Close this search box.

Comprehensive analysis of relay switches

What is a relay switch?

Relay switch is an electronic control device, which is usually used in circuits to achieve remote control and automatic control. Its basic function is to be used as a switch in a circuit, capable of controlling a larger current through a smaller current without direct contact with the circuit.

The operation of relay switches relies on electromagnetic principles. When the relay coil is energized, a magnetic field is generated, which attracts a mechanical switch and drives the switch to close or open, thus controlling a larger current. Relay switches can realize the isolation and amplification functions of circuits, which makes them very important in automated control systems, especially in situations where isolation control and high current control are required.

In addition, the use of relays also helps to improve the safety and stability of the circuit, because it can cut off high voltage or large current and protect the circuit from damage without affecting the main control system. These characteristics of relays make them widely used in many fields such as industrial automation, household appliances, and communication equipment.

working principle

The working principle of a relay switch involves electromagnetic action, in which a small current can control a larger current. Here are the detailed steps and instructions:

  1. Excitation coil: There is an electromagnetic coil in the relay. When passing through this coil, it generates a magnetic field.
  2. Iron Core and Action Mechanism: An iron core is usually installed around the coil to concentrate the magnetic field and enhance the magnetic force. The magnetic field generated by the coil attracts a moving iron piece connected to the iron core, causing it to move.
  3. Contact switch: The movement of the moving iron piece pushes the switch contacts to close or open directly or through a series of mechanical levers. These contacts are usually part of an electrical circuit, and the action of the relay (closing or opening) can control a larger current in the circuit.
  4. Control and Isolation: One of the main advantages of relays is control and isolation. Because the excitation coil and switch contacts are physically isolated, high-voltage or high-current circuits can be safely controlled with smaller currents, which is critical to ensuring the safety and stability of the circuit.
  5. De-excitation and reset: When the excitation coil is powered off, the magnetic field disappears, the moving iron piece returns to its original position through a mechanical device such as a spring, and the contacts return to their initial state (open or closed), thereby restoring the circuit to its original configuration. .

Types and structural characteristics of relay switches

There are many types of relay switches, each with different structural characteristics and application fields. The following are some common relay types and their structural characteristics:

  1. Electromagnetic relay: This is the most common type of relay. Its basic components include an electromagnetic coil, an iron core, an actuating arm (moving iron piece) and contacts. When current passes through the coil, a magnetic field is generated, causing the actuator arm to move, thereby changing the state of the contact (open or closed). Electromagnetic relays can be used to control larger currents and are widely used in industrial and household appliances.
  2. Solid State Relay (SSR): Solid state relays contain no mechanical moving parts and instead use semiconductor devices, such as transistors, to perform switching functions. This relay offers the advantages of fast response, high durability and noiseless operation. They are typically used in applications that require high-speed switching and high reliability, such as servers and communications equipment.
  3. Thermal Relay: Thermal relay uses the heat generated when electric current passes through a resistor to trigger operation. They are often used for overload protection and can automatically disconnect when a circuit is overloaded to protect the circuit from damage.
  4. Photorelay: A photorelay uses light as a trigger signal and typically includes a light-emitting diode (LED) and a light-sensitive element such as a photoresistor or phototransistor. This kind of relay is suitable for occasions where input and output need to be isolated, such as high-voltage control systems.
  5. Magnetic latching relay: This type of relay has a latching function, that is, after being activated, the relay can maintain its switching state even if the excitation current is removed. They are often used in applications where reduced energy consumption and extended battery life are required.
  6. Polarity Relays: Polarity relays rely on the direction of current flow to determine operating status. Their structure allows them to distinguish the positive and negative directions of current and are often used in communication systems to ensure the correct direction of signal transmission.

Applications of relay switches

Relay switches are widely used in many fields due to their high reliability and adaptability, especially in industrial control, transportation systems and household appliances. The following are specific uses of relays in these application areas:

Application in industrial control
In industrial environments, relay switches are key components for automation and increased production efficiency. They are used to control motor starting, stopping and direction switching, monitor production processes, and protect equipment from electrical faults. Relays can work with various sensors, such as temperature, pressure and flow sensors, to ensure precise control of processes. In addition, relays are also widely used in PLC (programmable logic controller) systems as executive elements to control important operating sequences.

Applications in transportation systems
In transportation systems, relay switches play a vital role. They are used to control railway signals, the raising and lowering of barrier gates and the switching of traffic lights. For example, in train control systems, relays are used to switch circuits between tracks to ensure safe and smooth train movement. Additionally, relays are used in automobiles to control power to headlights, starters, and other critical systems.

Applications in household appliances
In the field of household appliances, relays are widely used in washing machines, refrigerators, air conditioners and other equipment to control various functions of the equipment. For example, the relay can control the start and stop of the compressor of the refrigerator and adjust the indoor temperature; in the washing machine, the relay controls the switch of the water pump and the operating mode of the motor. The use of relays improves the operational convenience and energy efficiency of household appliances.

Relay switch VS traditional switch

Relay switches and traditional switches have their own characteristics and advantages in function and application. Here are the main comparisons between the two:

Relay switch


  1. Remote control: Relays can be operated remotely through tiny currents or voltages, and are suitable for occasions requiring remote or automated control.
  2. Isolation function: The relay realizes electrical isolation between input and output through the design of its coil and contacts, which increases the safety of the system.
  3. Adapt to high current and high voltage: The relay can control higher current and voltage without direct contact with high loads, thus reducing the risk to the operator.
  4. Versatility: Relays can be designed with multiple contacts and can control multiple circuits at the same time.


  • Mechanical wear: The contacts and moving mechanical parts in mechanical relays will wear out over time and frequency of use.
  • Response time: Compared to solid-state switches, mechanically operated relays have a slower response time.
  • Energy consumption: Relays require a certain amount of current in the excitation coil to maintain operation, and may have high energy consumption.


Traditional switch (mechanical switch)


  1. Simple and direct: The traditional switch is simple to operate, and the user directly controls it manually, making it easy to understand and use.
  2. Fast Response: Mechanical switch response is almost instantaneous with no delay.
  3. Low cost: Generally speaking, traditional mechanical switches cost less than relay systems.
  4. Low Maintenance: Maintenance is relatively simple as traditional mechanical switches have no electrical components.


  • No remote control: Traditional switches lack the ability to be remotely controlled and require physical contact.
  • Less safe: In high voltage or high current environments, direct operation of traditional switches may be a safety risk.
  • Single function: Traditional switches can usually only control the on/off of one circuit.

Causes of relay switch failure

Relay switches may experience various failures during long-term use. These failures are usually caused by the following reasons:

  1. Mechanical wear: The mechanical parts in the relay, such as contacts and actuating arms, will wear out due to long-term movement. Worn contacts can cause poor contact, which can cause circuit disconnection or instability.
  2. Contact Oxidation or Corrosion: Contacts may become oxidized or corroded when exposed to air for extended periods of time, which affects their conductive properties. Humidity and polluted environments can exacerbate this problem.
  3. Coil damage: If the relay’s coil is damaged due to overheating or excessive current, the relay will not work properly. A short circuit or open circuit in the coil will cause the relay to fail.
  4. Spring failure: The spring in the relay used to reset the action arm may fail due to fatigue or damage, preventing the relay from returning to its original position.
  5. Dust and Contaminants: Dust and other contaminants can accumulate on the contacts and mechanical parts of the relay, affecting its proper operation. These contaminants can cause poor electrical contacts or blockage of mechanical action.
  6. Power supply issues: Unstable power supply or voltage fluctuations can damage relays, especially those solid-state relays that are sensitive to power supplies.
  7. Improper Selection or Use: If a relay is not sized appropriately for its application environment (e.g. current, voltage, or switching frequency mismatch), it may fail prematurely.
  8. Installation errors: Improper installation of the relay, such as unstable fixation or incorrect wiring, may also cause its function to be affected.

How to test a relay switch

Testing a relay switch is an important step in ensuring its proper functionality and safe operation. Here are the basic methods for testing a relay switch:

1. Visual inspection
Before starting any detailed testing, start with a visual inspection:

  • Check the relay for obvious damage such as cracks, scorch, or corrosion.
  • Confirm whether the relay model and specifications meet the application requirements.

2. Test coil resistance
Use the resistance setting on a multimeter to measure the resistance of the relay coil:

  • Touch the red and black test leads of the multimeter to the two endpoints of the relay coil.
  • Read and record the resistance value, compare it with the standard resistance value on the relay specification sheet, and check whether there is an open circuit or short circuit.

3. Functional testing
Check the switching function of the relay contacts:

  • Power the relay coil and use the multimeter’s buzzer setting or voltage setting to test the contacts.
  • In the de-energized and energized states, check whether the status of the normally open (NO) and normally closed (NC) contacts switches correctly.

4. Detect contact resistance
Evaluate the resistance and contact quality of contacts:

  • With the relay in the activated state, use a multimeter to measure the resistance of the energized contacts (for example, after the normally open contact is closed).
  • If the resistance value is unusually high, it may indicate oxidation or burnt problems in the contacts.

5. Test the action time of the relay
For high performance applications, it may be necessary to measure the relay operating time:

  • Use an oscilloscope and specific test equipment to measure the time from energization of the coil to completion of contact action.
  • Compare the actual measured values with the requirements of the technical specifications to confirm whether the response speed of the relay meets the design standards.

6. Long-term stability testing
For critical applications, conduct long-term stability and durability testing:

  • Under simulated actual working conditions, the relay is continuously operated for a certain period.
  • Observe whether the performance of the relay is attenuated and check whether there is a fault or performance degradation.

Table of Contents