Varistor, especially metal oxide varistor (MOV), is a critical voltage protection component. Its core characteristic is nonlinear resistance: it exhibits high impedance (similar to an insulator) at normal operating voltage, and once the voltage exceeds its specific threshold (varistor voltage), its impedance will sharply decrease (similar to a conductor), thereby releasing excessive current and clamping the voltage at a relatively safe level. This characteristic makes it an expert in absorbing instantaneous overvoltage and surge current.
The following are some products and application areas that widely use varistors, and their existence is crucial for ensuring the safety and stable operation of equipment:
Household appliances and consumer electronics products:
Power adapter/switching power supply: This is one of the most common applications of varistors. Almost all AC powered devices (such as mobile phone chargers, laptop power supplies, televisions, game consoles, small appliances, etc.) have input terminals connected in parallel with varistors. It is used to absorb surge voltage in the power grid, such as voltage spikes caused by lightning induction and the start stop of nearby large equipment (motors, elevators), to prevent surge damage to sensitive components such as rectifier bridges, filter capacitors, and switch tubes in the later stage.
Home appliance motherboards: on control circuit boards inside washing machines, refrigerators, air conditioners, microwave ovens, etc., varistors are often deployed near power input parts or circuits that supply power to inductive loads such as motors and relays, providing surge protection.
Industrial equipment and automation:
Industrial power supply: Power supply fluctuations and interference in industrial environments are more complex and severe. High power switching power supplies, UPS uninterruptible power supplies, frequency converters, and other equipment extensively use varistors for primary and secondary protection.
Programmable Logic Controller: As the core of industrial control, PLC is often equipped with varistors in its power module and I/O module to protect it from power line surges and induced lightning strikes, ensuring the reliability of the control system.
Motor control and drive: In the control circuit of inductive loads such as contactors, relays, solenoid valves, etc., a high back electromotive force (voltage spike) will be generated at the moment of disconnection. The parallel connection of varistors at both ends of the coil can effectively absorb this energy and protect the driving switch tubes (such as thyristors, IGBTs) or control contacts from breakdown.
Electronic device interface protection:
Communication ports: Ethernet ports (RJ45), telephone line interfaces (RJ11), RS232/485, and other communication ports are exposed and highly susceptible to the threat of electrostatic discharge (ESD) or induced lightning strikes. Varistors (often used in conjunction with other protective devices such as TVS tubes) are connected in parallel between the signal line and ground, and can respond in nanoseconds to discharge dangerous overvoltages to ground, protecting the communication chip at the back end.
USB/HDMI and other peripheral interfaces: Hot plugging operations can easily generate ESD. Varistors are commonly used in the protection circuits of power and data lines for these interfaces.
Security monitoring equipment: The power and video signal lines (such as coaxial cables) of outdoor cameras, access controllers, and other devices are the main paths for introducing surges. Varistors are the fundamental components in lightning and surge protection schemes for these devices.
New energy and power equipment:
Photovoltaic inverter: Solar panels exposed outdoors are susceptible to direct or induced lightning strikes. Both the DC input side and AC output side of the inverter require strong surge protection, and varistors are an important component of it.
Electric vehicle charging stations: The power input, control circuit, and communication module of AC and DC charging stations require varistors for surge protection to ensure charging safety and equipment stability.
Electric energy meter (smart meter): Outdoor electric meters need to withstand lightning and overvoltage caused by grid operation, and varistors are key components for their power circuit protection.
Power surge protector: The core component inside a specialized SPD is usually a high current capacity varistor (or a combination of gas discharge tube and varistor), installed at the front end of the distribution box or equipment as the first or second level protection.
Automotive Electronics:
Car charger: The car charger with cigarette lighter interface uses varistors to protect its circuit from voltage fluctuations of the car battery (such as voltage spikes caused by sudden load drops).
Electric vehicle high-voltage system: In high-voltage components such as battery management systems, motor controllers, on-board chargers (OBCs), DC-DC converters, etc., varistors are used to absorb operating overvoltages and possible transient surges, protecting expensive power semiconductor devices (such as IGBT, SiC MOSFET).
Medical equipment:
Medical power supply and instruments: Precision medical equipment such as monitors, ultrasound devices, and analytical instruments have extremely high requirements for power supply stability and anti-interference. Varistors are used in their power modules to prevent equipment failures or measurement errors caused by grid surges, ensuring patient safety and accurate data.
Computers and servers:
Computer power supply: The internal input terminal of ATX power supply must have varistors as the first line of defense for surge protection.
Server power supply: Data centers have extremely high requirements for equipment reliability, and server power modules use high-quality varistors to resist grid interference.
In summary, the application logic of varistors is clear and visible:
Location: Mainly deployed at the power input end (AC or DC), interfaces susceptible to surge impact (communication, signal, control), and both ends of inductive loads.
Purpose: To absorb instantaneous overvoltage (surge, ESD, back electromotive force), clamp voltage, and discharge surge current, in order to protect expensive and sensitive electronic components and integrated circuits in the later stage from damage.
Core value: Providing cost-effective and responsive overvoltage protection is a fundamental component for improving the reliability, safety, durability, and electromagnetic compatibility of electronic and electrical equipment.
Therefore, it can be said that almost all electronic and electrical equipment connected to the power grid or with transient overvoltage risks silently use varistors as "voltage guardians" at a critical position in their circuit design.