Due to their isolation function, relays are widely used in industrial electronics, such as remote control, communication, automatic control, mechatronics equipment, and power electronics, becoming an indispensable control component. Their core functions are mainly reflected in the following aspects:
1. Expanded control range: By using multi-point relays, multiple points can be touched simultaneously when the control signal reaches the preset value, realizing the switching, opening and closing of the line.
2. Amplification function: Relays can drive high-power circuits with small control inputs, thereby amplifying the signal.
3. Signal integration and processing: When multiple signals are input to the relay, the predetermined control effect can be achieved through internal comparison and integration.
4. Automation Applications: Relays can be combined with program control circuits to realize the functions of remote control, monitoring and automation devices.
01 Introduction to Relays
Definition and function of relays
A relay is a self-controlling device that triggers a sudden change in output when the input quantity reaches a specific threshold. Its main function is to switch and control circuits. This automatic control device, the relay, can trigger a sudden change in output when the input quantity (such as electrical, magnetic, sound, light, heat, etc.) reaches a specific threshold. Its core function is to realize the switching and control of circuits.
02 Classification and Characteristics of Relays
Electromagnetic relay
Electromagnetic relays use components such as an iron core and coil to switch circuits on and off. The state of their normally open and normally closed contacts changes when the coil is energized or de-energized. The core components of an electromagnetic relay include an iron core, coil, armature, and contact springs. When a voltage is applied across the coil, a current is generated, which in turn triggers an electromagnetic effect. This effect causes the armature, under the influence of electromagnetic force, to overcome the tension of the return spring and adhere to the iron core, thereby causing the moving contact to engage with the stationary contact (normally open contact).
When the coil is de-energized, the electromagnetic attraction disappears, and the armature returns to its original position under the reaction force of the spring, causing the moving contact to engage with the original stationary contact (normally closed contact). This engagement and disengagement process achieves on/off control in the circuit.
Regarding the "normally open" and "normally closed" contacts of a relay, we can understand it as follows: when the relay coil is not energized, the stationary contact in the open state is called the "normally open contact"; while the stationary contact in the closed state is called the "normally closed contact".
Detailed Explanation of the Working Principle of Relays
thermal reed relay
Thermistor reed relays control switches using the magnetic force generated by a temperature-sensing magnetic ring, eliminating the need for coil excitation and achieving temperature detection and control. This innovative type of thermal switch has garnered significant attention for its unique temperature detection and control capabilities. Its core structure includes a temperature-sensing magnetic ring, a constant magnetic ring, a reed switch, a thermally conductive mounting plate, and a plastic substrate. Unlike traditional electromagnetic relays, thermistor reed relays do not require coil excitation; instead, they drive the switch's action through the magnetic force generated by the constant magnetic ring. The availability of this magnetic force depends entirely on the precise temperature control characteristics of the temperature-sensing magnetic ring.
Detailed Explanation of the Working Principle of Relays
solid-state relay
Solid-state relays utilize isolation devices to achieve electrical isolation and can be classified into various types according to the load power supply type and switching type. A solid-state relay, a four-terminal device, has two terminals as inputs and two as outputs, with electrical isolation achieved through isolation devices in the middle. It can be classified into AC and DC types according to the load power supply type, and into normally open and normally closed types according to the switching type. In terms of isolation type, it encompasses hybrid, transformer-isolated, and opto-isolated types, with opto-isolated type being the most common.
03 Solid State Relay Technical Parameters
Basic technical parameters
The technical parameters of solid-state relays cover input/output characteristics, switching characteristics, and isolation performance, such as rated operating voltage and DC resistance. As an important four-terminal device, the technical parameters of solid-state relays are crucial for understanding and applying them. These parameters include not only input/output characteristics, such as input voltage range and output current capacity, but also switching characteristics, such as switching speed and switching life. Furthermore, isolation performance is also one of the important indicators for evaluating solid-state relays.
Rated operating voltage: This refers to the specific voltage value required for the relay coil to operate normally. This parameter varies depending on the relay model and may be AC or DC voltage.
DC resistance: This refers to the DC resistance of the relay coil, which can be measured using a multimeter.
Pull-in current: Pull-in current refers to the minimum current required for a relay to stably generate a pull-in action. In practical applications, to ensure stable operation of the relay, the applied current is usually slightly higher than the pull-in current. At the same time, to protect the coil, the applied operating voltage should generally not exceed 1.5 times its rated operating voltage to prevent excessive current from burning out the coil.
Release current: Release current is the maximum current that causes a relay to return from the energized state to the de-energized state. This switching occurs when the current drops below a certain threshold. It's important to note that this current value is typically much lower than the energizing current.
Contact switching voltage and current: The contact switching voltage and current of a relay refers to the upper limit of voltage and current that the relay can safely control during normal operation. Exceeding this limit may damage the relay contacts. Therefore, in practical applications, it is essential to ensure that the applied voltage and current are always within this range.
011. Six types of relays
▲ 1.1 Electromagnetic Relay
Electromagnetic relays, as a type of relay, are based on the electromagnetic effect. In a circuit, when the current in the electromagnet coil reaches a certain value, a strong magnetic field is generated, which attracts the contacts to actuate, thus switching the circuit. They have a wide range of applications, not only for the protection and control of power systems but also extensively in communications, automotive, and aerospace fields. By understanding the principles and applications of electromagnetic relays, we can better utilize this key circuit component. Electromagnetic relays achieve circuit control through electromagnetic attraction and are suitable for power protection, home appliance control, and other fields. They have a simple structure but require regular maintenance.
▲ Working principle
An electromagnetic relay uses the attraction of an electromagnet to drive the armature to move, which in turn triggers the action of the contacts, thereby controlling the on/off state of the circuit.
▲ Uses and Features
Electromagnetic relays have important applications in many fields. For example, in home appliance control, they can be used to control the switching of appliances such as air conditioners and washing machines; they also play an indispensable role in industrial automation. Electromagnetic relays have a relatively simple structure and are moderately priced, but they may wear out after long-term use, requiring regular maintenance and replacement.
▲ 1.2 Solid State Relay
Solid-state relays, which use solid-state components to achieve relay functions, operate on a completely different principle than electromagnetic relays. They utilize the switching characteristics of electronic components to control a larger current with a small current, thereby switching the circuit on and off. Compared to electromagnetic relays, solid-state relays offer higher sensitivity, longer lifespan, and more stable performance. While relying on semiconductor components to control current switching, resulting in high sensitivity and long lifespan, the trade-off is higher cost and the risk of performance degradation at high temperatures.
▲ Working principle
Solid-state relays do not switch circuits through mechanical contacts, but rather use semiconductor components, such as thyristors or MOSFETs, to perform their switching function.
▲ Uses and Features
It is well-suited for applications requiring high-frequency switching or precision control, such as temperature control systems or PLC control. Solid-state relays have a relatively long lifespan and do not generate sparks during switching; however, because they generate heat during operation, they require a heatsink to ensure stable operation.
▲ 1.3 Time Relay
A time relay, a device capable of switching circuits according to a set time delay, is widely used in various automated control systems. Its working principle is that when the input signal reaches a set value, the relay starts its timing function; after a predetermined delay, it then executes the corresponding circuit switching action. Time relays can perform circuit switching with a delay, precisely controlling time, and have important applications in areas such as motor starting and lighting control.
▲ Working principle
A time relay controls the action of its contacts through a delay circuit, thereby achieving the function of timed on/off switching.
▲ Uses and Features
Time relays are widely used in fields such as delay control for motor startup and lighting control. For example, the automatic shut-off function of stairwell lights relies on the precise timing of time relays. Furthermore, time relays feature adjustable delay times and are available in both mechanical and electronic types to meet different application needs.
▲ 1.4 Thermal relay
Thermal relays, as important electrical components, play a protective role in circuits. Their working principle is based on the thermal effect of current; they control the action of contacts by monitoring changes in the circuit current. Once the current in the circuit exceeds a set value, the thermal relay quickly cuts off the circuit, thus providing protection. Thermal relays, through overload protection circuits, are mainly used for motor protection, and when used in conjunction with fuses, can achieve comprehensive circuit protection.
▲ Working principle
Thermal relays monitor changes in current in a circuit and utilize the property of a bimetallic strip bending when heated to quickly cut off the circuit when an overload is detected.
▲ Uses and Features
Thermal relays are widely used in motor protection to effectively prevent motors from burning out due to overload and ensure their safe and stable operation. Although thermal relays operate relatively slowly, their sensitivity to overload makes them an important means of motor protection.
▲ 1.5 Intermediate Relay
Intermediate relays are important control components, commonly used for signal conversion and transmission in circuits. Their working principle utilizes the attraction of an electromagnet to achieve circuit switching and control. Intermediate relays have multiple functions, such as signal amplification, circuit isolation, and logic conversion, and are widely used in various circuit systems and automation control fields. Intermediate relays enhance signal conversion, are used in complex control circuits, and often play a role in PLC expansion.
▲ Working principle
Intermediate relays are essentially enhanced versions of electromagnetic relays, primarily used for signal amplification and multiplexing.
▲ Uses and Features
Intermediate relays play a crucial role in PLC output expansion and are also frequently used for switching multiple signals. Intermediate relays have multiple contacts, making them well-suited for complex control circuits.
▲ 1.6 Polarized Relay
Polarized relays, as a special type of relay, differ from intermediate relays in their working principle and structure. They utilize external control signals to change their own state, thereby achieving circuit switching or control. Polarized relays, which change state based on external control signals, are suitable for industrial automation, communications, and other applications, and must be used in DC circuits.
▲ Working principle
A polarized relay contains a permanent magnet, and the direction of its contacts changes with the direction of the current.
▲ Uses and Features
Polarized relays are commonly used in applications such as polarity detection and automatic commutation, playing a crucial role in electric vehicle charging control. They are characterized by high sensitivity, but it's important to note that polarized relays are only suitable for DC circuits.
