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Jul 19 2019

What are Varistor and Metal-Oxide Varistor? Application and Selection of Varistor

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Varistor

1.Concept of Varistor

2.Basic performance of varistor

3.Types of varistor

4.Metal Oxide Varistor

5.Symbol of varistor

6.Main parameters of varistor

7.Varistor selection

8.Main applications of varistor

 

1. Concept of Varistor

 

The varistor is a voltage limiting type protection device. Using the non-linear characteristics of the varistor, when an overvoltage occurs between the two poles of the varistor, the varistor can clamp the voltage to a relatively fixed voltage value, thereby protecting the latter circuit. The main parameters of the varistor are: varistor voltage, current capacity, junction capacitance, response time and so on.

 

The response time of the varistor is ns, which is faster than the gas discharge tube and slightly slower than the TVS tube. Under normal circumstances, the response speed of the overvoltage protection for the electronic circuit can meet the requirements. The junction capacitance of a varistor is generally in the order of several hundred to several thousand Pf. In many cases, it should not be directly applied to the protection of high-frequency signal lines. When it is used in the protection of an AC circuit, it will increase leakage due to its large junction capacitance. The current needs to be fully considered when designing the protection circuit. The varistor has a large flow capacity but is smaller than a gas discharge tube. The varistor, abbreviated as VDR, is a voltage-sensitive nonlinear overvoltage protection semiconductor component.

 

2. Basic performance of varistor

 

(1) Protection characteristics. When the impact strength of the impact source (or the inrush current Isp=Usp/Zs) does not exceed the specified value, the limiting voltage of the varistor must not exceed the impulse withstand voltage (Urp) that the protected object can withstand. .

 

(2) Impact resistance, that is, the varistor itself should be able to withstand the specified inrush current, impact energy, and the average power of multiple impacts.

 

(3) There are two life characteristics, one is the continuous working voltage life, that is, the varistor should be able to work reliably for a specified time (hours) under the specified ambient temperature and system voltage conditions. The second is the impact life, that is, the number of times that the specified impact can be reliably withstood.

 

(4) After the varistor is involved in the system, in addition to the protection function of “safety valve”, it will bring some additional effects. This is called “secondary effect” and it should not reduce the normal working performance of the system. There are three main factors to consider at this time. One is the capacitance of the varistor itself (tens to tens of thousands of PF). The second is the leakage current at the system voltage, and the third is the effect of coupling on other circuits when the nonlinear current of the varistor through the source impedance.

 

3. Types of varistor

 

Varistors can be categorized by layout, manufacturing history, applied materials, and volt-ampere characteristics.

 

(1) According to the layout, the varistor can be classified into a junction varistor, a bulk varistor, a single-particle varistor, and a film varistor. The junction type varistor has the rare contact between the resistor body and the metal electrode, and the nonlinearity of the body type varistor is determined by the semiconductor nature of the resistor body.

 

(2) According to the application materials, varistor can be divided into zinc oxide varistor, silicon carbide varistor, metal oxide varistor, germanium (silicon) varistor, ferric acid, etc.

 

(3) According to their volt-ampere personality classification, varistor can be divided into symmetrical varistor (no polarity) and asymmetric varistor (with polarity) according to its volt-ampere personality.

 

4. Metal Oxide Varistor

 

The Metal Oxide Varistor or MOV for short, is a voltage dependant resistor in which the resistance material is a metallic oxide, primarily zinc oxide (ZnO) pressed into a ceramic like material. Metal oxide varistors consist of approximately 90% zinc oxide as a ceramic base material plus other filler materials for the formation of junctions between the zinc oxide grains.

 

Metal oxide varistors are now the most common type of voltage clamping device and are available for use at a wide range of voltages and currents. The use of a metallic oxide within their construction means that MOV’s are extremely effective in absorbing short term voltage transients and have higher energy handling capabilities.

 

As with the normal varistor, the metal oxide varistor starts conduction at a specific voltage and stops conduction when the voltage falls below a threshold voltage. The main differences between a standard silicon carbide (SiC) varistor and a MOV type varistor is that the leakage current through the MOV’s zinc oxide material is very small current at normal operating conditions and its speed of operation in clamping transients is much faster.

 

MOVs generally have radial leads and a hard outer blue or black epoxy coating which closely resembles disc ceramic capacitors and can be physically mounted on circuit boards and PCB’s in a similar manner. The construction of a typical metal oxide varistor is given as:

 

Metal Oxide Varistor Construction

 metal oxide varistor construction

metal oxide varistor construction


To select the correct MOV for a particular application, it is desirable to have some knowledge of the source impedance and the possible pulse power of the transients. For incoming line or phase borne transients, the selection of the correct MOV is a little more difficult as generally the characteristics of the power supply are unknown. In general, MOV selection for the electrical protection of circuits from power supply transients and spikes is often little more than an educated guess.

 

However, metal oxide varistors are available in a wide range of varistor voltages, from about 10 volts to over 1,000 volts AC or DC, so selection can be helped by knowing the supply voltage. For example, selecting a MOV or silicon varistor for that matter, for voltage, its maximum continuous rms voltage rating should be just above the highest expected supply voltage, say 130 volts rms for a 120 volt supply, and 260 volts rms for a 230 volt supply.

 

The maximum surge current value that a varistor will take depends on the transient pulse width and the number of pulse repetitions. Assumptions can be made upon the the width of a transient pulse which are typically 20 to 50 microseconds (μs) long. If the peak pulse current rating is insufficient, then the varistor may overheat and become damaged. So for a varistor to operate without any failure or degradation, it must be able to quickly dissipate the absorbed energy of the transient pulse and return safely to its pre-pulse condition.

 

5. Symbol of varistor

 

"Varistor" is a resistive device with nonlinear volt-ampere characteristics. It is mainly used to clamp the voltage when the circuit is under overvoltage, and absorb excess current to protect sensitive devices. The full name is “Voltage Dependent Resistor” and is abbreviated as “VDR” or “Varistor”.

 

The sign of the varistor is as follows: (We have listed several common representations)

 symbol 1

 symbol 2

 symbol 3

 symbol 4

symbols and graphics of varistor

 

6. Main parameters of varistor

 

The main parameters of the varistor are nominal voltage, voltage ratio, maximum control voltage, residual voltage ratio, flow capacity, leakage current, voltage temperature coefficient, current temperature coefficient, voltage nonlinear coefficient, insulation resistance, static capacitance and so on.

 

(1) Varistor voltage: The so-called varistor voltage, is breakdown voltage or threshold voltage. It refers to the voltage value at the specified current which is measured when a 1 mA DC current is applied to the varistor in most cases, and the varistor voltage range of the product can range from 10-9000V. Generally, V1mA=1.5Vp=2.2VAC, where Vp is the peak value of the rated voltage of the circuit. VAC is the rms value of the rated AC voltage. The choice of the voltage value of the ZnO varistor is crucial, and it is related to the protection effect and service life.

 

(2) Maximum allowable voltage (maximum limit voltage): This voltage is divided into AC and DC. If it is AC, it refers to the effective value of the AC voltage allowed by the varistor which is expressed in ACrms. The varistor with the maximum allowable voltage should be selected under the effective value. In fact, V1mA and ACrms are related to each other. Knowing the former also knows the latter, but ACrms is more direct to the user, and the user can directly select the appropriate varistor according to ACrms. In the AC loop, there should be: min (U1mA) ≥ (2.2 ~ 2.5) Uac, where Uac is the effective value of the AC operating voltage in the loop. The above value principle is mainly to ensure that the varistor has an appropriate safety margin when it is applied in the power supply circuit. For DC, in the DC loop, there should be: min(U1mA) ≥(1.6~2)Udc, where Udc is the DC rated working voltage in the loop. In the AC loop, there should be: min (U1mA) ≥ (2.2 ~ 2.5) Uac, where Uac is the effective value of the AC operating voltage in the loop. The above value principle is mainly to ensure that the varistor has an appropriate safety margin when it is applied in the power supply circuit. In the signal loop, there should be: min (U1mA) ≥ (1.2 ~ 1.5) Umax, where Umax is the peak voltage of the signal loop. The flow capacity of the varistor should be determined according to the design specifications of the lightning protection circuit. In general, the varistor has a flow capacity greater than or equal to the flow capacity of the lightning protection circuit design.

 

(3) Flow capacity: The so-called flow capacity is the maximum pulse current value when the ambient temperature is 25°C, the variation of the varistor voltage does not exceed ± 10% for the specified inrush current waveform and the specified number of inrush currents. In order to extend the life of the device, the magnitude of the surge current absorbed by the ZnO varistor should be less than the maximum flux of the product given in the manual. However, starting from the protection effect, it is required that the selected flow rate is larger. In many cases, the actual flow rate that is actually generated is difficult to calculate accurately. Simply put--flow capacity, also known as flow-through, refers to the maximum pulse (peak) current value allowed across a varistor under specified conditions (at a specified time interval and number of times, applying a standard inrush current). Generally, overvoltage is one or a series of pulse waves. There are two kinds of shock waves used in the experimental varistor, one is 8/20μs wave, that is, the so-called wave head is 8μs pulse wave with a tail time of 20μs, and the other is a square wave of 2ms.

 

(4) Maximum limiting voltage: The maximum limiting voltage is the highest voltage that can be withstand across the varistor. It indicates the voltage generated at the two ends when the specified inrush current Ip passes through the varistor. This voltage is also called residual voltage. Therefore, the residual voltage of the selected varistor must be less than the withstand voltage Vo of the protected object, otherwise it will not achieve a reliable protection purpose. Usually the impact current Ip value is large, such as 2.5A or 10A, thus the corresponding maximum limit voltage Vc is quite large, for example, MYG7K471 has Vc=775 (Ip=10A).

 

(5) Maximum energy (energy tolerance): The energy absorbed by the varistor is usually calculated as follows: W=kIVT(J)

 

Where I - the peak of the varistor flowing through

 

V——voltage across the varistor when current I flows through the varistor

 

T——current duration

 

K——the waveform coefficient of current I


2ms square wave k=1

 

8/20μs wave k=1.4

 

10/1000μs k=1.4

 

The varistor can absorb energy up to 330J per square centimeter for a 2ms square wave; the current density can reach 2000A per cubic centimeter for a 8/20μs wave, which indicates that his flow capacity and energy tolerance are very large.

 

Generally speaking, the larger the chip diameter of the varistor, the greater the energy tolerance and the greater the withstand current. When using the varistor, it should also consider the overvoltage that often encounters less energy but has a higher frequency. If there is an overvoltage of one or more times in a few tens of seconds or one or two minutes, then the average power that the varistor can absorb should be considered.

 

(6) Voltage ratio: 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.

 

(7) Rated power: The maximum power that can be consumed at a specified ambient temperature.

 

(8) Maximum peak current Once: The maximum current value of the current with a standard waveform of 8/20μs. At this time, the rate of change of the varistor voltage is still within ±10%. 2 times: The maximum current value of the impact of the standard waveform of 8/20μs is twice, and the interval between the two impacts is 5 minutes. At this time, the rate of change of the varistor voltage is still within ±10%.

 

(9) Residual voltage ratio: When the current flowing through the varistor is at a certain value, the voltage generated at both ends is called the residual voltage. The ratio of the residual voltage to the nominal voltage of the residual voltage ratio.

 

(10) Leakage current: Leakage current, also known as wait current, refers to the current flowing through the varistor at a specified temperature and maximum DC voltage.

 

(11) Voltage temperature coefficient: The voltage temperature coefficient refers to the rate of change of the nominal voltage of the varistor in a specified temperature range (temperature is 20~70 °C), that is, when the current through the varistor is kept constant, the temperature changes. Relative change at both ends of the varistor at 1 °C.

 

(12) Current Temperature Coefficient: The current temperature coefficient is the relative change in current flowing through the varistor when the temperature changes by 1°C while the voltage across the varistor remains constant.

 

(13) Voltage nonlinear coefficient: The voltage nonlinear coefficient refers to the ratio of the static resistance value to the dynamic resistance value of a varistor under a given applied voltage.

 

(14) Insulation resistance: The insulation resistance is the resistance between the lead wire (pin) of the varistor and the insulating surface of the resistor.

 

(15) Static Capacitance: Static capacitance refers to the capacitance capacity inherent in the varistor itself.

 

The principle of use of the varistor is that after it is connected to the protected device, it cannot affect the normal operation of the device, and can effectively perform instantaneous overvoltage protection on the device. To this end, in addition to the technical parameters of the varistor, the following issues should be considered in the actual selection:

 

1) varistor voltage selection

 

Taking into account the deviation between the actual varistor voltage of the varistor and the nominal voltage (should be considered 1.1 to 1.2 times the nominal voltage), the possible fluctuation range of the power supply voltage in the AC circuit (it should be considered 1.4 to the rated voltage) 1.5 times), the relationship between the peak value of the AC voltage and the effective value (1.4 times should be considered), so a varistor with a varistor voltage of 2.2 to 2.5 times the rated voltage should be used. In the DC circuit, a varistor having a varistor voltage of 1.8 to 2 times the rated value of the DC voltage is often selected.

 

2) Flow capacity selection

 

In principle, it should be chosen according to the maximum transient inrush current that may be suffered, but it is difficult to do this. In practice, the varistor is selected according to the application or according to the test level specified in the product test standard.

 

According to the former, 1kA (8/20μs current wave) varistor can be used for the protection of thyristor; 3kA for surge absorption of electrical equipment; 5kA for overvoltage absorption of lightning and electronic equipment Upper; 10kA is used for the protection of lightning strikes. According to the latter, the common integrated wave (a voltage wave of 1.2/50μs is generated when the generator is open-circuited; a current wave of 8/20μs is generated when the short-circuit is output; the internal resistance of the generator is 2Ω) to evaluate the device against lightning surge interference. ability. In the 4kV test, the maximum current absorbed by the protector can reach 2kA; for the 6kV test, the maximum value of the absorbed current is 3kA. However, in actual selection, the flow capacity of the selected varistor should also be appropriately increased. Because of the large varistor with high current capacity, there should be a relatively small residual voltage drop when absorbing the same magnitude of surge current. At the same time, there is a large protection margin for the selected varistor.

 

3) Intrinsic parasitic capacitance

 

Varistors have an inherent capacitance problem that ranges from a few hundred to several thousand pF depending on the form factor and nominal voltage. The inherent capacitance of the varistor determines that it is not suitable for use in high frequency applications, otherwise it will affect the normal operation of the system; it is suitable for use in power frequency systems, such as protection for power supply lines, protection of thyristor rectifiers, etc.

 

The instantaneous power of the varistor is relatively large, but the average continuous power is small, so it cannot be operated for a long time.

 

The role of zinc oxide varistor (Carbon resistor): also known as the "surge absorber is a voltage attribute resistor with voltage and current symmetry characteristics, its main design is to protect all electronic products. Or the component is protected from the surge caused by switching or lightning strikes, and the characteristics of the nonlinear index.

 

Features: fast reaction time; low leakage current; superior voltage ratio; wide voltage to energy ratio; low standby power and no subsequent current; high-performance surge current processing capability; stable performance of suppression voltage characteristics.


7. Varistor selection

 

When selecting a varistor, the specific conditions of the circuit must be considered. Generally, the following principles should be followed:

 

(1) Selection of varistor voltage V1mA

 

According to the power supply voltage, the power supply voltage continuously applied across the varistor cannot exceed the “maximum continuous operating voltage” value listed in the specification. That is, the maximum DC working voltage of the varistor must be greater than the DC working voltage VIN of the power line (signal line), that is, VDC ≥ VIN; for the pressure sensitive selection of the 220V AC power supply, the fluctuation range of the working voltage of the power grid should be fully considered. When the varistor voltage value of the varistor is sufficient, leave a sufficient margin. The domestic power grid generally has a fluctuation range of 25%. A varistor with a varistor voltage of 470V to 620V should be selected. Selecting a varistor with a higher varistor voltage can reduce the failure rate and prolong the service life, but the residual voltage is slightly increased.

 

(2) Selection of flow rate

 

The nominal discharge current of the varistor should be greater than the surge current required to withstand or the maximum surge current that may occur during operation of the equipment. The nominal discharge current should be calculated from the value of the shock resistance life cycle number of the pressure sensitive resistor 10 times or more, which is about 30% of the maximum impact flow rate (ie 0.3 IP).

 

(3) Selection of clamp voltage

 

The clamp voltage of the varistor must be less than the maximum voltage (ie the safe voltage) that the protected component or device can withstand.

 

(4) the choice of capacitor Cp

 

For high frequency transmission signals, the capacitance Cp should be smaller, and vice versa. 5. Internal resistance matching (ResistanceMatch)

 

The internal resistance R (R ≥ 2 Ω) of the protected component (line) and the transient internal resistance Rv of the varistor: R ≥ 5Rv; for the protected component with small internal resistance, without affecting the signal transmission rate Try to use a large capacitor varistor.

 

 

8. Main applications of varistor

 

(1) lightning protection

 

Lightning strikes can cause atmospheric overvoltages. Most of them are inductive overvoltages. The overvoltage generated by the lightning strike on the transmission line is called the direct lightning overvoltage, and its voltage value is particularly high, which is extremely harmful due to 102~104V. Therefore, for outdoor power systems and electrical equipment, measures must be taken to prevent overvoltage.

 

The use of ZnO varistor arresters is very effective in eliminating atmospheric overvoltages. It is generally connected in parallel with electrical equipment. If the electrical equipment requires a low residual voltage, multi-level protection can be used. The following figure shows a common protection circuit for eliminating atmospheric overvoltages using ZnO arresters. Figure (a) shows the connection method of the ZnO arrester for three-phase electrical equipment, Figure (b) shows the connection method of the ZnO arrester for the solenoid valve control system, and Figure (c) shows the connection method of the ZnO arrester between the power supply and the load.

 

  protection circuit

Varistor for lightning protection of electrical equipment

 

(2) circuit protection

 

In practical applications, various electronic circuits and electrical equipment are often affected by operating overvoltages. The so-called operating overvoltage is the suppression of overvoltage generated when the electromagnetic energy is rapidly converted and the electrical energy is rapidly transmitted when the operating state of the circuit suddenly changes. In order to prevent such overvoltage, high-energy ZnO varistor can be used to protect various large-scale power supply equipment, large electromagnets and large-sized motors. For automotive circuits, communication lines and many civil electrical circuits, low-voltage ZnO varistors or other types can be used. The low voltage varistor is protected.

 

The following figure is a few examples of using a varistor to prevent operation of an overvoltage protection circuit. Figure (a) shows the protection mode of the three-phase rectifier circuit; Figure (b) shows the protection of the single-phase bridge rectifier circuit; Figure (c) uses the varistor to cooperate with the vacuum switch to suppress the operation overvoltage to the high-voltage motor. Protection; and Figure (d) and Figure (e) are varistor protection circuits for micro motors and DC motors, respectively.

 

  circuit protection

Varistor for circuit protection

(a) three-phase rectification; (b) single-phase rectification; (c) with vacuum switch; (d) three-phase motor; (e) DC motor

 (3) switch protection

 

(3) Varistor for switch protection

When a circuit with an inductive load is suddenly disconnected, its overvoltage can exceed several times the supply voltage. Overvoltage can cause arcing and spark discharge between contacts, which can damage contacts such as contactors, relays, and electromagnetic clutches, and shorten the service life of the device. The varistor has a shunt at high voltages and can therefore be used to protect the contacts by preventing spark discharges at the moment the contacts are opened. The connection method of the varistor protection switch or contact is shown in the figure below. When the varistor is connected in parallel with the inductor, the dry voltage of the switch and the dry voltage of the varistor are the sum of the residual voltage of the varistor. The energy absorbed by the varistor is the energy stored by the inductor. When the varistor is connected in parallel with the switch, the overvoltage on the switch is equal to the residual voltage of the varistor, and the energy absorbed by the varistor is slightly larger than the energy stored in the inductor.


  Varistor for switch protection

(a) in parallel with the inductor; (b) in parallel with the switch

 

(4) device protection

In order to prevent the semiconductor device from being burnt due to overvoltage for some reason, it is often protected by a varistor. The figure below shows the application circuit of the varistor protection transistor. The damage of the overvoltage to the transistor can be effectively suppressed between the collector and the emitter of the transistor or the primary shunt varistor of the transformer. At normal voltages, the varistor is in a high-impedance state with minimal leakage current. When subjected to an overvoltage, the varistor quickly changes to a low resistance state, and the overvoltage energy is absorbed by the varistor in the form of a discharge current. After the surge voltage is passed, when the circuit or component is subjected to a normal voltage, the varistor returns to a high resistance state.

 

  device protection

Transistor overvoltage protection circuit 

(a) in parallel with the triode; (b) in parallel with the inductor


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