Basic Knowledge of MOV Varistors: How They Provide Overvoltage Protection in Circuits and Prevent Surge Damage to Devices
In electronic devices, overvoltage and surge currents are the “invisible killers” of circuit stability. A varistor (Voltage Dependent Resistor, VDR) is a key circuit protection component that can quickly respond to abnormal voltage levels and protect sensitive devices from damage.
This article will delve into the working principle, key parameters, application scenarios, and selection guide for varistors, helping you build more reliable electronic systems.
1. What is a MOV Varistor?
MOV (Metal Oxide Varistor) is a nonlinear resistor whose resistance value changes significantly with external voltage. It is commonly written as “MOV,” which stands for Metal Oxide Varistor.
Core Features: Voltage Threshold Response: When the voltage is below the threshold, the resistance is very high (acting as an insulator); once the voltage exceeds the threshold, the resistance drops rapidly (acting as a conductor), quickly absorbing surge energy.
Self-Restoration: After transient voltage disappears, it automatically recovers to a high-resistance state without manual intervention.
Materials and Structure: The main components are zinc oxide (ZnO) or silicon carbide (SiC); SMD Varistor manufactured using a ceramic sintering process in multi-layer structures.
2. How Does a MOV Varistor Work?
- Normal Voltage:Resistance is up to megaohms, with leakage current very low (<1μA), causing minimal impact on circuit operation.
- Overvoltage or Surge OccursWhen the voltage exceeds the threshold (e.g., 470V), the internal grain boundary breaks down, causing resistance to drop instantly to a few ohms, creating a current path to dissipate energy in the form of heat.
- Response Speed: Nanosecond-level response (<1ns), capable of withstanding lightning strikes, electrostatic discharge (ESD), and other instantaneous high-voltage impacts.
3. Key Performance Parameters
| Parameter | Definition & Significance | Typical Examples |
| Varistor Voltage(Vn) | The critical voltage at which the resistance drops sharply | 470V、14V、320V |
| Surge Current Capacity(Ip)) | The maximum surge current the varistor can withstand (8/20μs pulse waveform) | 50A、100A、20kA、5kA |
| Energy Absorption(J)) | 单次脉冲可吸收的最大能量 | 10J、50J、200J |
| Leakage Current(IL)) | The current leakage under normal operating voltage | <1μA |
4. Four Major Applications of MOV Varistors
- Power Protection: Used at AC/DC power inputs (e.g., adapters, chargers) to suppress lightning surge and switching noise.
- Communication Equipment: Protects Ethernet ports and RS485 interfaces from ESD and electrical fast transients (EFT).
- Industrial Control: Absorbs back EMF generated by inductive loads in motor drives and PLC modules.
- Consumer Electronics: Provides electrostatic discharge protection for USB ports and button circuits in smartphones and laptops.
5. Selection Guide: How to Match Circuit Needs?
- MOV Voltage Selection: The MOV voltage should be 1.21.5 times higher than the maximum operating voltage of the circuit (e.g., for a 220V AC circuit, choose a 470V MOV).
- Size and Package: For high-frequency circuits (e.g., HDMI interfaces), choose low-capacitance surface-mount MOVs (e.g., 0402 package).
- Environmental Suitability: For high-temperature and high-humidity environments, select MOVs with anti-sulfuration design (e.g., Boarden VSR series).
- Safety Certification: Automotive electronics must meet AEC-Q200 standards, and industrial equipment must comply with UL and other certifications.
6. Boarden MOV Varistor Solutions
As an innovator in circuit protection,Boardenoffers a full range of MOV products, covering high, medium, and low voltage ranges to meet various industry needs:
- High Reliability:Verified through 1000 surge tests and 85℃/85%RH durability tests to ensure stability in extreme environments.
- SMD Packaging: Miniaturized, surface-mount, replacing traditional leaded MOV products.
- Custom Services: Provides specialized models with low capacitance, high energy, and anti-sulfuration.
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