Varistor-Voltage Dependent Resistor

Varistor-Voltage Dependent Resistor
Varistor-Voltage Dependent Resistor

Electronic components called Varistor Voltage Dependent Resistor safeguard electrical circuits from overvoltage. High resistance at normal working voltages; low resistance when voltage crosses a threshold. This allows varistors to control voltage spikes and transient surges that could damage sensitive electronics. Varistors protect equipment from voltage disturbances and improve electrical system reliability by shunting excess voltage away from the protected circuit.


Varistor, commonly known as a voltage dependent resistor (VDR), is an electrical component that varies resistance with applied voltage. Low voltage causes a varistor’s resistance to rise, stopping current flow. When the voltage exceeds its threshold, the varistor’s resistance rapidly drops, enabling current to flow. This protects the circuit by absorbing voltage surges from the varistor. Semiconducting materials allow varitors to execute their voltage-dependent resistance function.

Varistor-Voltage Dependent Resistor
Varistor-Voltage-Dependent Resistor


Varistor protects electrical circuits against transitory voltage. Lightning, switching activities, and electromagnetic interference can cause sudden voltage spikes. A Varistors are meant to clamp high voltages, protecting sensitive components. Varistors protect electronic systems by absorbing and dissipating voltage spike energy. Varistors also respond swiftly to voltage variations, protecting against voltage anomalies.


Varistors are used in many sectors and electronics to prevent voltage variations. Power distribution, telecommunications, consumer electronics, industrial automation, and automotive systems use them extensively. Varistors protect transformers, circuit breakers, and other power distribution equipment against lightning and fault surges. A Varistors safeguard modems and communication lines against voltage transients in telecom networks. Varistors protect computers, TVs, and audio equipment against power spikes. Varistors safeguard ECUs and other components against alternator load dumps and electrical system problems in the automotive sector.

Varistor, what? Definition of Voltage-Dependent Resistor

A varistor, commonly known as a voltage-dependent resistor, is an electronic component whose resistance changes with terminal voltage. Its resistance decreases with voltage. Varistors usually protect sensitive circuits from power spikes and overvoltage.

Varistor resistance is high at low voltages, limiting current flow. However, its resistance reduces fast as voltage increases, diverting surplus current from the circuit. The varistor’s nonlinear resistance fluctuates with voltage, hence its name.

Most varistor types are metal oxide varistor (MOV). Its zinc oxide granules are sintered to generate electrical barriers. These barriers stop current at low voltages. But at high voltage, the walls break down and enable more conduction. MOVs can deflect thousands of amps for milliseconds.

Different varistors include:

SiC varistors work at higher temperatures than MOVs. Frequently used in automobile.

Polymer-based varistors are flexible, lightweight, and cheap but have limited voltage and energy handling. For ESD protection.

Titanium dioxide-bismuth oxide ceramic varistors. Max operating temperature is high yet costly. Used in power electronics.

Varistors prevent transitory overvoltage situations and are cheap. Under sustained high-voltage circumstances, overheating and thermal runaway can cause catastrophic failure. Their limited operating temperature range must be considered for an application. Varistors protect delicate electronics when operated within their ratings.

A Varistor Works How? Understanding Nonlinearity

How can varistors dynamically modify their resistance based on voltage? Their nonlinearity and voltage-dependent resistance are the key.

Varistors are ceramic or polymer materials that conduct at high voltages but are insulators at low voltages. As voltage rises, resistance falls. Varistors have unique properties due to their nonlinear current-voltage relationship.

Voltage-dependent resistance

Varistor resistance is strong and functions as an insulator below its threshold. After reaching the threshold, its resistance reduces considerably, enabling current to flow freely. This occurs when the substance electrically breaks down, creating electron channels.

Higher voltage creates more pathways and lowers resistance. The varistor’s characteristic curve demonstrates its nonlinear resistance at different voltage levels. As resistance reduces at higher voltages, the curve flattens from steep.

Nonlinear Features

Varistors can clamp transient voltages extremely well while allowing standard voltage signals through. They instantly change resistance to voltage spikes and revert to high resistance. Varistors are important for overvoltage protection and surge suppression in electronics and power systems due to their nonlinear behavior.

A Varistors protect circuits from voltage surges and spikes by being nonlinear and voltage-dependent. Varistors are suitable for many overvoltage protection applications because they can dynamically change resistance.

Varistors: MOVs to Polymer-Based

A Varistors have numerous types for different uses. For particular applications, silicon carbide, polymer, and ceramic varistors are available. Metal oxide varistors (MOVs) are the most prevalent.

MOVs is metal oxide varitors

Zinc oxide MOVs are the most common varistor. Simple construction and low cost make them excellent for high-volume commercial applications like surge prevention in power strips or AC line filters. MOVs have high surge current and fast response. Their performance declines with time and with significant energy surges. MOVs are cheap and effective for numerous applications.

SiC Varistors

Utility and industrial power applications use silicon carbide varistors because they resist degradation and can withstand greater voltages. SiC varistors work slower and cost more. They excel in high-power, high-voltage equipment that needs performance.

Variants Made of Polymers

Conductive polymer varistors are flexible, low-cost, and easy to produce. Most polymer varistors offer lesser energy handling, shorter response times, and more parameter variation than ceramic or silicon carbide ones. ESD protection and other low-power purposes are possible with polymer varistors, but high-energy surge suppression is not.

Ceramic Varistors

Doped ceramic varistors give non-linear resistance. At reasonable cost, they are more stable and responsive than polymer varistors. Some ESD and low-to-medium power surge prevention uses ceramic varistors. Their energy capacity is usually lower than MOVs or silicon carbide varistors.

You have many varistors with different performance, cost, and application trade-offs to choose from. Choose the proper varistor for optimal protection and reliability by learning the benefits and downsides of each type.

Varistor Applications: Overvoltage Protection

Varitors safeguard delicate electrical circuits against voltage spikes and surges. Their voltage-dependent resistance diverts excess voltage to ground, shielding components from harm.

Safeguarding Circuits

Integrators, switches, relays, and other circuit elements can fail due to high voltage transients. Varistors safeguard these. They mitigate harm from:

Utility company power surges

Lightning strikes nearby.

Motor, transformer inductive load switching

Discharge (static shock)

When voltage exceeds a threshold, a varistor immediately reduces its resistance, sending additional voltage to ground. This limits voltage to safeguard the circuit.

Selection of Varistor

Varistor selection depends on several factors:

  • The varistor’s threshold voltage should be slightly higher than the circuit’s operational voltage. This limits its conductance to overvoltage situations.
  • Energy rating—How much energy the varistor can absorb at once. Energy-rich varistors can endure stronger voltage spikes.
  • High-frequency transient protection requires millisecond response times. Utility power applications may use slower Varistor Voltage Dependent Resistor.
  • Operating temperature—The Varistor Voltage Dependent Resistor must be rated for all installation temperatures. High-temperature varistors work in harsh settings.
  • Choose a varistor with the right ratings for your application and installation conditions to protect sensitive devices from overvoltage. Varistors protect all circuits and components cheaply but effectively.

Limitations and Failure Modes in Varistor Design

You must consider varistor limits and failure modes while building circuits with them. Varistors have flaws that affect performance and longevity.

Varistors can only tolerate so much voltage before breaking down, so choose one with a voltage rating higher than your system’s maximum transient voltage. When a transient spike exceeds the varistor’s voltage rating, it shorts. It can harm linked components and disable your system. Choose a varistor 50-100% greater than the maximum transient to avoid overvoltage failure.

Varistors can only absorb so much energy before overheating. Varistor energy ratings in joules reflect how much energy they can handle per pulse. More energy than rated can cause the varistor to overheat and fail. Choose a varistor with a much greater energy rating than the highest estimated transient energy to avoid overheating.

Varistor performance and lifetime depend on temperature. Temperature accelerates varistor aging and reduces voltage and energy handling. Every 10°C temperature increase can reduce a variable’s energy rating by 50%. Place varistors away from heat sources and in cool, airy spaces. Thermal management like heat sinks helps high-energy applications.

Watch for other failure modes:

Exposure to transients causes gradual degradation. Varistor Voltage Dependent Resistor should be replaced every 5-10 years for best protection.

Moisture and chemicals can degrade Varistor Voltage Dependent Resistor materials and connections, increasing resistance. Varistors should match the environment.

Even within its ratings, repeated high-energy transients can exhaust a Varistor Voltage Dependent Resistor. Derating varistors extends their longevity in essential systems.

Understanding varistor limitations and failures helps you design your circuit to maximize voltage-dependent resistor protection and service life. Select the proper varistor, operate it within safe ratings, and inspect it often to detect degradation before system harm. With adequate protection and maintenance, varistors can shield your equipment from transient voltages.

Be the first to comment

Leave a Reply

Your email address will not be published.