How Metal Oxide Varistors Work in Circuit Protection

What Is a Metal Oxide Varistor (MOV)?

A Metal Oxide Varistor is a voltage-dependent, nonlinear resistive device. It is primarily made from zinc oxide (ZnO) grains and small amounts of other metal oxide additives, sintered at high temperatures. Its core electrical characteristics are:

• Under normal operating voltage: Exhibits an extremely high resistance (megaohm range), generating only microamps of leakage current—functioning like a "closed valve."
• During a transient over-voltage event: Its resistance drops dramatically, quickly conducting and shunting surge current away, clamping the voltage to a safe level.
• Response speed: Nanosecond range (typically 1-50ns), allowing it to quickly respond to voltage spikes caused by lightning, grid switching, and other phenomena.

This unique nonlinear voltage-current characteristic makes the MOV an ideal choice for suppressing abnormal over-voltage in circuits and protecting sensitive downstream components.

Basic Working Principle of an MOV

Microstructure and Conduction Mechanism

An MOV has a complex polycrystalline structure composed of zinc oxide grains and grain boundary layers. A potential barrier, similar to a bidirectional PN junction, forms between adjacent ZnO grains:

• At low voltage: The grain boundary barrier prevents electrons from passing through, and the device is in a high-resistance state.
• When voltage exceeds the threshold: Electron tunneling occurs at the grain boundaries, the barrier breaks down, and the device switches to a low-resistance, conductive state.
• After the surge is dissipated: The MOV automatically returns to its high-resistance state.

This process is repeatable. However, each surge impact causes gradual stress accumulation on the grain boundaries, ultimately leading to device aging.

Voltage-Current (V-I) Characteristic Curve

The V-I characteristic of an MOV can be divided into three operating regions:

Region	Voltage State	Current State	Description
Leakage Region	Below rated voltage	Microamps (µA)	High impedance, almost no current flow
Nonlinear Region	Near rated voltage	Milliamps to kiloamps	Small voltage change, large current change, best clamping effect
Saturation Region	Significantly above rated voltage	Extremely high current	Linear resistor behavior, may lead to thermal runaway damage

In design applications, the MOV should operate within the nonlinear region to avoid entering the saturation region, which could cause overheating and destruction.

Important Parameters When Selecting an MOV

Key Technical Specifications

Engineers usually evaluate the following core parameters when selecting an MOV:

Parameter	Symbol	Description	Typical Range
Varistor Voltage	V₁mA	Voltage across the device at 1mA DC current	18V ~ 1800V
Max. Continuous Operating Voltage	Vᵢ (AC/DC)	Max. AC/DC voltage that can be applied continuously	14V ~ 750V AC
Max. Clamping Voltage	VCLAM	Voltage across the MOV at a specified surge current	Related to V₁mA
Peak Surge Current	Iₚ (8/20µs)	Max. single 8/20µs waveform surge current capability	100A ~ 70kA
Rated Energy	E (10/1000µs)	Max. single-pulse energy absorption capability	Few joules to hundreds of joules
Leakage Current	Iₗ	Leakage current at max. continuous operating voltage	< 20µA

Selection Calculation Formula

Varistor Voltage Calculation:

For an AC circuit: V₁mA ≥ 2.2 × Vnom (Vnom is the RMS AC voltage)

Or V₁mA ≥ 1.5 × Vp (Vp is the peak voltage)

Example: For a 220V AC power system

• V₁mA = 2.2 × 220V = 484V
• Choose a MOV with a nominal value of 470V (e.g., 471KD series)

Max. Allowable Voltage Verification:

• VACrms ≈ 0.64 × V₁mA
• VDCr ≈ 0.83 × V₁mA

Surge Current Capability Calculation:
When the surge test voltage level is known:
Iₚ = (Vtest - VCLAM) / 2
In practice, select a MOV with a surge current rating at least 3 times higher than the calculated requirement to ensure reliability after multiple surges.

Package Specifications and Naming Conventions

The MOV's package diameter is directly related to its surge current capability:

Model Series	Diameter (mm)	Peak Surge Current Range	Typical Applications
5D Series	5	100A ~ 400A	Low-power, small supplies
7D Series	7	500A ~ 1750A	Chargers, adapters
10D Series	10	500A ~ 2500A	LED drivers, switching supplies
14D Series	14	1000A ~ 4500A	Industrial supplies, appliances
20D Series	20	2000A ~ 6500A	Power meters, telecom equipment
25D+ Series	25+	4500A ~ 15000A	Surge protective devices (SPDs), distribution systems

Naming Rule Example (471KD14):

• 471: Varistor voltage, 47 × 10¹ = 470V
• K: Tolerance (K=±10%, L=±15%, M=±20%)
• D: Package shape (D=Disc, S=Square)
• 14: Diameter (14mm)

Typical MOV Application Circuit

Basic Application Topology

An MOV is typically connected in parallel between the live (L) and neutral (N) lines at the power input, or between the line and ground. Its typical location is after the fuse and before the bridge rectifier or switching power supply.

plaintext

AC Input ──┬── Fuse ──┬── MOV ──┬── Downstream Circuit (Bridge Rectifier / Power Supply)

         │            │         │

         └────────────┴─────────┘

                     │

                    Ground (Optional)

Integrated Protection Solutions

To improve protection reliability and safety, MOVs are often used together with the following components:

1. Thermal Cutoff (TCO) : Connected in series with the MOV. It cuts off the circuit if the MOV overheats due to abnormal over-voltage, preventing thermal runaway.
2. Overcurrent Protection (Fuse) : Melts to isolate the circuit if the MOV fails in a short circuit.
3. Gas Discharge Tube (GDT) : Used in series with an MOV for high-energy surge scenarios, improving overall withstand voltage.
4. TVS Diode : Used for low-voltage, fine clamping protection in sensitive precision circuits.

Example of Integrated Device : Some manufacturers integrate a PPTC (Polymeric Positive Temperature Coefficient) resettable fuse with an MOV, providing resettable protection under both over-voltage and over-current conditions.

Advantages and Limitations of MOVs

Key Advantages

1. Fast Response: Nanosecond response time, faster than GDTs.
2. High Surge Current Capability: Up to 70kA (8/20µs waveform), suitable for lightning protection.
3. Bidirectional Symmetry: No polarity concerns, ideal for AC circuits.
4. Cost-Effective: Better price/performance ratio than other over-voltage protection devices.
5. Wide Voltage Range: From low-voltage signal circuits (18V) to high-voltage power systems (1800V).

Limitations and Mitigation Measures

Limitation	Description	Mitigation
Leakage Current	Microamp leakage current exists during normal operation	Pay special attention in ultra-low-power designs
High Capacitance	Hundreds to thousands of pF, affects high-frequency signals	Not suitable for high-frequency signal lines; use MLV or TVS instead
Aging Failure	Performance degrades after multiple surge events	Design with sufficient margin; use with thermal protection
High Clamping Voltage	Can be 1.5-2 times the varistor voltage	Ensure downstream circuits have sufficient voltage margin

Common Failure Modes and Thermal Runaway Protection

Two Main Failure Modes

1.Progressive Aging Failure (Most Common):

Repeated surge impacts damage grain boundaries; varistor voltage gradually decreases.

When varistor voltage drops below the peak normal grid voltage, the MOV enters a continuous conduction state.

Continuous AC current flow causes thermal runaway and eventual burnout.

 

2.Instantaneous Overstress Failure (Less Common):

A single, extremely large surge (e.g., direct lightning strike) exceeds the MOV's absolute rating.

The MOV is instantly punctured or cracked, typically failing as a short circuit.

Thermal Runaway Protection Mechanisms

To prevent fire hazards from a failing MOV, protection circuits should incorporate:

• Thermal Fuse Disconnection: Monitors MOV temperature and permanently disconnects the circuit if a safe threshold is exceeded.
• Series Fuse: Melts to isolate the fault if the MOV short-circuits.
• Flame-Retardant Packaging: Housings the MOV in a flame-retardant case to limit damage if it fails.

Introduction to Multilayer Varistors (MLVs)

For low-voltage, high-frequency signal line ESD protection, Multilayer Varistors (MLVs) are often a more suitable choice:

Feature	MOV	MLV
Structure	Single-layer disc	Multilayer SMD
Response Speed	Nanoseconds	Sub-nanoseconds
Capacitance	Higher (pF-nF)	Lower (pF range)
Application	Power lines, high-energy surges	Signal lines, ESD protection
Typical Use Cases	AC input, lightning protection	USB2.0, HDMI, GPIO

MLVs are suitable for data ports up to 480Mbps, such as USB2.0, computer peripherals, digital cameras, and mobile phones.

Common MOV Application Problems

 

• Aging from Repeated Surges: The MOV's protective performance degrades over time, requiring derating or eventual replacement.
• Inadequate Energy Rating: Using an MOV with insufficient energy handling capacity may lead to immediate failure.
• Thermal Runaway: Without proper thermal protection, a failing MOV can overheat and potentially cause a fire.
• Incorrect Voltage Selection: If the varistor voltage is too low, the MOV will conduct prematurely; if too high, it will not clamp effectively.

To improve protection, MOVs are often used together with:

• Thermal fuses (TCO)
• Fuses or circuit breakers
• Gas discharge tubes (GDTs)
• TVS diodes for final stage clamping

MOV Solutions from Topdiodes

Topdiodes provides a wide range of Metal Oxide Varistors for over-voltage protection in consumer, industrial, and power electronics applications.

Product Range Includes:

• Standard disc-type MOV series (5D/7D/10D/14D/20D/25D)
• High-surge withstanding MOV series
• Multilayer varistor (MLV) SMD series
• Thermally protected MOV (TMOV) series

Key Benefits:

• Varistor voltage range: 18V ~ 1800V covering all application scenarios
• Peak surge current: up to 15kA (8/20µs)
• Safety certifications: UL, cUL, TUV, CQC
• RoHS and REACH compliant
• Short lead times and cross-reference replacement support

Recommended Applications:

Application Area	Recommended Series	Key Parameters
Chargers/Adapters (5-20W)	7D Series	470V, 500-1750A
LED Driver Power Supplies	10D/14D Series	470-680V, 2500-4500A
Industrial Switching Supplies	14D/20D Series	470-820V, 4500-6500A
Home Appliance Control Boards	10D/14D Series	270-470V
Surge Protective Devices (SPD)	25D+ Series	470-820V, 10kA+
Signal Line ESD Protection	MLV Series	Low capacitance, fast response