What is a Varistor?

The varistor is a passive two-terminal solid state semiconductor device used to protect electrical and electronic circuits.

Unlike the fuse or circuit breaker, which provides overcurrent protection, the varistor provides overvoltage protection through voltage tightening, as in zener diode.

Varistor is derived from the words VARI-able resi-STOR. Due to its name, it is necessary not to confuse it with reosta and posiometer because its value is not modifiable like them.

Varistor

However, unlike a variable resistance, whose resistance value can be changed manually between its minimum and maximum values, the varistor automatically changes the resistance value with the change in voltage, making it a nonlinear resistance or VDR for short, depending on the voltage.

Today, the resistant body of a varistor is made of semiconductor material that makes it a kind of semiconductor resistance with non-omic symmetrical voltage and current properties suitable for both AC and DC voltage applications.

In many ways, the varistor looks similar in size and design to a capacitor and is often mixed as a single.However, a capacitor cannot suppress voltage fluctuations in a way that a varistor can.When high voltage fluctuation is applied to a circuit, the result is usually catastrophic for the circuit, so the varistor plays an important role in protecting sensitive electronic circuits from sudden switching and overvoltage transitions.

Transient fluctuations are caused by various electrical circuits and sources, regardless of whether they are operating from an AC or DC source, usually because they are produced in-house of the circuit itself or transmitted to the circuit from external sources.Transient events within a circuit can rise rapidly by increasing the voltage to several thousand volts, and varistors are used in such cases to prevent supply between sensitive electronic circuits and components.

One of the most common sources of transient voltages is the L(di/dt) effect caused by switching inductive coils and transformer magnetizing currents, DC motor switching applications and the opening of fluorescent lighting circuits or other supply fluctuations.

AC Waveform Transient Events

Varistors are connected to circuits via a mains supply in a phase-neutral, phase-phase or DC operation in a positive-negative manner for AC operation and have a voltage rating suitable for their application.A varistor can also be used for DC voltage stabilization and electronic circuit protection, especially against excessive voltage pulses.

Varistor Static Resistance

Under normal operation, the varistor has a very high resistance, so it works similarly to the zener diode, allowing lower threshold voltages to pass unaffected as part of its name.

However, when the voltage on the varistor (both polarities) exceeds the rated value of the varistors, its effective resistance is strongly reduced by increased voltage, as shown.

From the Ohm Act, we know that the current-voltage (IV) properties of a constant resistance are a straight line, provided that the R is kept constant.

But the IV curves of a varistor are not a straight line, since a small voltage change causes a significant current change.Below is a typical normalized voltage-current characteristic curve for a standard varistor.

Varistor Characteristic Curve

Varistor Characteristic Curve

From above, we can see that the variator has symmetrical bidirectional properties, that is, the varistor works in both directions (section Ι and ΙΙΙ) of a sinusoidal waveform that behaves similarly to two consecutively connected zener diodes.When there is no conductor, the IV curve indicates a linear relationship, since the current flowing from the varistor remains constant and low in only a few micro-amps "leakage" current.This is due to its high resistance, which acts as an open circuit, and the voltage on the varistor (both polarities) remains constant until it reaches a certain "rated voltage".

This nominal or clamping voltage is the voltage on the varistor measured by the specified 1mA DC current.That is, the DC voltage level applied throughout its terminals, which allows a stream of 1mA to flow through the resistant body of varistors connected to the materials used in its construction.At this voltage level, the varistor begins to move from insulating state to conductor state.

When the temporary voltage on the varistor is equal to or greater than the nominal value, the resistance of the device suddenly becomes very small, turning the varistor into a conductor due to the avalanche effect of the semiconductor material.The small leakage current flowing from the varistor rises rapidly, but the voltage on it is limited to a level just above the varistor voltage.

In other words, the varistor self-regulates the temporary voltage by allowing more currents to pass through it, and due to the nonlinear upright IV curve, it can pass widely varying currents in a narrow voltage range, trimming any voltage increase.

Varistor Capacity Values

Since the main conductive zone between the two terminals of a varistor behaves like a dielectric, under the compression voltage the varistor acts more like a capacitor than resistance.Each semiconductor variator has a capacitance value that is directly connected to its area and varies inversely proportional to its thickness.

When used in DC circuits, the capacitance of the varistor remains more or less constant, provided that the applied voltage does not rise above the clamping voltage level and suddenly drops towards the maximum nominal continuous DC voltage.

However, in AC circuits, these capacitances can affect the body resistance of the device in the non-conductive leakage zone of IV properties.Since they are normally connected in parallel with an electrical device to protect against excessive voltages, the leakage resistance of varistors decreases rapidly with the increase in frequency.

This relationship is approximately linear with frequency and resulting parallel resistance, AC reactance can be calculated using 1/(2πεC) for a normal capacitor of Xc.In addition, as the frequency increases, so do the leakage current.

But in addition to the silicon semiconductor-based varistor, metal oxide varistors have been developed to overcome some limitations associated with silicon carbide species.

Metal Oxide Varistor

Metal Oxide Varistor, or MOV, is a resistance to voltage settings, made using zinc oxide (ZnO).Metal oxide varistors consist of other filler materials for the formation of connections between approximately 90% zinc oxide and zinc oxide grains as ceramic basic material.

Metal oxide varistors are now the most common type of voltage clamping device and are available for use in a wide range of voltages and currents.The use of metallic oxide in their structure means that MOV is extremely effective at absorbing short-term voltage transitions and has higher energy processing capabilities.

As with the normal varistor, the metal oxide variator begins transmission at a certain voltage and stops transmission when the voltage falls below a threshold voltage.The main difference between a standard silicon carbide (SiC) variator and an MOV-type varistor is that the leakage current passing through the zinc oxide material of the MOV is very small current under normal operating conditions and the operating speed in clamping transitions is much faster.

MOV's usually have non-hard blue or black epoxy coating, which is very similar to radial inserts and disc ceramic capacitors and can be physically mounted similar to circuit boards and PCBs.The structure of a typical metal oxide varistor is given as follows:

Metal Oxide Varistor Structure

To choose the right MOV for a particular application, it is necessary to have some knowledge about the source impedance and the possible impact force of transient events.For incoming line or phase-induced transitions, the correct MOV is slightly more difficult to choose, as the characteristics of the power supply are generally unknown.In general, the choice of MOV for electrical protection from the power supply transient currents and spikes of circuits is usually little more than an informed estimate.

However, metal oxide varistors are available at a wide range of varistor voltages, from about 10 volts to AC or DC over 1,000 volts, so knowing the feed voltage can help with the choice.For example, choosing an MOV or silicon varistor in this regard, for voltage, the maximum continuous rms voltage rating should be just above the expected maximum feed voltage, for example, 130 volt rms for a 120 volt feed and 260 volt rms for 230 volts are required.

The maximum pulse current value that a varistor receives depends on the temporary pulse width and the number of pulse repetitions.Assumptions can be made on the width of a transient pulse, which is typically between 20 and 50 microseconds (μs) long.If the peak pulse current rating is insufficient, the varistor may overheat and be damaged.Therefore, in order for a varisttor to operate without any malfunction or deterioration, it is necessary to be able to quickly dispense the absorbed energy of the temporary pulse and safely return to its pre-impact state.

Varistor Applications

Varistors have many advantages and can be used in many different applications for suppressing grid-sourced transitions on both AC and DC power lines, from household appliances and lighting to industrial equipment.Varistors can be connected directly through network sources and semiconductor switches for the protection of transistors, MOSFETs and thyristor bridges.

Varistor Applications

Summarize

In this tutorial, we found that the main function of Voltage-Dependent Resistance/varistor or VDR is to protect electronic devices and electrical circuits from voltage fluctuations and rises, such as those created by inductive switching transitions.

Such varistors are used in sensitive electronic circuits to ensure that the varistur becomes an effective short circuit to protect the circuit from overvoltage, since they can withstand hundreds of peak currents, if the voltage suddenly exceeds a predetermined value.

Varistors are a type of resistance characterized by nonlinear, non-omic current voltage and are a reliable and economical way to protect against overvoltage transient currents and fluctuations.

They do this by acting as a high-resistance blocking device at lower voltages and as a good low-resistance conductive device at higher voltages.The effectiveness of a varistor in maintaining an electrical or electronic circuit depends on the correct selection of the varistor in terms of voltage, current and energy distribution.

Metal Oxide Varistors or MOV's are typically made of small disc-shaped metal zinc oxide material.They are available in many values for certain voltage ranges.The voltage rating of an MOV, called "varistor voltage", is the voltage on a varistor when a current of 1mA passes through the device.This varistor voltage level is essentially the point on the IV characteristic curve when the device starts transmission.Metal oxide varistors can also be connected in series to increase the degree of compression voltage.