Metal Oxide Varistor：Overview,Principle and Applications

A varistor, also known as a VDR (voltage-dependent resistor), is a type of electronic component with VI characteristics similar to those of a diode. Its primary function is to protect devices from high transient voltages. When a high voltage generates a large current, the arrangement of an MOV (Metal Oxide Varistor) allows it to short itself, thereby protecting the current-dependent components from unexpected surges. Unlike rheostats and potentiometers, which are ohmic variable resistors, varistors are non-ohmic variable resistors. Among the various types of varistors, the metal oxide varistor is the most commonly used. This article provides an overview of MOVs (metal oxide varistors).

What is a Metal Oxide Varistor?

A metal oxide varistor is a varistor made from a combination of zinc oxide and other metal oxides such as manganese and cobalt. These materials are placed between two metal plates or electrodes to interact with each other. Metal oxide varistors are used to protect heavy devices from transient voltages.

MOVs are similar to resistors in that they consist of two leads and do not have polarity, allowing them to be connected in either direction. These components cannot resist transient voltages that exceed their rated limits. Once these components absorb transient voltage, they dissipate it as heat.

If this process continues for a short period, the device may become exhausted due to extreme heat. Varistors are connected in parallel to provide better energy-handling capacities. Metal oxide varistors can also be connected in series to achieve high voltage ratings.

Working Principle of metal oxide varistor

The term MOV or Metal Oxide Varistor refers to a variable resistor. Unlike a potentiometer, its resistance changes automatically based on the voltage. As the voltage across the varistor increases, the resistance decreases. This property is particularly useful for protecting circuits from high voltage spikes.

MOV Specifications

The specifications of MOVs include the following, which play an essential role when selecting metal oxide varistors:

Working voltage (maximum) of metal oxide varistor

• Varistor voltage
• Peak voltage and maximum clamping voltage obtained once a pulse current is applied
• Leakage current
• Capacitance
• Highest working voltage
• Highest AC voltage
• Clamping voltage
• Surge current
• Surge shift
• Response time
• Energy absorption: the maximum energy dissipated for a specific waveform without issues
• Surge shift: change within the voltage when surge current is applied

Features  of metal oxide varistor

The features of MOV include the following:

• AC voltage range: 130V to 1000V
• DC voltage range: 175V to 1200V
• Insulation resistance: 1000MΩ
• Operating temperature range: -55 to +85 °C
• Metal Oxide Varistor Circuit

A metal oxide varistor is frequently used in various circuits along with a fuse. These two components are connected in parallel to the protected circuit. The circuit diagram of the MOV is shown below. The main components used to protect the circuit are the fuse and the varistor.

Once the voltage is within the fixed range, the MOV's resistance will be extremely high. Therefore, current flows through the circuit but not through the MOV. However, when a voltage spike occurs within the main voltage, it appears directly across the varistor because it is connected in parallel with the AC mains. This large voltage decreases the resistance value in the MOV to an extremely low level, causing current to flow through the varistor and prompting the fuse to disconnect the circuit from the supply.

During voltage spikes, the high faulted voltage returns to normal values immediately. In such cases, the duration of current flow is not long enough to damage the fuse, and the circuit returns to its normal state once the voltage stabilizes. However, when a voltage spike is detected, the varistor temporarily disconnects the circuit by damaging itself due to the large current. If the circuit experiences multiple voltage spikes, the varistor in the circuit will eventually be damaged.

MOV Performance

The primary function of an MOV is to act as a surge suppressor. When the voltage across the varistor is below the clamping voltage, the varistor will not conduct.

The performance of the varistor degrades over time, even with the passage of small surges. Another factor affecting the varistor's performance is its energy rating. Connecting multiple varistors in parallel can enhance their performance.

A key feature of this type of varistor is its response time, as it can short voltage spikes in nanoseconds. However, the response time can be influenced by the mounting design technique and the inductance of the component leads.

Metal Oxide Varistor Applications

The applications of MOV include the following:

• Metal oxide varistors are used to protect against voltage spikes, over-voltage, line-to-line faults, arcing, and switching.
• These varistors can protect various devices from faults.
• They are used for single-phase line-to-line and line-to-ground protection within electrical circuits.
• These are used to protect switching devices such as transistors, thyristors, MOSFETs, etc.
• They are used in circuits to protect against voltage spikes and surges.
• In many cases, they are used in power strips, adaptors, etc.
• These varistors are used in common electronic devices like digital cameras, cell phones, MP3 players, etc.
• MOVs are used to protect industrial AC lines, power systems, data systems, etc.

Thus, this is an overview of metal oxide varistors, including their working, circuit, specifications, and applications. An MOV is a protection component that can safeguard power supply circuits from surges by changing its resistance. These circuits can be powered through AC mains. Here is a question for you: What is the clamping voltage of a metal oxide varistor?

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