What are Varistors?

Varistors, short for variable resistors, are electronic components that exhibit a nonlinear current-voltage relationship. They are designed to protect electronic circuits from overvoltage transients, such as those caused by lightning strikes or power surges. The most common type of varistor is the Metal Oxide Varistor (MOV), which is widely used in various applications due to its excellent surge suppression capabilities and cost-effectiveness.

How do Metal Oxide Varistors work?

Metal Oxide Varistors are composed of a ceramic material, typically zinc oxide (ZnO), with small amounts of other metal oxides such as bismuth, cobalt, or manganese. These metal oxides are mixed together, pressed into a disc shape, and sintered at high temperatures. The resulting material has a polycrystalline structure with numerous grain boundaries, which are responsible for the varistor’s unique electrical properties.

Under normal operating conditions, the varistor acts as a high-resistance device, allowing only a small leakage current to flow through it. However, when the applied voltage exceeds a certain threshold, known as the varistor voltage, the resistance of the device dramatically decreases, allowing a large current to flow through it. This action effectively diverts the surge current away from the protected circuit, preventing damage to sensitive components.

Key characteristics of Metal Oxide Varistors

1. Varistor voltage (V_1mA): The voltage at which the varistor’s resistance drops significantly and conducts a current of 1 mA.
2. Maximum continuous operating voltage (MCOV): The maximum voltage that can be applied continuously to the varistor without causing degradation or failure.
3. Energy absorption capability: The amount of energy the varistor can absorb during a surge event without being damaged.
4. Response time: The time taken for the varistor to switch from its high-resistance state to its low-resistance state when a surge occurs.

Applications of Metal Oxide Varistors

Metal Oxide Varistors find applications in a wide range of industries and devices, including:

1. Power supply protection
2. Telecommunications equipment
3. Automotive electronics
4. Industrial control systems
5. Home appliances
6. Medical devices

Power supply protection

MOVs are commonly used in power supplies to protect against voltage spikes and transients that may occur on the AC mains. They are typically connected in parallel with the input of the power supply, between the live and neutral lines, or between either line and ground. In this configuration, the varistor clamps any voltage surges to a safe level, preventing damage to the power supply and the connected equipment.

Telecommunications equipment

In telecommunications systems, MOVs are used to protect sensitive equipment from voltage surges caused by lightning strikes or other transient events. They are often used in conjunction with gas discharge tubes (GDTs) and transient voltage suppression (TVS) diodes to provide comprehensive protection against various types of surges.

Automotive electronics

Varistors are increasingly used in automotive electronics to protect against voltage transients caused by load dumps, alternator field decay, and other events. They are used to protect sensitive electronic control units (ECUs), such as engine control modules, transmission control modules, and infotainment systems.

Industrial control systems

In industrial environments, MOVs are used to protect control systems, sensors, and actuators from voltage surges caused by switching of inductive loads, such as motors and solenoids. They are also used to protect against surges caused by lightning strikes on outdoor equipment.

Home appliances

MOVs are used in various home appliances, such as televisions, washing machines, and air conditioners, to protect against voltage surges on the AC mains. They help to prevent damage to the electronic components and improve the overall reliability of the appliance.

Medical devices

In medical devices, varistors are used to protect sensitive electronic components from voltage surges and electrostatic discharge (ESD) events. They are particularly important in life-critical devices, such as pacemakers and defibrillators, where reliable operation is essential.

Selecting the right Metal Oxide Varistor

When choosing a Metal Oxide Varistor for a specific application, several factors must be considered:

1. Varistor voltage: The varistor voltage should be selected based on the maximum continuous operating voltage of the protected circuit. Typically, the varistor voltage should be about 1.2 to 1.5 times the nominal operating voltage.

2. Energy absorption capability: The varistor’s energy absorption capability should be sufficient to handle the expected surge energy in the application. This can be determined by considering factors such as the surge source, the impedance of the circuit, and the duration of the surge event.

3. Package size: MOVs are available in various package sizes, ranging from small surface-mount devices (SMDs) to large disc-type packages. The package size should be selected based on the required energy absorption capability and the available space in the circuit.

4. Response time: The response time of the varistor should be fast enough to protect the circuit from the expected surge events. Most MOVs have response times in the range of a few nanoseconds, which is sufficient for most applications.

5. Operating temperature range: The varistor should be selected to operate reliably over the expected temperature range of the application. High temperatures can cause the varistor’s characteristics to degrade over time, leading to reduced protection levels.

Varistor failure modes and protection strategies

While Metal Oxide Varistors are designed to protect against voltage surges, they can also fail under certain conditions. The two main failure modes of MOVs are:

1. Thermal runaway: If the varistor is subjected to a continuous overvoltage or a surge event that exceeds its energy absorption capability, it can enter a state of thermal runaway. In this state, the varistor’s resistance drops to a very low value, leading to high current flow and excessive heat generation. This can cause the varistor to short-circuit or even explode, potentially damaging the protected circuit.

2. Gradual degradation: Over time, the varistor’s characteristics can degrade due to repeated exposure to voltage surges or high operating temperatures. This degradation can cause the varistor voltage to decrease, leading to increased leakage current and reduced protection levels.

To mitigate these failure modes and ensure reliable protection, several strategies can be employed:

1. Proper varistor selection: As discussed earlier, selecting a varistor with the appropriate voltage rating, energy absorption capability, and operating temperature range can help to prevent failures.

2. Series impedance: Adding a small series impedance, such as a resistor or an inductor, can limit the current flow through the varistor during a surge event, reducing the risk of thermal runaway.

3. Thermal protection: In some cases, a thermal fuse or a positive temperature coefficient (PTC) device can be connected in series with the varistor. If the varistor enters thermal runaway, the increased temperature will cause the thermal fuse to open or the PTC to increase its resistance, disconnecting the varistor from the circuit.

4. Regular maintenance: In critical applications, it may be necessary to perform regular inspections and replace varistors that show signs of degradation or have been subjected to a significant number of surge events.

Comparing Metal Oxide Varistors with other surge protection devices

Metal Oxide Varistors are just one of several types of surge protection devices available. Other common devices include:

1. Gas Discharge Tubes (GDTs): GDTs are spark gap devices that can handle very high surge currents but have a relatively slow response time compared to MOVs.

2. Transient Voltage Suppression (TVS) diodes: Tvs Diodes are semiconductor devices that offer fast response times and low clamping voltages but have limited energy absorption capabilities compared to MOVs.

3. Multilayer Varistors (MLVs): MLVs are similar to MOVs but have a multilayer structure that provides better energy absorption and faster response times.

The table below compares the key characteristics of these surge protection devices:

In practice, these devices are often used in combination to provide comprehensive protection against various types of surge events.

Frequently Asked Questions (FAQ)

1. Q: Can Metal Oxide Varistors be used for DC voltage protection?
   A: Yes, MOVs can be used for DC voltage protection. However, the varistor voltage should be selected based on the maximum continuous DC voltage, and the energy absorption capability should be sufficient for the expected DC surge events.

2. Q: How do I know if a varistor has failed?
   A: A failed varistor may exhibit a short circuit or a significant increase in leakage current. In some cases, a failed varistor may show visible signs of damage, such as cracks or burn marks. Regular inspections and testing can help to identify failed varistors.

3. Q: Can I use multiple varistors in parallel for increased energy absorption?
   A: While it is possible to use multiple varistors in parallel, it is generally not recommended. Paralleling varistors can lead to current sharing issues and may cause one varistor to take more stress than the others, leading to premature failure. If higher energy absorption is required, it is better to use a single varistor with a higher energy rating.

4. Q: How do I choose the right varistor voltage for my application?
   A: The varistor voltage should be selected based on the maximum continuous operating voltage of the protected circuit. A general rule of thumb is to choose a varistor voltage that is about 1.2 to 1.5 times the nominal operating voltage. However, it is essential to consider the specific requirements of the application and consult the varistor manufacturer’s datasheets for guidance.

5. Q: Can varistors protect against electrostatic discharge (ESD) events?
   A: Yes, varistors can provide protection against ESD events. However, for the best protection, it is recommended to use varistors in combination with other ESD protection devices, such as TVS diodes or multilayer varistors (MLVs), which have faster response times and lower clamping voltages.

Conclusion

Metal Oxide Varistors are essential components in the field of surge protection, offering a cost-effective and reliable solution for protecting electronic circuits from voltage transients. By understanding the key characteristics, applications, and selection criteria of MOVs, engineers and designers can effectively incorporate these devices into their designs, ensuring the reliability and longevity of electronic systems.

As technology continues to advance, the demand for surge protection will only increase, and Metal Oxide Varistors will continue to play a crucial role in safeguarding electronic devices from the damaging effects of voltage surges.