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A "varistor" is a resistive device with non-linear volt-ampere characteristics. It is mainly used to clamp the voltage when the circuit is subjected to overvoltage, and absorb excess current to protect sensitive devices. The English name is "Voltage Dependent Resistor" abbreviated as "VDR", or "Varistor". The material of the resistor body of a varistor is a semiconductor, so it is a variety of semiconductor resistors. The "zinc oxide" (ZnO) varistor, which is now widely used, has a main material composed of the divalent element zinc (Zn) and the hexavalent element oxygen (O). So from the perspective of materials, the zinc oxide varistor is a kind of "II-VI oxide semiconductor". In Taiwan, varistors are called "surge absorbers" and sometimes "electric shock (surge) suppressors (absorbers)."
A varistor is a voltage-limiting protection device. Utilizing 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 achieving protection for the subsequent circuit. The main parameters of the varistor are: varistor voltage, current capacity, junction capacitance, response time, etc.
The response time of the varistor is ns level, which is faster than the gas discharge tube and slightly slower than the TVS tube. Generally, the response speed of overvoltage protection for electronic circuits can meet the requirements. The junction capacitance of a varistor is generally in the order of hundreds to thousands of Pf. In many cases, it should not be directly applied to the protection of high-frequency signal lines. When applied to the protection of AC circuits, the larger the junction capacitance will increase leakage Current needs to be fully considered when designing the protection circuit. The varistor has a larger flow capacity, but is smaller than a gas discharge tube. The varistor is abbreviated as VDR, which is a kind of voltage-sensitive non-linear overvoltage protection semiconductor element.
How varistors work
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 infinite resistance. In other words, when the voltage applied to it is lower than its threshold, it is equivalent to an off-state switch.
When the voltage applied to the varistor exceeds its threshold, the current flowing through it increases sharply, which is equivalent to an infinitely small resistance. In other words, when the voltage applied to it is higher than its threshold, it is equivalent to a closed-state switch.
Selection and precautions of varistor
Before selecting a varistor, you should first understand the following related technical parameters: the nominal voltage refers to the voltage value across the varistor at the specified temperature and DC current. Leakage current refers to the value of the current flowing in the varistor when the maximum continuous DC voltage is applied at 25 ° C. Grade voltage refers to the peak voltage of the voltage across the varistor when it passes an 8/20 grade current pulse. The flux is a peak current when a predetermined pulse current (8/20 μs) waveform is applied. Surge environmental parameters include the maximum surge current Ipm (or the maximum surge voltage Vpm and the surge source impedance Zo), the surge pulse width Tt, the minimum time interval Tm between two adjacent surges, and the predetermined value in the varistor. During the working life, the total number of surge pulses N and so on.
Generally speaking, a varistor is often used in parallel with the protected device or device. Under normal circumstances, the DC or AC voltage across the varistor should be lower than the nominal voltage, even when the power supply fluctuations are the worst. It should not be higher than the maximum continuous working voltage selected in the rated value. The nominal voltage value corresponding to the maximum continuous working voltage value is the selected value. For the application of overvoltage protection, the varistor voltage value should be greater than the voltage value of the actual circuit. Generally, the following formula should be used for selection: VmA = av / bc where: a is the fluctuation coefficient of the circuit voltage; v is the DC operating voltage of the circuit It is the effective value when AC); b is the varistor voltage error; c is the aging coefficient of the component; the actual value of VmA calculated in this way is 1.5 times the DC operating voltage, and the peak value must be considered in the AC state, so the calculation result should be Expanded 1.414 times.
In addition, you must pay attention to:
(1) It must be ensured that the continuous working voltage will not exceed the maximum allowable value when the voltage fluctuation is maximum, otherwise the service life of the varistor will be shortened;
(2) When a varistor is used between the power line and the ground, the voltage between the line and the ground may increase due to poor grounding. Therefore, a varistor with a higher nominal voltage than the line-to-line application is usually used.
The surge current absorbed by the varistor should be less than the maximum throughput of the product.
Application range
Power system
Surge suppressor
security system
Motor protection
Automotive electronics
Household appliances
The role of resistance
What is the use of a varistor? The biggest feature of a varistor is that when the voltage applied to it is lower than its threshold "UN", the current flowing through it is extremely small, which is equivalent to a closed valve. When the voltage exceeds UN, its The resistance value becomes smaller, so that the current flowing through it increases sharply without affecting the other circuits much, thereby reducing the impact of overvoltage on subsequent sensitive circuits. With this function, abnormal overvoltages that often occur in circuits can be suppressed, and the circuit can be protected from damage caused by overvoltages.
For example: The zinc oxide varistor is used in the power circuit of our home color TV. The varistor voltage used here is 470V. When the maximum transient surge voltage (non-effective value) exceeds 470V, the voltage The varistor reflects his clamping characteristics, pulls the excessive voltage down, and allows the subsequent circuit to work within a safe range.
Application Type
The voltage / current applied to the varistor should not be the same for different applications.
Therefore, the requirements for varistor are also different. It is very important to distinguish this difference for correct use.
According to the purpose of use, varistor can be divided into two categories: ① varistors for protection, ② varistor for circuit function.
Protection resistance
(1) Differentiate whether it is a varistor for power supply protection or a varistor for signal line and data line protection. They must meet the requirements of different technical standards.
(2) Depending on the continuous working voltage applied to the varistor, the varistor used across the power line can be divided into two types: AC or DC. Different performance.
(3) The varistor can be divided into three types: surge suppression type, high power type and high energy type according to the different overvoltage characteristics of the varistor.
★ Surge suppression type: refers to a varistor used to suppress transient overvoltages such as lightning overvoltages and operating overvoltages. The appearance of such transient overvoltages is random, non-periodic, and the peak value of current and voltage may be Great. Most varistors fall into this category.
★ High power type: It refers to a varistor used to absorb continuous pulse groups that appear periodically. For example, a varistor connected in parallel to a switching power converter. Here the impulse voltage appears periodically, and the period can be known. The energy value can generally be It is calculated that the peak value of the voltage is not large, but because of the high frequency of occurrence, its average power is quite large.
★ High-energy type: It refers to a varistor used to absorb the magnetic energy in a large inductive coil such as an excitation coil of a generator, a lifting electromagnet coil, etc. For this type of application, the main technical indicator is energy absorption capacity.
The protection function of the varistor can be repeatedly used 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 the load of some current transformers.
effect
The varistor is mainly used for transient overvoltage protection, but its volt-ampere characteristics similar to that of a semiconductor zener tube also make it have a variety of circuit element functions, such as:
(1) DC high-voltage small-current voltage stabilizing element, its stable voltage can be as high as thousands of volts or more, which cannot be achieved by silicon voltage stabilizing tube.
(2) Voltage fluctuation detection element.
(3) DC level shifting element.
(4) Equalizing element.
(5) Fluorescent starting element
Basic performance
(1) Protection characteristics: When the impact strength of the impact source (or the impulse current Isp = Usp / Zs) does not exceed the specified value, the limit voltage of the varistor must not exceed the impulse withstand voltage (Urp) that the protected object can withstand. .
(2) Shock resistance, that is, the varistor itself should be able to withstand the specified surge current, impact energy, and average power when multiple shocks occur one after the other.
(3) There are two life characteristics. The first is the continuous working voltage life, that is, the varistor should be able to work reliably for the specified time (hours) at the specified ambient temperature and system voltage conditions. The second is the impact life, which can reliably withstand the specified number of impacts.
(4) After the varistor is involved in the system, in addition to the protective function of the "safety valve", it will 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 of PF), the second is the leakage current at the system voltage, and the third is the non-linear current of the varistor through the source impedance. The effect of coupling on other circuits.
