MOV Selection and Size

"Varistor" is a resistor device with nonlinear voltage-current characteristics, primarily used for voltage clamping when a circuit is subjected to overvoltage, absorbing excess current to protect sensitive components. Its English name is "Voltage Dependent Resistor", abbreviated as "VDR", or simply "Varistor". The resistive material of a varistor is semiconductor-based, making it a type of semiconductor resistor. The widely used zinc oxide (ZnO) varistor is composed of the divalent element zinc (Zn) and the hexavalent element oxygen (O). From a material perspective, the zinc oxide varistor is classified as a "Group II-VI oxide semiconductor".

 

In Taiwan varistors are referred to as "surge absorbers" or sometimes as "electrical surge suppressors (absorbers)".

 

A varistor is a voltage-limiting protective device. By leveraging its nonlinear characteristics, when overvoltage occurs between the two terminals of the varistor, it can clamp the voltage to a relatively fixed value, thereby protecting the subsequent circuit. Key parameters of varistors include: varistor voltage, current capacity, junction capacitance, response time, etc.

 

The response time of a varistor is in the nanosecond range, faster than gas discharge tubes but slightly slower than TVS diodes. In most cases, its response speed is sufficient for overvoltage protection in electronic circuits. The junction capacitance of a varistor typically ranges from hundreds to thousands of picofarads, making it unsuitable for direct use in high-frequency signal line protection. In AC circuit protection, its relatively large junction capacitance increases leakage current, so this must be carefully considered in protective circuit design. The current capacity of a varistor is relatively large, though smaller than that of gas discharge tubes.

 

 

 

Working Principle of Varistors

 

When the voltage across the varistor is below its threshold, the current flowing through it is extremely small, equivalent to a resistor with near-infinite resistance. In other words, it acts like an open switch.

 

When the voltage exceeds its threshold, the current surges, and the varistor behaves like a resistor with near-zero resistance, acting like a closed switch.

 

Selection and Considerations for Varistors

 

Before selecting a varistor, the following technical parameters should be understood:

 

• Nominal voltage (V): The voltage across the varistor under specified temperature and DC current conditions.
• Leakage current: The current flowing through the varistor at 25°C when the maximum continuous DC voltage is applied.
• Clamping voltage: The peak voltage across the varistor when subjected to a standard 8/20μs current pulse.
• Current capacity: The maximum peak current the varistor can handle under specified pulse conditions (e.g., 8/20μs waveform).
• Surge environmental parameters: Include maximum surge current (Ipm), surge pulse width (Tt), minimum interval between surges (Tm), and the total number of surges (N) over the varistor's expected lifespan.

 

Additional considerations:

 

1. Ensure that the maximum continuous operating voltage is not exceeded under worst-case voltage fluctuations, as this shortens the varistor's lifespan.
2. When used between a power line and ground, higher nominal voltage varistors are recommended due to potential ground voltage rise.
3. The surge current absorbed by the varistor must be less than its maximum current capacity.

 

Advantages

 

Various sizes: SMD, 5mm, 7mm, 10mm, 14mm, 20mm, 25mm, 32mm, 34mm, 40mm, 53mm

Wide varistor voltage range: 18V–1800V

Multiple surge withstand levels: Standard, high surge, ultra-high surge

High current handling and energy absorption capability

Single-unit current capacity up to 70KA or higher

Fast response time

Low leakage current

Multiple lead types: Straight, bent, and other special configurations

Various packaging options: Bulk, reel, tape

 

Applications

 

Power systems

Surge suppressors

Security systems

Motor protection

Automotive electronics

Household appliances

 

 

 

Function of Varistors

 

The key feature of a varistor is that when the voltage across it is below its threshold (UNUN​), the current flow is minimal, acting like a closed valve. When the voltage exceeds UNUN​, its resistance drops sharply, allowing a surge current to pass while minimizing impact on other circuits. This suppresses abnormal overvoltage, protecting sensitive components.

 

Example: Zinc oxide varistors (e.g., 470V rating) are used in household color TV power circuits to clamp transient surge voltages, ensuring safe operation of downstream components.

 

Application Types

 

Depending on usage scenarios, varistors can be categorized into two main types:

1. Protective varistors
2. Circuit-functional varistors

 

Protective Varistors

 

(1) Power line protection vs. signal/data line protection (different technical standards apply).
(2) AC vs. DC applications (different aging characteristics under continuous voltage stress).
(3) Based on overvoltage characteristics:

 

Surge suppression type: For transient overvoltages (e.g., lightning, switching surges).

High-power type: For periodic pulse absorption (e.g., in switch-mode power supplies).

High-energy type: For absorbing magnetic energy in large inductive loads (e.g., generator coils).

Most varistors are reusable, but some are designed as one-time protective devices (e.g., those with short-circuit contacts).

 

Functional Uses

 

Varistors can also serve as:

1. High-voltage, low-current DC stabilizers (up to thousands of volts).
2. Voltage fluctuation detectors.
3. DC level shifters.
4. Voltage equalizers.
5. Fluorescent starter elements.

 

Key Performance Metrics

 

1. Protection characteristics: Limits voltage below the protected device’s withstand level.
2. Surge endurance: Must withstand specified surge currents, energy, and repetitive pulses.
3. Lifespan: Includes continuous voltage lifespan and surge cycle lifespan.
4. Secondary effects: Considerations like capacitance, leakage current, and nonlinear coupling effects.

 

Basic Parameters

 

1. Nominal varistor voltage (V): Voltage at 1mA current.
2. Voltage ratio: Ratio of voltages at 1mA and 0.1mA currents.
3. Maximum clamping voltage (V): Peak voltage under maximum surge current.
4. Residual voltage ratio: Clamping voltage relative to nominal voltage.
5. Current capacity (kA): Maximum surge current withstand.
6. Leakage current (mA): Current under maximum DC voltage.
7. Voltage temperature coefficient: Voltage change rate with temperature.
8. Current temperature coefficient: Current change rate with temperature.
9. Voltage nonlinearity coefficient: Static vs. dynamic resistance ratio.
10. Insulation resistance: Resistance between leads and body.
11. Static capacitance (pF): Inherent capacitance.
12. Rated power: Maximum power for stable operation at 85°C over 1000 hours.
13. Maximum surge current (8/20μs): Peak current without significant degradation.

 

Operation Principle

 

In standby mode, the varistor has high impedance (megohms), minimally affecting the circuit. When surge voltage exceeds its breakdown level, impedance drops (to a few ohms), diverting excess current and protecting downstream components.

 

Metal Oxide Varistors (MOVs)

 

The most common type, MOVs, consist of zinc oxide grains with other metal oxides or polymers, forming a ceramic block between two metal plates. The grain boundaries create diode-like junctions, resulting in nonlinear behavior: high resistance at low voltage, low resistance at high voltage.

MOVs may also be called:

ZNR (Zinc-Oxide Nonlinear Resistor)

ZOV (Zinc-Oxide Varistor)

CNR (Composite Nonlinear Resistor)

TNR (Titanium-Oxide Nonlinear Resistor)

 

Material Types

 

Zinc Oxide (ZnO)

Silicon Carbide (SiC)

Titanium Oxide (TiO₂)

 

Zinc Oxide Varistor Application Principle

 

Connected in parallel with protected equipment, the varistor responds within nanoseconds to surges, clamping the voltage far below the threat level (VsVs​), ensuring safety.

 

Common Models

 

Standard varistor models include: 5D, 7D, 10D, 14D, 20D, 32D, etc. Some also offer explosion-proof capabilities.

 

UF Capacitors supply MOV many years, we have standard series, Z-series, 6KV3KA Series.

 

 

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