In the daily electrical appliances, for our equipment used in the pressure sensitive resistor, the original choice of models for the 14D101K, the actual operation for 3 months, this model is often burned. After the 14D121K, the actual operation of 3 months, found no burn. Therefore, in order to guide the work, finishing and learning this information, and in the finishing process, it is found that the pressure sensitive resistance should not be directly connected to the input end of the component. Details of the specific pressure sensitive resistance are as follows:
First, the principle of the pressure sensitive resistance
The pressure sensitive resistance means "the value of the resistance varies with the voltage in a certain current voltage range", or "the resistance value is sensitive to the voltage". The corresponding English is called "VoltageDependentResistor" or "VDR".
With the increase of the voltage above it, its resistance value can be changed from M () to m (Ohm). When the voltage is low, the varistor in leakage current region, showing great resistance, leakage current is very small; when the voltage increases in the linear region, the current changes in a wide range, the voltage changes little, showing better characteristics of pressure limit; voltage rise, voltage sensitive resistor in the saturation region, showing a small linear resistance, due to the current large, will be a long time to make the varistor overheating even burst. When the normal use of the pressure sensitive resistance is in the leakage current area, the surge current into the nonlinear area, generally can not enter the saturation zone
The material of the resistor is a semiconductor, so it is a variety of the semiconductor resistor. Now a large number of use of "Zinc Oxide" (ZnO) pressure sensitive resistor, which is the main material has two valence elements (Zn) and six valence element oxygen (O) constitute. So from the material point of view, Zinc Oxide varistor is a kind of "II-VI semiconductor oxide". Two, the role of the pressure sensitive resistance
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, the current flowing through it in the equivalent of the valve open. By using this function, the abnormal overvoltage can be suppressed in the circuit, which can protect the circuit from excessive voltage.
The voltage sensitive resistor is a kind of element which has the function of transient voltage suppression. It can be used in place of the combination of transient suppression diode, Qi Na diode and capacitor. The voltage sensitive resistor can protect the circuit of the IC and other equipment, prevent the damage to them due to electrostatic discharge, surge and other transient current (such as lightning strike). The varistor is connected to the protected equipment or IC circuit, when the voltage is higher than a certain value, the varistor resistance decreased rapidly, the conduction current, thereby protecting the IC or electrical equipment; when the voltage is lower than the varistor voltage value, varistor resistance is extremely high, almost open. It will not affect the normal work of the electrical equipment or devices.
Three, the nominal parameters of the pressure sensitive resistance
Varistor with the letter "MY", such as J for home, behind the letters W, G, P, L, H, Z, B, C, N and K are respectively used for voltage regulation, overvoltage protection, high frequency circuit, lightning protection, arcing, noise elimination, compensation, or high reliability and high degaussing hand. Although the pressure sensitive resistance can absorb a great surge of electrical energy, but can not withstand the current level of more than Ma, in the use of overvoltage protection must be taken into account.
Four, the characteristic parameters of the pressure sensitive resistance
Voltage UN (U1mA): usually in the pressure sensitive resistance through the 1mA DC current when the voltage to indicate whether it is a sign of voltage, this voltage is known as the voltage sensitive UN. Voltage sensitive voltage is also commonly used symbol U1mA. The error range of voltage sensitive voltage is generally + 10%. In the experiment and practical application, the pressure sensitive voltage is generally decreased from the normal value of 10% as the criterion of the failure of the pressure sensitive resistance.
The maximum continuous operating voltage UC: maximum AC voltage sensitive resistor can withstand the pressure of long-term (RMS) Uac or maximum DC voltage Udc. General Uac = 0.64U1mA, Udc = 0.83U1mA.
The flow rate (maximum impact current) IP: acupressure resistance can withstand 8/20 S wave maximum impact current peak. "Able to bear" the meaning is that the impact of pressure after the pressure change rate is not greater than 10%. The technical specifications in the current is usually given 1 times IP impact value.
The maximum clamping voltage (voltage limit) VC: given the technical specifications the maximum clamping voltage value refers to the 8/20 s IX is applied to wave current provisions of varistor (A) voltage varistor has.
Actual use, the higher the voltage, the greater the impact of the impact of the current limit voltage (or the residual pressure) is higher, can be found on the V-I curve given by the product.
The rated energy E: rated energy refers to the varistor can withstand maximum energy impulse current waveform impact time (rate of change of varistor voltage after impact is not greater than 10%), can be expressed as:
E=K*IP*VC*T
Type: IP, VC, T for the pulse width, K, and the waveform of the constant. For 8/20 s and 10/1000 S waves, K=1.4; for 2ms square wave, K=1.
The rated power (maximum average power): Pm pressure sensitive resistance at room temperature, continuous under repeated impact, and the impact of short time interval, and heat accumulation effect, can withstand the maximum average power. Although the pressure sensitive resistance can withstand a lot of pulse power, but can withstand the average power is very small.
The capacitance C0: acupressure resistance between two electrodes of capacitor in the range of pF ~ nF in several hundred. The smaller the volume, the higher the voltage, the smaller the capacitance.
The leakage current of Il varistor: current to exert maximum DC voltage Udc flows. When measuring the leakage current, the voltage of the Udc=0.83U1mA is usually added to the voltage (sometimes with 0.75U1mA). General requirements of static leakage current (Il less than or equal to 20 mu A is also required less than or equal to 10 mu A). In practical use, it is more concerned with the value of the static leakage current than the value itself, but its stability, that is, the rate of change after impact test or under high temperature conditions. After impact test, or at high temperature, the rate of change is not more than doubled, that is, the stability of the.
To the nonlinear exponent: refers to the ability of the change of voltage effect on the current formula is available:
I=KU alpha or alpha =loglog
By the front type visible, the greater the effect of the voltage change on the current capacity of the current, the better the nonlinear characteristics. After visible, the alpha is the reciprocal of the slope of the various points on the volt ampere characteristic, the more flat the characteristic is, the bigger the alpha (the drain current area and the saturated zone alpha =1, also known as the low alpha region). When using the instrument measurement, generally set I2=1mA, I1=0.1mA, so
Alpha T=1/log (U1mA/U0.1mA)
Five, the amount of pressure drop characteristics
When the impact test of the pressure sensitive resistance, with the increase of the number of shocks to be carried out, the impact of the current applied to the corresponding reduction. For example: the standard 20 varistor substrate diameter (U1mA = 82V), the derating characteristics are shown in the following table (which can be found from the surge of lifetime rating curve given by the manufacturer.):
Allow the impact of the number of times 2 times 10 times 100 times 1000 times 10000 times 1 times
Each impact current 6500A4000A2000A1000A430A200A
Six, the measurement of the pressure sensitive resistance
When measuring the multimeter 10K file, to probe connected to the resistor multimeter should show marked varistor on resistance, if beyond this great value, the varistor has been damaged.
Seven, the selection of the pressure sensitive resistance
The selection of the pressure sensitive resistance, the general choice of nominal voltage sensitive voltage V1mA and the flow capacity of two parameters.
1, the so-called pressure sensitive voltage, that is, the breakdown voltage or threshold voltage. Refers to the voltage value in the specified current, in most cases with 1mA DC current into the voltage value of the voltage value of the voltage, the voltage range of its products can range from 10-9000V. According to the specific needs of the correct choice. General 1mA= "1".5Vp= "2".2VAC, type, Vp for the peak voltage of the circuit. VAC effective value for rated AC voltage. The choice of voltage value of ZnO is very important, it is related to the protection effect and service life. As with a rated supply voltage is 220V, electric appliance, varistor voltage V1mA= "1".5Vp= "1".5 * 220V= "476V" V1mA= "2".2VAC= "2".2 * 220V= "484V", so the varistor breakdown voltage between 470-480V optional.
2, the current capacity, the maximum peak pulse current is 25 C ambient conditions, the number of impulse current provisions for the impact of current waveform and regulations, change of varistor voltage does not exceed a maximum value when the pulse current + 10%. In order to prolong the service life of the device, the amplitude of surge current absorbed by ZnO is less than the maximum flow rate of the product given in the manual. However, from the protection of the effect of starting, the demand for the flow of a large number of good. In many cases, the actual traffic is difficult to accurately calculate the flow, then the use of 2-20KA products. Such as hand products flow can not meet the application requirements, can be only a single parallel varistor varistor after use, parallel flow through the same, for each single varistor and the numerical. It is required that the voltage sensitive resistance of parallel connection is the same as that of the same, otherwise it is easy to cause the shunt is not uniform and the damage is caused by the pressure.
Eight, the use of the pressure sensitive resistance
The pressure sensitive resistance is generally used in parallel in the circuit, when the voltage of both ends of the resistance changes rapidly, the resistance short circuit fuse the current fuse, which plays a protective role. The voltage sensitive resistor is used in the circuit, which is often used in the power supply overvoltage protection and voltage regulation.
The reliability and safety of the power supply are greatly dependent on the correct use of the pressure sensitive resistance. In particular, in the power supply lightning protection should also be considered in the design of various parts of the power quality difference, lightning frequency and intensity difference, the protected equipment installation and use and impact tolerance and other differences, can not use a formula to copy sets. Design of lightning protection devices must be in the field of use conditions or as close as possible under the conditions of the actual simulation test.
Calculation of pressure sensitive voltage:
General availability calculation:
U1mA=KUac
Formula: K for the quality of the power quality related coefficient, generally take K= (2 ~ 3), the better quality of the power supply of small, poor quality of rural areas (especially the mountains) is desirable. Uac for AC power supply voltage. For 220V ~ 240V AC power supply lightning protection device, the pressure sensitive voltage should be selected as 470V ~ 620V. The use of a high voltage sensitive voltage of the pressure sensitive resistance, can reduce the failure rate, prolong the service life, but the residual pressure slightly increased.
Calculation of nominal discharge current:
The nominal discharge current of the pressure sensitive resistance shall be greater than the surge current required to withstand or the maximum surge current that may occur each year. The nominal discharge current should be calculated according to the numerical value of the shock resistance surge 10 times in the time of surge life of the pressure sensitive resistance, which is about 30% (i.e., 0.3IP) of the maximum impact flow.
Parallel connection of the pressure sensitive resistance:
When a pressure sensitive resistor can not meet the requirements of the nominal discharge current, it should be used in parallel. Sometimes in order to reduce the limit voltage, even if the nominal discharge current meets the requirement, the multi - voltage - sensitive resistors are used in parallel. We must pay special attention to, varistor used in parallel, must be strictly selected consistent parameters (for example: U1mA = 3V, delta alpha = 3) are matched to ensure the uniform current distribution.
Nine, the use of the pressure sensitive issues
The failure mode of the pressure sensitive resistance is usually short circuit, in order to prevent the failure of the pressure sensitive resistance caused by short circuit and fire, can be in series on each of the pressure sensitive resistance of a temperature insurance tube or thermal detachment mechanism. Temperature insurance tube should have good thermal coupling and varistor, when the varistor failure (high impedance short circuit), it is the heat generated by the temperature insurance tube fuse, so that the voltage sensitive resistor and circuit isolation failure, ensure the safety of equipment. When the higher power frequency transient overvoltage on the pressure sensitive resistance, it may cause the breakdown of the voltage sensitive resistance short circuit (low impedance short circuit), and the temperature of the fuse is too late to fuse, may also be on fire. In order to avoid this kind of phenomenon, it can be used in each of the pressure sensitive resistance in series with an impact power frequency fuse (single power frequency fuse may not fuse in the aging failure). Can also put the varistor and ceramic gas discharge tube connected in series with the normal work of ceramic gas discharge tube is not conductive, varistor without leakage current, can greatly prolong service life; the surge, the first breakdown ceramic gas discharge tube, and then by limiting the varistor surge voltage, the total residual pressure for both and, slightly increased (tens of volts); the impact of the past, as the varistor limiting current, discharge tube can not maintain the conduction to extinguish the arc, restore to normal working condition; when the varistor short-circuit failure, because the ceramic gas discharge tube current flowing through the great power will soon fail, but the failure mode of it most are open, so it is not easy to cause a fire.
Therefore, the selection of varistor in our equipment basically no mistake, should be selected according to the formula, namely the varistor voltage nominal voltage 130V varistor, would not be according to the principle of the actual 14D151 model should be selected. But, in actual use, we should not directly to the varistor is connected, according to the actual situation, and should put varistor ceramic gas discharge tube used in series. A varistor and a gas discharge tube of the two products of our company strong push the main core, if the selection is not too understands friends can contact our online customer service. Let customer service to solve your needs!