Metal Oxide Varistors: Everything You Need to Know

What are Varistors?

Varistors, short for variable resistors, are voltage-dependent resistors used for protecting electronic circuits against excessive transient voltages. Their resistance varies with the applied voltage – at low voltages, they have a high electrical resistance which decreases as the voltage is increased.

The most common type are metal oxide varistors (MOVs), which contain a ceramic mass of zinc oxide grains in a matrix of other metal oxides (such as small amounts of bismuth, cobalt, manganese) sandwiched between two metal plates (the electrodes). The boundary between each grain and its neighbour forms a diode junction, which allows current to flow in only one direction. The mass of randomly oriented grains is electrically equivalent to a network of back-to-back diode pairs, each pair in parallel with many other pairs.

How do MOVs work?

When a small or moderate voltage is applied across the electrodes, only a tiny current flows, caused by reverse leakage through the diode junctions. When a large voltage is applied, the diode junctions break down due to a combination of thermionic emission and electron tunneling, and a large current flows. The result of this behavior is a highly nonlinear current-voltage characteristic, in which the MOV has a high resistance at low voltages and a low resistance at high voltages.

A varistor remains non-conductive as a shunt mode device during normal operation when the voltage across it remains well below its “clamping voltage”, thus varistors are typically used for suppressing high-voltage transients. When subjected to a high voltage transient, its resistance drops and it becomes highly conductive, thus limiting the transient voltage to a safe level. The device reverts back to being non-conductive after the transient threat is gone. This behavior is similar to that of a pair of back-to-back Zener diodes.

Key Characteristics and Ratings of MOVs

Some of the key characteristics and ratings to consider when selecting MOVs include:

Continuous Operating Voltage

The maximum steady state voltage an MOV can withstand continuously without degrading or failing. It is typically around 80-90% of the varistor voltage. Operating at or above this voltage for extended periods can cause the MOV to overheat and fail.

Varistor Voltage

The voltage across the MOV measured at a specified pulsed DC test current, typically 1 mA. This is usually printed on the MOV body. Standard values range from a few volts to several kilovolts.

Energy Rating

The maximum amount of energy the MOV can absorb from transient impulses of a specified waveform before failure. It is measured in Joules. Higher energy ratings enable the MOV to handle larger and more frequent transient events.

Disk Size (mm)	Energy Rating (Joules)
5	0.1 – 1
7	0.25 – 5
10	1 – 30
14	10 – 110
20	50 – 400

Peak Current

The maximum transient current the MOV is rated to handle for an 8/20μs impulse. This is important for ensuring the MOV can safely shunt the expected transient currents in the application.

Response Time

The time required for the MOV to clamp the transient overvoltage to its rated level. MOVs have very fast response, typically in the sub-nanosecond range.

Maximum Clamping Voltage

The peak voltage that will appear across the MOV terminals when measured at a specified pulsed current (typically 5-20 times the nominal current). It’s an important indicator of the protective level provided by the MOV.

MOV Failure Modes and Protection Methods

MOVs can fail in two main ways:

1. Short-circuit failures occur when excessive current permanently damages the metal oxide material, causing the MOV to become a short-circuit. This is a potentially catastrophic failure that can lead to further circuit damage or fires if not protected against.

2. Open-circuit failures occur when repeated transient voltages or surges gradually degrade the MOV’s materials until it loses its clamping ability and becomes an open-circuit.

To protect against these failures, various methods can be used:

• Fusing: A Fast-Blow Fuse in series with the MOV can prevent overheating and fires in case of short-circuit failure. The fuse must be properly rated to allow the MOV to clamp transients but still open quickly on failure.

• Thermal fuses: Thermal fuses or thermally protected MOVs have built-in thermal disconnects which isolate the MOV when it reaches a certain temperature due to overcurrent. These provide additional protection compared to standard fuses.

• Arc protection: In some high energy applications, arc-proof ceramic cases or potting compounds may be used to contain any arcing failures.

• Redundancy: Using multiple MOVs in parallel can provide redundancy in case one fails open. The parallel MOVs share current and must each be rated to handle the full transient for reliable protection.

Proper MOV sizing and placement is also critical to achieving effective circuit protection while avoiding MOV failures. Consulting manufacturer application notes and derating guidelines is recommended.

Applications for MOVs

MOVs are widely used for overvoltage protection in various electrical and electronic systems, including:

Power Supplies

MOVs are commonly used at the input of power supplies to suppress transients coming from the AC power lines. They clamp the input voltage to a safe level, protecting the sensitive downstream components.

Communication Lines

High-speed communication ports like Ethernet, RS-232, and telephone lines are susceptible to transients coupled from nearby lightning strikes or switching. MOVs, often in combination with TVS diodes or gas discharge tubes, are used to clamp these transients and protect the communication ICs.

Automotive Electronics

The harsh electrical environment in vehicles with inductive loads like motors, solenoids, and ignition systems can generate large voltage spikes. MOVs protect sensitive electronics like control modules, sensors, and displays from these transients.

Industrial Equipment

Industrial controls, motors, relays, and switchgear are all sources of significant transients. Properly rated MOVs are used to harden the inputs and outputs of PLCs, drives, contactors, and other industrial electronic devices against overvoltages.

Consumer Electronics

MOVs are used for secondary surge protection in consumer devices like TVs, appliances, and smart home devices. They supplement the primary protection typically provided by gas discharge tubes or spark gaps at the AC mains input.

The table below summarizes some common MOV applications and the typical voltage ranges used:

Application	Typical Voltage Range
AC Mains	120 – 500 VAC
DC Power	5 – 1000 VDC
Telecom	5 – 200 V
Automotive	12 – 48 VDC
Industrial	12 – 690 V
Consumer Devices	5 – 240 VAC

Specifying and Selecting MOVs

When choosing an MOV for a specific application, several factors must be considered:

1. Operating voltage: The MOV’s continuous operating voltage must be higher than the circuit’s nominal operating voltage, typically by a 10-20% margin. This ensures it does not affect normal circuit operation.

2. Clamping voltage: The MOV should clamp transients to a level that is within the withstand capability of the protected circuit.

3. Energy rating: The MOV must be able to safely absorb the expected transient energy in the application. This depends on factors like the transient source, exposure levels, and coupling paths.

4. Response time: MOVs have very fast response, but the actual clamping time depends on the transient waveform and MOV characteristics. It should be fast enough to protect the downstream components.

5. Mounting and placement: The MOV should be placed as close as possible to the protected circuit to minimize parasitic inductance and provide the lowest clamping voltages. The layout and mounting must also allow proper heat dissipation.

6. Environmental factors: Temperature, humidity, altitude, and other environmental parameters can affect MOV performance and life expectancy. Rated limits must be observed.

7. Regulatory standards: Some applications may require MOVs to meet certain regulatory standards for safety, EMC, or hazardous environment use.

Manufacturers provide selection guides and online tools to help choose the right MOV based on these parameters. Proper MOV selection is critical for achieving reliable circuit protection.

Frequently Asked Questions

1. How do I know if my MOV is working?

An MOV is a passive device that is normally non-conductive. It only becomes conductive when the voltage across it exceeds its clamping level. One way to check if an MOV is functional is to use an ohmmeter to measure its resistance. A good MOV should have a very high resistance, typically in the megaohm range, when measured with a low-voltage ohmmeter. If it measures as a short-circuit, it may have failed and needs replacement.

2. Can MOVs be used in series or parallel?

MOVs can be connected in parallel to increase current handling capacity or provide redundancy. When paralleling MOVs, it’s important to use devices with closely matched characteristics to ensure proper current sharing. MOVs should generally not be used in series as small differences in their clamping voltages can cause one MOV to take all the transient current and fail prematurely.

3. What happens when an MOV fails?

An MOV can fail in short-circuit or open-circuit mode. In short-circuit failure, the MOV becomes a low resistance path that can draw excessive current and potentially cause a fire hazard. In open-circuit failure, the MOV loses its clamping ability and leaves the circuit unprotected. Failed MOVs must be replaced promptly with proper safety precautions.

4. How long do MOVs last?

The life expectancy of an MOV depends on factors like the operating environment, the frequency and severity of transient events, and the MOV’s ratings. In a typical surge protection application, an MOV may last 5-10 years or more. However, every transient event gradually degrades the MOV’s materials, and it may fail after a large surge or many smaller ones. Periodic inspection and replacement of MOVs is recommended for critical applications.

5. Can MOVs be used for AC and DC applications?

Yes, MOVs are suitable for both AC and DC applications. For AC circuits, the MOV’s continuous operating voltage rating must be chosen based on the AC voltage’s peak value, not RMS. For DC applications, the MOV rating must exceed the maximum expected DC voltage. Proper polarity must also be observed for DC use, as MOVs are not bi-directional devices.

Conclusion

Metal oxide varistors are a key component in protecting circuits from transient overvoltages. Their fast response, high energy handling, and compact size make them ideal for many surge protection applications. However, proper selection and application of MOVs is critical to ensure reliable protection without compromising the MOV’s life or creating additional hazards. By understanding the key characteristics, ratings, failure modes, and applications of MOVs, designers can effectively utilize these versatile devices to enhance the reliability and robustness of electronic systems.