A "varistor" is a resistive device with nonlinear volt-ampere characteristics that is used to clamp a voltage when the circuit is subjected to an overvoltage, absorbing excess current to protect sensitive devices. The English name is “Voltage Dependent Resistor” and is abbreviated as “VDR” or “Varistor”. The resistor material of the varistor is a semiconductor, so it is a variety of semiconductor resistors. A "zinc oxide" (ZnO) varistor, which is currently used in large quantities, is composed of a divalent element zinc (Zn) and a hexavalent element oxygen (O). Therefore, from the material point of view, the zinc oxide varistor is a "II-VI oxide semiconductor". In Taiwan, varistors are called "surge absorbers" and are sometimes referred to as "electric shock (surge) suppressors (absorbers)".

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.

The working principle of varistor

When the voltage applied to the varistor is below its threshold, the current flowing through it is extremely small, which is equivalent to a resistor with an infinite value. That is, when the voltage applied to it is below its threshold, it is equivalent to an open state switch [1].

When the voltage applied to the varistor exceeds its threshold, the current flowing through it surges, which is equivalent to an infinitely small resistance. That is, when the voltage applied to it is above its threshold, it is equivalent to a closed state switch [2].

 

advantage

Various diameter sizes: SMD or DIP, 5mm, 7mm, 10mm, 14mm, 20mm, 25mm, 32mm, 34mm, 40mm, 53mm

Wide range of variable resistor voltages: 18V-1800V

Multiple surge withstand: standard, high surge, ultra high surge

High current handling and energy absorption

Monomer flow rate can reach 70KA or higher

Fast response time

Low leakage current

Multiple lead forms: straight, bent and other special lead types

A variety of packaging formats: bulk, package, roll packaging

 

Application range

Power Systems

Surge suppressor

security system

Motor protection

Automotive electronics system

Household appliances

 

Basic performance

(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 the "safety valve", it will bring some additional effects. This is called the "secondary effect", which 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 nonlinear current of the varistor through the source impedance. The effect of coupling on other circuits.