1, what is the "varistor": "varistor is the term in mainland China, meaning" in a certain current and voltage range of resistance with the voltage and change ", or" resistance value of the voltage-sensitive " . The corresponding English name is "Voltage Dependent Resistor" abbreviated as "VDR." The varistor is made of semiconductors, so it is a variety of semiconductor resistors. Now a large number of "zinc oxide" (ZnO) pressure (Zn) and hexavalent elemental oxygen (O), so that the zinc oxide varistor is a kind of "Ⅱ-Ⅵ oxide" from the material point of view Semiconductor. "In China TW, varistors are named according to their use, called" surge absorbers. "Varistors are sometimes referred to as" electric shock (surge) suppressors according to their use (absorption Device ".
2, varistor circuit "safety valve" role: What is the use of varistor? Varistor is the biggest characteristic is added to it above the voltage below its threshold "UN", the flow of its current Very small, the equivalent of a closed valve, when the voltage exceeds UN, the current surge through it, the equivalent of the valve open. With this feature, you can suppress the circuit often occurs in the abnormal over-voltage, protection circuit from over-voltage damage.
3, the application type: the use of different occasions, the application of the purpose of varistor, the role of the varistor voltage / current stress is not the same, so the requirements of the varistor is not the same, pay attention to distinguish this difference, for Proper use is very important. According to the purpose of the use of varistors can be divided into two categories: ① protection with varistor, ② circuit function with varistor.
3.1 Protective varistors for protection
(1) to distinguish between power protection, or signal lines, data line protection with varistors, they have to meet the requirements of different technical standards.
(2) Depending on the continuous operating voltage applied to the varistor, the varistor for the power supply line can be divided into two types: AC or DC, and the varistor under these two kinds of voltage stresses Aging performance is different.
(3) According to the varistor bear the abnormal over-voltage characteristics of different varistors can be divided into surge suppression, high power and high energy type of these three types.
Surge suppression type: refers to the suppression of lightning overvoltage and operating overvoltage and other transient overvoltage varistor, the occurrence of such transient overvoltage is random, non-periodic, the current and voltage peak may be Big. Most varistors all fall into this category.
High power type: refers to the varistor used for the absorption cycle of the continuous group of varistors, such as connected to the switching power supply varistor, where the impact voltage cycle occurs, and the cycle we can see that the energy value can generally be calculated Out, the peak voltage is not large, but because of the high frequency, the average power is quite large.
High energy type: refers to the absorption of generator excitation coil, lifting solenoid coil and other large inductance coil magnetic energy in the voltage transformer, for such applications, the main technical indicators of energy absorption capacity.
Varistor protection, the vast majority of applications, it can be repeated many times, but sometimes it will be made of current fuse as a "one-time" protection device. Such as a short-circuit contact varistor connected to some current transformer loads.
3.2 varistor for circuit function: Varistor is mainly used for transient overvoltage protection, but it is similar to the volt-ampere characteristics of the semiconductor regulator, but also it has a variety of circuit components, such as can be used as:
(1) DC high voltage small current regulator components, the stable voltage can be as high as several thousand volts or more, which is silicon regulator can not be achieved.
(2) voltage fluctuation detection element.
(3) DC battery shift components.
(4) pressure equalizer.
(5) Fluorescence start element
4, the basic performance of protective varistors
(1) protection characteristics, when the impact of the impact of strong (or impact current Isp = Usp / Zs) does not exceed the specified value, the varistor limit voltage is not allowed to withstand the object can withstand the withstand voltage (Urp) The
(2) impact resistance, that is, the varistor itself should be able to withstand the specified impact current, impact energy, and multiple shocks have appeared in the average power.
(3) life characteristics of two, one is the continuous working voltage life, that is, the varistor in the provisions of the ambient temperature and system voltage conditions should be able to reliably work for the specified time (hours). Second, the impact of life, that is able to reliably withstand the required number of shocks.
(4) varistor intervention system, in addition to play a "safety valve" protection, but also bring some additional effects, which is the so-called "secondary effect", it should not reduce the normal performance of the system. At this time to consider the main factors are three, one of the varistor itself capacitance (tens to tens of thousands of PF), the second is the system voltage leakage current, the third is the varistor nonlinear current through the source impedance The effect of coupling on other circuits.