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Varistor Basics– What is Varistor and How They Work

Warm hints: The word in this article is about 3660 and  reading time is about 25 minutes.   
Guidance: Varistor (VSR,) is a sort of nonlinear overvoltage security semiconductor component which is delicate to voltage. It is spoken to by the content image "RV" or "R" in the circuit, and figure 1 is its circuit realistic image. Varistors are not the same as normal varistors in that they are made by the nonlinear attributes of semiconductor materials.This paper describes the encyclopedic description of what is a varistor, including the type of varistor, its use and its role.Warm hints: The word in this article is about 3200 and  reading time is about 25 minutes.   
Guidance: Varistor (VSR,) is a sort of nonlinear overvoltage security semiconductor component which is delicate to voltage. It is spoken to by the content image "RV" or "R" in the circuit, and figure 1 is its circuit realistic image. Varistors are not the same as normal varistors in that they are made by the nonlinear attributes of semiconductor materials.

What is Varistor

Varistor is the shortened form of voltage touchy resistor, and it is a sort of nonlinear opposition component. The varistor opposition is identified with the voltage connected at the two closures. At the point when the voltage added to the varistor is inside its ostensible worth the obstruction of the resistor is in an interminable state with no present going through. At the point when the voltage at the two parts of the bargains is somewhat bigger than the ostensible voltage, the varistor gets through the conduction rapidly, and its obstruction worth declines rapidly, which makes the resistor in the conduction state. At the point when the voltage is diminished beneath the ostensible voltage, the obstruction estimation of the voltage-delicate resistor is expanded, and the voltage-touchy resistor is again in a high-opposition state. At the point when the voltage at the two parts of the bargains surpasses its most extreme farthest point voltage, it will get through the harm totally and won’t almost certainly recuperate without anyone else.

varistor image
varistor image

Varistor (VSR,) is a sort of nonlinear overvoltage security semiconductor component which is delicate to voltage. It is spoken to by the content image “RV” or “R” in the circuit, and figure 1 is its circuit realistic image. Varistors are unique in relation to conventional varistors in that they are made by the nonlinear attributes of semiconductor materials.

Figure 1 Circuit Graphic Symbol of Varistor
Figure 1 Circuit Graphic Symbol of Varistor

Varistor is a perfect defensive component with the attributes of high value, little volume, wide working voltage run, quick reaction to overvoltage beat, solid protection from drive current, low spillage current (not exactly a couple microamperes to several microamperes), little opposition temperature coefficient, etc. It is broadly utilized in family machines and other electronic items, and is regularly used to frame the overvoltage security circuit, the de-noising circuit, the counter sparkle circuit, the lightning assurance circuit, the flood voltage assimilation circuit and the defensive semiconductor segment.

Figure 2 The Shape of Varistor
Figure 2 The Shape of Varistor

Varistor Types

Varistors can be classified by structure, manufacturing process, use material and volt-ampere characteristics:

  • (1) According to the structure of varistors, varistors can be isolated into intersection varistors, single molecule varistors and slight film varistors. The intersection varistor has nonlinear qualities due to the exceptional contact between the resistor and the metal terminal, and the nonlinear of the varistor is controlled by the semiconductor properties of the resistor itself.
  • (2) Varistors can be partitioned into zinc oxide varistors, silicon carbide varistors, metal oxide varistors, germanium (silicon) varistors, barium titanate varistors, etc.
  • (3) According to its volt-ampere attributes, varistors can be isolated into even varistors (non-polar) and deviated varistors (polar).

Varistor Parameters

(1) What is Varistor voltage

Breakdown voltage or threshold voltage, refers to the voltage value at a specified current. In most cases, the voltage values measured by 1mA DC current when passing through the varistor can range from 10 to 9000V. It can be selected correctly according to the specific needs.

The varistor has an articulation point in the pre-breakdown territory of its V ≤ I trademark bend, which compares to a particular affectation point voltage and a particular expression point voltage. At the point when the connected voltage is higher than this expression point voltage, the varistor enters the “on” express (the opposition worth decreases); When the connected voltage is lower than this affectation point voltage, the varistor enters the “cut-off” express (the obstruction worth turns out to be huge). The most significant normal for varistor is that the opposition worth differs with the connected voltage. The emphasis point voltage in the V ≥ I trademark bend can best mirror this significant normal for varistor, so we can comprehend the intonation point voltage as the varistor voltage UN of the varistor (the limit voltage between the on and off states). Since varistor is a sort of earthenware component with totally uniform inside, regardless of whether it is a varistor of a similar particular, the emphasis purpose of every component is unique. So as to institutionalize the need, the International Electrotechnical Commission(IEC) misleadingly indicated two DC reference flows I0-1mA and 0.1mA for estimating the enunciation purpose of the varistor. Practically the majority of the varistor producers presently use U1mA or U0.1 mA to speak to the weight delicate voltage.

Under normal circumstances,  there should be:

V (1 mA) = 1.5Vp = 2.2 VAC

 where Vp is the peak of the circuit’s rated voltage. VAC is the effective value of the rated AC voltage.

For DC in the DC loop, there should be:

min (U1mA) ≥ (1.6-2) Udc

Udc is the DC rated operating voltage in the loop.

The selection of voltage value of ZnO varistor is very important, which is related to the protection effect and service life.

(2) Maximum continuous operating voltage(MCOV)

Because the varistor has positive and negative symmetry volt-ampere characteristics, it can be applied to both DC circuit and AC circuit. MCOV refers to the maximum DC voltage (UDC )or the maximum AC voltage effective value (URMS) that the varistor can withstand for a long time.

Varistors have a very special characteristic: long-term static power is very small, and instantaneous dynamic power is very large. Because the static power of varistor is very small, the long-term working voltage applied to both ends of varistor is absolutely smaller than its varistor voltage UN, otherwise the varistor will burn out due to overburden.

If the varistor is used in a AC circuit, the principle for determining the URMS is: the peak value (1.41URMS) of the maximum continuous alternating voltage is greater than the lower limit of tolerance (±10%) of the varistor voltage UN, and the formula is expressed as follows:

Figure 4 Relation between MCOV and Input Voltage under AC Power Supply of Varistor
Figure 4 Relation between MCOV and Input Voltage under AC Power Supply of Varistor

If the varistor is used in a DC circuit, the principle for determining the UDC is: The power consumption of the varistor under the action of UDC is roughly equal to or slightly smaller than the power consumption under the action of URMS and its power consumption under the action of URMS, and the empirical formula obtained by the invention is as follows:

Relation between MCOV and Input Voltage under DC Power Supply of Varistor
Figure 4 Relation between MCOV and Input Voltage under DC Power Supply of Varistor

(3) Maximum peal current

When the ambient temperature is 25 C, the varistor voltage does not exceed (+10%) of the specified impulse current waveform and the maximum impulse current value at the specified number of impulse currents.

In order to prolong the service life of the equipment, the amplitude of surge current absorbed by the zinc oxide varistor should be less than the maximum flow rate of the product given in the manual. However, from the point of view of protection effect, the flow required to be selected is larger.

In many cases, it is difficult to calculate the actual flow accurately. In fact, flow, also known as flow, refers to the maximum pulse (peak) current allowed to pass through varistors under specified conditions (applying standard pulse current at specified time intervals and times). Generally, overvoltage is one or a series of pulses. There are two kinds of shock waves used in the experimental rheostat, one is 8/20 u s wave, the other is 8 U S wave head, 20 u s tail pulse wave, and the other is 2 ms square wave, as shown in the following figure.

Figure 6 Impulse Current Waveform Used in Test Varistor
Figure 6 Impulse Current Waveform Used in Test Varistor

(4) Residual Voltage(UR)

Residual voltage Ur refers to the peak voltage at both ends of a specific waveform when surge current flows into varistors. Generally speaking, the peak surge current flowing into varistor is over 1 mA. The specific waveforms of general varistors and protective varistors refer to the 8/20 mus standard lightning current waveforms specified in IEC 60060 < 2 < 1973 standard, as shown in the figure.

01 represents the apparent origin, TS is called the apparent wavefront time, TR is called the apparent half peak time, IM is called the electric peak. Because it is difficult to find the origin 01 accurately on the oscilloscope, the approximate method will be used to measure the origin at the front-end time t s and half-peak time t R. The specific methods are as follows: first, the T1 value is measured on the oscilloscope, and then the TS value is measured on the oscilloscope. From the formula ts=1.25*T1 approximation, the actual starting point of Tr measurement at half peak is changed from 01 to 0. In addition, the IEC allows a small anti-polarity oscillation in the measured surge current waveform.

Figure 7 Schematic Diagram of Surge Waveform
Figure 7 Schematic Diagram of Surge Waveform

(5) Residual Voltage ratio(Kp)

When the current flowing through the varistor is a certain value, the voltage generated at both ends of the varistor is called the residual voltage of this current value. The ratio of residual voltage to nominal voltage is the ratio of residual voltage to nominal voltage: KR=UR/UN

The residual voltage ratio reaction varistor limits the energy of overvoltage, which has been widely used in the research of varistor materials. It has become the standard electrical performance parameter in lightning protection varistor, arrester valve plate and high energy varistor valve plate.

(6) Maximum limit voltage(Up)

Limiting voltage Up is a special form of residual voltage UR, and it is also a characteristic index to evaluate the ability of varistors of specific specifications to suppress transient overvoltage. Firstly, a basic equivalent assessment current IP shall be specified for the varistor of different chip diameters, and the limiting voltage Up of the varistor of each chip diameter shall correspond to the specified good assessment current(as shown in Table). Secondly, the limited voltage Up is not the residual voltage measured by IP, but the upper limit value of the residual voltage stipulated by each manufacturer. Therefore, the limited voltage Up is actually the protection voltage level of each specification that manufacturers promised to the users. In the IEC standard, the limited voltage is also called the voltage under the grade current.

(7) Leakage current

Leakage current, also known as waiting current, refers to the current that the varistor flows through the varistor at the specified temperature and maximum DC voltage.

(8) Ratio-voltage

The voltage ratio is the ratio of the voltage generated when the varistor current is 1 mA to the voltage generated when the varistor current is 0.1 mA.

(9) Ceiling capacity(Em)

The maximum energy, Em, is the maximum energy of a surge current or a pulse current that can be dissipated by the varistor. The meaning of bearing is that the varistor voltage UN after shock is less than ±10% compared with that before shock, and visual and visible mechanical damage can occur at the same time.

The energy absorbed by the varistors is generally calculated as follows: W=kIVT(J)

I: the peak flow through the varistor

V: The voltage across the varistor when the current I flows through the varistor

T: Current duration

Square waves of 2ms: K=1

8 / 20 μ s wave: K=1.4

10 / 1000 μ s: K=1.4

Em is closely related to current waveform. The energy test waveform specified by IEC is 2ms standard square wave, as shown in the figure.

Figure 9 Waveform Parameters of 2ms Standard Square Wave
Figure 9 Waveform Parameters of 2ms Standard Square Wave

TD is called effective square wave duration (also called T0.9), TT is called effective square wave total time (also written as T0.1), and I2ms is called square wave (average) current. IEC60060-2:1973 stipulates that the tolerances of TD are + 20% and-0%, TT ≤ 1.5 TD, and I’/ I and I “/ I do not exceed 10%. When the 2ms standard square wave current flows over the varistor, the residual voltage waveform of the varistor is 2ms voltage wave. Moreover, it is more regular than 2ms electric wave. After the average residual voltage U2ms of varistor in 2ms range can be measured by a method similar to I2ms measurement, the actual dissipative energy of varistor can be calculated by using the following formula:

E2ms=U2msI2ms×2×10-3(J)

For 2ms square wave, the absorption energy of varistor can reach 330 J per square centimeter. For 8 / 20 μ s wave, the current density can reach 2000 A per cubic centimeter, which indicates that its current flow ability and energy tolerance are very large. Generally speaking, the larger the chip diameter of varistor, the greater its energy tolerance and shock current resistance.

(10) Power rating(Po)

Rated power Po refers to the maximum average power that varistors can withstand and maintain thermal stability and no structural failure under the action of current pulse group. The maximum number of shocks per second N is calculated according to the lower formula:

lower formula
lower formula

(11) Temperature Coefficient(Tc)

The voltage temperature coefficient refers to the change rate of the nominal voltage of the varistor in the specified temperature range (the temperature is 20 ≤ 70 ℃). That is, when the current through the varistor is kept constant, the relative change in voltage across the varistor is changed by 1 °C as the temperature changes.

Figure 10 Voltage and Temperature Coefficient of Varistor
Figure 10 Voltage and Temperature Coefficient of Varistor

Tupper is the upper class temperature of the varistor (°C), the maximum allowable operating temperature. The definition formula of voltage temperature coefficient Tc actually only indicates the average voltage temperature coefficient in the range from room temperature to its upper limit category temperature, which is generally greater than-0.05% ℃. Strictly speaking, the voltage temperature coefficient is not a constant, and the TC value is different at different temperatures, but it is usually not necessary to give the relationship curve between Tc and temperature.

(12) Current temperature coefficient

The current temperature coefficient refers to the relative change in current across the varistor is changed by 1 °C as the temperature changes when the voltage through the varistor is kept constant.

(13) Insulation resistance

Insulation resistance refers to the resistance value between the lead line (pin) of the varistor and the insulation surface of the resistor.

(14) Voltage non-linear coefficient(α)

Nonlinear index α is a sign of whether the resistance value of a element varies with voltage or electric flux and whether the change is sensitive or not. The general resistor (linear resistor) is a voltage-sensitive resistor with a value of 1. The geometric meaning is the reciprocal of the slope of the V ≥ I characteristic curve drawn by the double logarithmic coordinate method.

IEC provides that:

IEC provide
IEC provide

(15) Static capacitance

Static capacitance refers to the inherent capacitance capacity of varistor itself.

(16) Response time(T)

In IEEE C62.33 ≤ 1982 standard, the response time tau of varistor is defined as shown in the figure. The voltage Vc in the figure refers to the residual voltage of 8 / 20 μ s standard lightning current wave by varistor. When the peak value of surge current is equal, but at the front time TS is shorter than 8 μ s, the residual voltage V1 is higher than that of Vc, (V1-Vc) called voltage overshoot VOS. The time widthτ (t2-t1) between the peak time T1 of V1 and the overshoot time T2 of 50%VOS is called the “response time” of varistor, and its measured value is generally within 25ns.

(17) Pulse current stability(10,000 impact life)

The 8 / 20 μ s standard lightning current wave of Ia applies the peak value to the varistor, which strikes 104 times in one direction and has an interval of 10 s. The specified value of Ia is shown in Table, followed by recovery at room temperature for 1-2 hours. After recovery, the varistor shall meet the following requirements:

Visual inspection: No visible damage, and the sign is clear.

Varistor voltage (voltage under specified current): The change rate shall no more than±10%.

Basic performance of varistor

  • (1) Protection characteristics: When the impulse strength (or impulse current isp=usp/zs) of the impulse source does not exceed the specified value, the limiting voltage of the rheostat shall not exceed the impulse withstand voltage (urp). Protected objects are affordable.
  • (2) Shock resistance: The rheostat itself should be able to withstand the specified impulse current, impulse energy and the average power of multiple impulses.
  • (3) Life characteristics: First of all, continuous working voltage life, that is, rheostat should be able to work reliably under specified ambient temperature and system voltage conditions (hours). The second is impact life, i.e. the number of times that a specified impact can be reliably withstood.
  • (4) Secondary effect: When voltage sensitive resistance is added to the system, it not only has the protective effect of “safety valve”, but also brings some additional effects, namely “secondary effect”, which should not reduce the normal working performance of the system. There are three factors to be considered, one is the capacitance of the rheostat itself (tens of thousands to tens of thousands of pF), the other is the leakage current under the system voltage, and the non-linear current of the voltage sensitive resistor is affected by the source impedance coupling. CE is connected to other circuits.

Varistor Functions

The main function of the rheostat is to protect the transient voltage in the circuit. Because of the above working principle, the rheostat is equivalent to a switch. Only when the voltage is higher than the threshold value, the resistance value is very small. When the switch is closed, the current flowing through it will surge, and the effect on other circuits will not be affected. Therefore, the influence of Overvoltage on subsequent sensitive circuits is reduced. The protective function of this rheostat can be used repeatedly, or it can be used as a one-time protective device similar to current fuse.

Rheostat is mainly used for transient overvoltage protection in circuit, but because its volt-ampere characteristic is similar to that of semiconductor voltage regulator, rheostat also has many functions of circuit elements. For example, voltage sensitive resistor is a DC high voltage and small current regulator. Varistor can be used as voltage fluctuation detection element, DC level drift element, fluorescent start element, voltage equalizing element, etc. Varistors are widely used in household appliances and other electronic products, such as overvoltage protection, lightning protection, surge current suppression, peak pulse absorption, current limiting, high voltage arc extinguishing, noise elimination, semiconductor device protection, etc.

Figure 13  Typical Application Circuit of Varistor
Figure 13  Typical Application Circuit of Varistor

The protection function of rheostat has been widely used. For example, the power supply circuit of home color TV is to use rheostat to complete the over-voltage protection function. When the voltage exceeds the threshold value, the varistor reflects its clamping characteristics, reduces the overvoltage, and makes the back circuit work within the safe voltage range.

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