Introduction to varistor

    2. The "safety valve" function of the varistor circuit.
What is the use of the varistor? The biggest feature of the varistor is that when the voltage applied to it is lower than its threshold "UN", the current flowing through it is extremely small, equivalent to a closed valve. When the voltage exceeds UN, the current flowing through it Its current surges, equivalent to a valve opening. Using this function, abnormal overvoltage that often occurs in the circuit can be suppressed and the circuit can be protected from overvoltage damage.

    3. Application type
: In different usage situations, the purpose of applying varistor is different, and the voltage/current stress acting on the varistor is different,
so the requirements for the varistor are also different. Pay attention to distinguish this difference, and for the correct Use is very important.
According to the different purposes of use, varistors can be divided into two categories: ① varistor for protection, ② varistor for circuit function.

    4.1 Varistors for protection
(1) Differentiate between varistors for power supply protection, signal line protection, and data line protection. They must meet the requirements of different technical standards.
(2) Depending on the continuous operating voltage applied to the varistor, the varistor used across power lines can be divided into two types: AC or DC. The performance of the varistor under these two voltage stresses Aging characteristics behave differently.
(3) According to the different abnormal overvoltage characteristics of the varistor, the varistor can be divided into three types: surge suppression type, high power type and high energy type.
    ★ Surge suppression type: refers to a varistor used to suppress transient overvoltages such as lightning overvoltage and operating overvoltage. The occurrence of this transient overvoltage is random and non-periodic, and the peak value of current and voltage may Very big. The vast majority of varistors fall into this category.
    ★High power type: refers to a varistor used to absorb continuous pulse groups that appear periodically, such as a varistor connected in parallel to a switching power supply converter. Here the impulse voltage appears periodically, and the period can be known, and the energy value can generally be Calculated, the peak voltage is not large, but due to its high frequency, its average power is quite large.
    ★High-energy type: refers to varistor used to absorb magnetic energy in large inductor coils such as generator excitation coils and lifting electromagnet coils. For such applications, the main technical indicator is energy absorption capacity.
The protective function of a varistor can be repeated many times in most applications, but sometimes it is also made into a "disposable" protection device like a current fuse. For example, a varistor with a short-circuit contact connected in parallel to some current transformer loads.
    4.2 Varistors for circuit functions
Varistors are mainly used for transient overvoltage protection, but its volt-ampere characteristics similar to those of semiconductor voltage regulators also enable it to have a variety of circuit component functions, such as:
(1) The stable voltage of DC high-voltage and low-current voltage stabilizing components can be as high as several thousand volts, which is beyond the reach of silicon voltage regulator tubes.
(2) Voltage fluctuation detection component.
(3) DC battery shift component.
(4) Voltage equalizing components.
(5) Fluorescence starting component

    5. Basic performance of protective varistor
(1) Protection characteristics. When the impact strength of the impact source (or impact current Isp = Usp/Zs) does not exceed the specified value, the limit voltage of the varistor is not allowed to exceed the protected object. The impulse withstand voltage (Urp) it can withstand.
(2) Impact resistance characteristics, that is, the varistor itself should be able to withstand the specified impact current, impact energy, and the average power when multiple impacts occur one after another.
(3) There are two life characteristics. One is the continuous operating voltage life, that is, the varistor should be able to work reliably for the specified time (number of hours) under the specified ambient temperature and system voltage conditions. The second is the impact life, that is, the number of times it can reliably withstand the specified impact.
(4) After the varistor is inserted into the system, in addition to playing a protective role as a "safety valve", it will also bring some additional effects. This is the so-called "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 PF), the other is the leakage current under the system voltage, and the third is the nonlinear current of the varistor through the source impedance. The coupling effect on other circuits.