VARISTOR – THE SILENT CIRCUIT PROTECTOR

Varistor (short for variable resistor, also called a voltage‑dependent resistor or VDR) is a two‑terminal, nonlinear electronic component whose resistance drops sharply when the voltage across it exceeds a threshold, making it ideal for clamping voltage transients and protecting circuits. Its main function is to protect electronic components from surges.

The most common type is the metal‑oxide varistor (MOV), made from sintered metal oxide grains with thin insulating boundaries; under high field these boundaries break down locally, creating many parallel micro‑conduction paths. MOS varistors have revolutionized arrester technology over the last 50 years. Panasonic introduced ZnO varistor (ZNR– Zinc Oxide Non-linear Resistor) in 1968, followed by commercial introduction in 1972 by General Electric in 1972

Varistor has a non‑ohmic, symmetric I–V characteristics. At normal operating voltages it has very high resistance and draws negligible current. When a surge appears, its resistance drops down by several orders of magnitude, shunting the excess energy and clamping the voltage to a relatively fixed level.  

Unlike a normal fixed resistor that obeys Ohm’s Law, a VDR has high resistance at normal operating voltages, which drops exponentially when the voltage exceeds a threshold. This shunts destructive surge energy away from sensitive circuit semiconductors and power supplies.

Behavior of varistors is often modeled by a power law:

I = k V^α

where α (nonlinearity coefficient) is large for good varistors, giving a very steep transition from “off” to “on”.

Construction of Varistor (MOV)

Metal Oxide Varistors are made from ceramic powders of oxides of zinc, cobalt, manganese, bismuth, etc. The ceramic is made into a disc, with metal plates on both sides, and encapsulated to get sealed unit. A zinc oxide varistor consists of approximately 90% of zinc oxide and small quantities of other metal oxides. Ceramic powders of metal oxides are placed between two metal plates (electrodes).

Grain boundaries in zinc oxide allow many charge carriers, enabling the device to conduct when subjected to a high voltage. The MOV/ VDR starts conducting when voltage crosses a certain limit, and stops conducting when it falls below threshold voltage.

Main parameters for selection of VDR

• Clamping voltage / varistor voltage (V_VDR): Voltage above normal peak operating voltage, but below damage threshold of device under protection, while carrying a specified test current (e.g. 2 mA).
• Maximum Continuous Voltage (AC/DC): Highest continuous DC/AC voltage the VDR can withstand without excessive leakage or ageing effect.
• Energy Rating (J) and Peak Surge Current (I_peak): Usually mentioned for 8/20 μs or 10/100 μs waveform, the ability to survive single or repeated transients (like lightning, switching spikes etc.)
• Capacitance: MOVs have tens to thousands of pF between terminals-significant for high frequency signal lines, and can distort load RF circuits.
• Leakage Current and Ageing : Near threshold events can degrade MOVs, increasing leakage currents and causing early ageing. A limit for the leakage currents is mentioned, below which there it bis safe without causing thermal runaway, short, or degradation.

Varistor Types

While the radial leaded Metal Oxide Varistor (MOV) is the mainstay of AC power supplies, there are other technologies available for VDR. Selecting the wrong type for a specific job is a common reason for premature field failures. Technologies available and their uses are given below.

Thermally protected varistors (TMOV) integrate MOV with thermally activated elements to provide overvoltage protection. They break the circuit in response to heating from overvoltage events.

Failure modes of Varistors

• When subjected to extreme surges (e.g. direct lightning), VDR can fail by overheating, charring, or may even catch fire if not provided by thermal protection, fuses ot thermal cutoff.
• To get long life, V_VDR, energy level, peak current ratings should be judiciously chosen keeping expected transient levels in view. VDR may also be provided with thermal protection or fuses etc.

Note: Varistors are often placed directly across AC mains lines (120V/230V AC) or high-voltage DC bus capacitors. For safety purpose, the circuit should be de-energized, and any parallel capacitors should be discharged]

Varistor Markings and Specifications

Varistors are marked with standardized symbols and alphanumeric codes stamped on the housing.

• Colour: Most varistors are blue in colour, though other colours are available
• Shape: D for disc type, S for square shape., These are most common, though others are available- like R for radial, B for bar, T for tube. Larger diameters usually means higher thermal mass and greater energy absorption (Joules).
• Size: This affects the ability of varistor to absorb or drain away sudden voltage surge. The bigger the size, more is the capacity. VDR size also affects voltage tolerances and clamping voltage. Generally, size is measured in mm, standard sizes being 05, 07, 10, 14, 18, 20, or larger.
• Voltage tolerances: This is another critical value. It creates a boundary of the voltage. For example, if varistor has rated clamping voltage of 275, with tolerance of 10%. The clamping voltage may be between 247.5 V and 302V. The tolerance is mentioned by a letter – J for 5%, K for 10%, L for 15% and M for 20%. K is most common.
• Clamping voltage: The highest voltage limit reached before varistor starts to conduct and protect a device / equipment.

Applications of Varistors

In many designs, varistors are combined with gas discharge tubes (GDTs) and/or TVS diodes to handle different parts of surge spectrum (very high energy vs. very fast, low‑energy events). Nonlinear characteristic of VDR make them ideal surge protector devices. High voltage transients can be due to inductive discharge from motors or transformers lightning strikes, electrostatic discharge (ESD) etc.

Varistors are common in surge protector strips in circuits, and special types with low capacitance are used in communication lines. VDRs are useful for a wide range of applications. Few typical applications are given below.