during operation, so the noise is lower. Solid state relays usually have good input and output isolation and can provide electrical isolation between input and output to protect circuits and equipment.
Signal relays are a special type of relay primarily used to transmit low power, low current signals. They are commonly used in applications that require isolating, amplifying, switching, or converting signals. Compared with traditional electromagnetic relays or solid state relays, signal relays are more focused on handling control signals rather than high power loads. Signal relays are specially designed to handle low power signals, such as control signals with small voltage or current.
These signals usually come from sensors, control circuits, logic circuits, etc. Similar to other types of relays, signal relays can also provide electrical isolation between the input and output. This protects the input signal source from output circuitry and improves system stability. Signal relays typically have fast response times because they are used for control signals that need to communicate state changes in a timely manner. Since signal relays primarily handle low-power signals, they are usually relatively small for space-constrained applications. Since signal relays are mainly used for low power signal processing, they usually have a long life and high reliability.
Safety relay is a kind of relay specially used to realize the safety control of machine, equipment or system. They are designed to ensure the safety of personnel and equipment during operation to prevent accidents and injuries. The safety relay has a special safety logic function, which can monitor the input signal and judge according to the predetermined safety logic. For example, if a safety door is opened or other dangerous conditions occur, the safety relay can quickly cut off the power and stop the operation of the equipment.
Safety relays usually have a mandatory control function, that is, when a safety event occurs, the relay will forcibly cut off the power supply of the device to ensure that the device stops running. The safety relay can monitor the input signals of various safety devices, such as emergency stop buttons, safety light barriers, safety door switches, etc.
A reed relay is a special type of relay whose contacts are made of reed switches. A reed switch is a tiny switch made of two conductive metal sheets (dry reeds) encapsulated in glass. When an external magnetic field acts on the reeds, they will attract together to close the circuit. Since the contacts of the reed switch are made of lightweight metal sheets, the response speed is very fast, and the closing action can be completed within milliseconds.
Reed switches dissipate only a tiny amount of power when the contacts are closed or open, so reed relays are particularly effective in low power applications. Due to the absence of mechanical contacts, reed relays are more reliable than traditional electromagnetic relays, enabling longer life and stability. Reed relays provide good electrical isolation when the contacts are closed, effectively isolating the input and output circuits.
A power relay is a relay specially designed to handle high-power circuits, and is usually used to control high-current, high-voltage equipment and loads. Compared with signal relays, power relays have larger capacity and are suitable for high power loads. Similar to other relays, power relays provide electrical isolation between the control circuit and the controlled circuit to protect the control circuit from high power circuits.
Since power relays are required to handle higher currents and voltages, they are typically larger than signal relays and are suitable for scenarios where large components can be accommodated. Power relays generally have high durability and can withstand frequent switching operations and high load conditions.
An I/O relay is a relay used to input and output control signals, and is usually used to connect the interface between external equipment and the control system. I/O relays are used to receive input signals from devices such as external sensors, switches, and buttons. These input signals can be of various types such as switch state, temperature, pressure, light, etc.
I/O relays usually have galvanic isolation between input and output to ensure that external signals do not affect the control system, while also protecting external devices from the control system. I/O relays can control external actuator devices such as motors, solenoid valves, lights, etc. By controlling the output signal, operations such as start-stop and adjustment of the equipment can be realized.
A high frequency relay is a relay specifically designed to handle high frequency and radio frequency (RF) signals. They are commonly used in wireless communications, radar systems, microwave equipment, broadcasting, antenna switching, and other applications that require processing of high-frequency signals. High-frequency relays need to have low insertion loss, that is, in the closed state of the relay, signal attenuation should not be introduced as much as possible.
High frequency relays require high isolation to avoid signal crosstalk between the relay's input and output. High-frequency relays need to have fast response time to adapt to the transmission rate of high-frequency signals. High frequency relays should minimize intermodulation distortion to maintain high signal quality. The packaging and design of high-frequency relays need to consider the electromagnetic characteristics of the signal to reduce signal reflection and interference.
8. Nano electromechanical relay
Nanoelectromechanical relays are electrically actuated switches which are built at the nanometer scale using semiconductor fabrication techniques. They are designed to replace or be used in conjunction with traditional semiconductor logic. The mechanical properties of NEM relays make them switch much slower than solid-state relays, but they have a number of favorable properties, such as zero current leakage and low power consumption, that make their use in next-generation computing possible.
Nanoelectromechanical relays are typically fabricated using surface micromachining techniques typical of microelectromechanical systems (MEMS). Laterally actuated relays are constructed by first depositing two or more layers of material on a silicon wafer. The upper structural layer is photolithographically patterned to form isolated blocks of uppermost material. The underlying layers are then selectively etched away, leaving thin structures (such as the beams of a relay) suspended above the wafer and free to bend laterally. A common set of materials used in this process is polysilicon as the upper structural layer and silicon dioxide as the sacrificial lower layer.
Nanoelectromechanical relays can be fabricated using back-end compatible processes, allowing them to be built on top of CMOS. This property allows the use of NEM relays to significantly reduce the area of certain circuits. For example, a CMOS-NEM relay hybrid inverter occupies 0.03 µm, which is one-third the area of a 45 nm CMOS inverter.
9. Protection relay
A protective relay is a relay that protects a circuit from sudden changes in current and voltage. Also known as a protective relay. Protective relays, power systems constituting power plants and substations, power transmission and distribution lines, and instrument transformers for short-circuit faults and ground faults that occur in load equipment, in order to minimize the impact of faults on other places by detecting Minimizing the impact of the above fluctuations, it plays the role of sending a control signal to the circuit breaker to select the faulty part and quickly disconnect it from the power system.
When setting the run time and run value for each relay, set it so that it ends up running for a short period of time to avoid propagating accidents to higher level equipment as described above. It is necessary to conduct research with reference to the characteristics of each relay in advance, and actually conduct a characteristic test of each relay on site to study settlement.
Also, depending on the condition of the equipment, it may not be possible to establish proper coordination just by setting the relays, so it is possible to adjust the operation time by using relays for relays (called delay relays).
Types of protective relays:
Overcurrent Relay (OCR): It operates when a certain value or more of current flows. There are two elements: overcurrent detection (called the timing element) and short circuit current detection (as the instantaneous element).
Overvoltage Relay (OVR): The relay operates when a voltage exceeding the expected value is applied to the circuit and protects equipment from overvoltage caused by generator failure, etc. In particular, it can be used in power purchasing circuits in customers with critical voltages.
Undervoltage Relay (UVR): A relay that operates when the voltage of a circuit drops below a planned value, to warn of a voltage drop due to a power failure or short circuit to a load, and to start a backup generator.
Ground fault overcurrent relay (OCGR, GR): It will operate when the ground fault current (zero phase current) reaches a certain value or more. When detecting a ground fault, the zero-phase current transformer (ZCT) detects the zero-phase current. It is desirable to use a Ground Fault Directional Relay (DGR) in circuits with high capacitance to ground, such as when cable lengths are long due to fear of failure.
Differential Relay (DFR): It operates when the vector difference between the currents flowing into and out of the protected part exceeds a predetermined value. Under normal conditions, in the event of an accident outside the protected area, the CT secondary currents I1 and I2 will be in balance and will not work, but in the event of an internal accident, the balance will be lost and the differential current I 1 -I 2 flow operation.
Ground Fault Directional Relay (DGR): Zero-phase voltage and zero-phase current determine the direction of the ground fault and operate. If the DGR is installed in a distribution substation with multiple feeders, the zero-phase current is detected by the secondary side current of the ZCT, the zero-phase voltage is detected by the transformer that is grounded, and the phase relationship is to selectively cut off the grounded distribution line.
Short Circuit Relay (DSR): The combination of line voltage and phase current determines the direction of the short circuit location. It is a relay that operates by selecting a faulty part in a loop system or a system with power sources that are difficult to detect with overcurrent relays on both sides.
Underfrequency Relay (UFR): It will operate when the frequency drops below the commercial frequency (50/60Hz). Mainly used in generator circuits.
Distance Relay (DR): Distance relays take voltage and current as input quantities and operate when the function of the ratio of voltage to current drops below a predetermined value. This ratio is called the impedance seen by the repeater, also known as the distance relay because this impedance is an electrical measure of the distance of the transmission line.
