Varistor is a kind of resistance device with nonlinear volt ampere characteristics, which is mainly used to clamp the voltage when the circuit is under voltage and absorb the excess current to protect the sensitive device. English is called "Voltage Dependent Resistor" or "VDR" or "Varistor". The varistor is a semiconductor material, so it is a variety of semiconductor resistor. Now a large number of "Zinc Oxide" (ZnO) varistors, its main material has two valence element zinc (Zn) and the valence element oxygen (O) constitute. So from the material point of view, Zinc Oxide varistor is a kind of "II-VI semiconductor oxide". China in Taiwan, known as the "varistor surge absorber", sometimes referred to as "electric shock (surge suppressor) (absorber)".
Varistor is a kind of voltage limiting protection device. By using the nonlinear characteristic of the varistor, the varistor can clamp the voltage to a relatively fixed voltage value when the overvoltage appears at the two poles of the varistor. The main parameters of varistor are voltage sensitive, current capacity, junction capacitance, response time and so on.
The response time of the varistor is ns grade, faster than the gas discharge tube, tube slightly slower than TVS, electronic circuit for overvoltage protection of the response speed can meet the requirements of the general situation. Junction capacitance varistors in hundreds to thousands of Pf magnitude range, not many cases direct application in the protection of high frequency signal line, used in the protection circuit in communication, because of its larger capacitance increase in leakage current, give full consideration to the needs in the design of protection circuit. Varistor flow capacity is larger, but the ratio of the gas discharge tube is small. VDR is a kind of voltage sensitive nonlinear overvoltage protection semiconductor device.
Working principle of varistor
When added to the varistor voltage is lower than its threshold, the minimum current flowing through it, it is equivalent to a resistance value of infinity. In other words, when the voltage above it is below its threshold, it is equivalent to a switch off state.
When added to the pressure sensitive resistor voltage exceeds its threshold, the current flowing through it surge, it is equivalent to the resistance of infinitesimal. That is, when the voltage above it is higher than its threshold, it is equivalent to a closed state switch.
Selection and precautions of varistor
Prior to the selection of varistors, the following technical parameters should be understood: the nominal voltage is the voltage at the two ends of the varistor at the specified temperature and DC current. The leakage current is the current flowing through the varistor when the maximum continuous DC voltage is applied at 25 DEG C. Rated voltage is the voltage peak at the two ends of the varistor when the 8/20 current pulse is used. The flow rate is the peak current when the specified pulse current (8/20 s) is applied. Surge environment parameters including the maximum surge current Ipm (or the maximum surge voltage and surge impedance of Vpm Zo), the minimum time interval of Tm surge pulse width Tt, two adjacent surge and the varistor scheduled work life period, the total number of N surge pulse.
Generally speaking, the varistor is often used and protected device or devices in parallel, under normal circumstances, pressure sensitive resistor DC or AC voltage should be less than the nominal voltage, the power fluctuation even in the worst, also should not be higher than the rated maximum working voltage in the selection, the maximum continuous working voltage nominal value the corresponding voltage value is selected value. For the application of overvoltage protection, the varistor voltage value of the voltage should be greater than the actual value of the circuit, general should choose the use type: VmA=av/bc type: a circuit voltage fluctuation coefficient; V circuit for DC voltage (AC for RMS); B varistor voltage error; C for aging coefficient, element the actual value calculated by VmA; this is 1.5 times the DC voltage, AC in the state but also consider the peak, so the calculated results should be expanded 1.414 times.
In addition, the choice must also pay attention to:
(1) it is necessary to ensure that the continuous operating voltage will not exceed the maximum allowable value at the maximum voltage fluctuation, otherwise the service life of the varistor will be shortened;
(2) when the varistor is used between the power line and the earth, the voltage between the ground and the ground is increased due to the bad grounding, so the utility model is usually used as a varistor with a higher nominal voltage than the line and the line.
The surge current absorbed by the varistor should be less than the maximum flow rate of the product.
power supply system
Automotive electronic system
Household Electric Appliances
Effect of resistance
What is the use of varistors? The biggest characteristic of the varistors with them when the voltage is lower than the threshold it "UN", the minimum current flowing through it, the equivalent of a closed valve, when the voltage exceeds UN, its resistance value is reduced, so that the current flowing through it in the impact on other circuit changes not to reduce the impact on subsequent small voltage sensitive circuit. By using this function, we can suppress the abnormal overvoltage in the circuit and protect the circuit from over-voltage damage.
For example: a power supply circuit of our home TV in Zinc Oxide on the use of varistor varistor varistor voltage, here used to 470V, when the maximum transient voltage surge (non effective value) more than 470V, the varistor is the embodiment of his clamping performance, the high voltage low, make the circuit work in a safe range.
For different applications, the voltage / current acting on the varistor should not be the same as the application of the varistor,
Therefore, the requirements of the varistor are not the same, it is very important to distinguish the difference.
According to the purpose of the use of different, can be divided into two major categories: 1.
(1) the distinction between the power supply protection varistors, or signal lines, data line protection varistors, to meet the requirements of different technical standards.
(2) according to the continuous working voltage applied on the varistor on different cross power line can be divided into communication with varistors or DC with two types of varistor in the two voltage aging characteristics under different stress.
(3) according to the characteristics of the abnormal overvoltage, the varistor can be divided into the following three types: surge suppression, high power and high energy, which can be divided into three types.
It is used for surge suppression varistors: suppressing lightning overvoltage and switching overvoltage and transient overvoltage, the transient overvoltage appeared to be random, non periodic, peak current and voltage can be very large. The vast majority of varistors fall into this category.
This high power type: refers to the varistor continuous pulse group absorption cycle, for example, and then varistor in switching power converters, this impulse voltage cycle, and the cycle shows that the energy values can be calculated, the peak voltage is not large, but because of the high frequency, the average power considerable.
B: refers to high energy absorption excitation coil, which is heavy energy varistor voltage solenoid coil and other large inductance coil in, for this type of application, the main technical indicators is the energy absorption capacity.
The protection function of the varistor, under the vast majority of applications, can be repeated many times, but sometimes it is made into a current fuse, such as a "one-time" protection device. For example, a short circuit contact voltage sensitive resistor connected to a certain current transformer load.
The varistor is mainly used in transient overvoltage protection, but it is similar to the volt ampere characteristics of the semiconductor voltage regulator, it also has a variety of circuit components, such as:
(1) the high voltage DC current regulator, the stable voltage can be as high as several thousand volts, which is the silicon regulator can not achieve.
(2) voltage fluctuation detection element.
(3) DC level shift element.
(4) equalizing element.
(5) fluorescent start element
(1) protection characteristics, when the impact of the impact of the source of strong (or shock current Isp=Usp/Zs) does not exceed the specified value, the varistor voltage limit does not allow the object to be protected by the impact withstand voltage (Urp).
(2) shock resistance, that is, the varistor itself should be able to withstand the prescribed impact current, impact energy, as well as the impact of repeated power average power.
(3) there are two life characteristics, one is the continuous working voltage life, that is, the varistor should be able to work reliably in the specified environment temperature and system voltage conditions (hours). The two is the impact of life, that is able to reliably withstand the impact of the number of times.
(4) after the intervention system, in addition to the "safety valve" protection, but also bring some additional effects, which is called the "two effect", it should not reduce the normal performance of the system. The factors to consider when there are three, one is capacitance varistor itself (tens to tens of thousands of PF), the two is the leakage current in the system voltage, three nonlinear current varistor effect on other circuits by coupling the source impedance.
Contact: Tina Zhang
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