The function parameters of the pressure sensitive resistor and the matters needing attention
What are the parameters and type selection of the pressure sensitive resistance? Voltage sensitive resistor, VSR, is a kind of voltage sensitive nonlinear overvoltage protection semiconductor device. It is expressed in the circuit with the character symbol "RV" or "R"! 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".
The effect of the pressure sensitive resistance
The biggest characteristic of the varistor is when added at a voltage above it below its threshold "UN". The minimum current passing through it, equivalent to a shut the valve, when the voltage exceeds the UN and the current flowing through it surge, quite in the valve open. By using this function, the abnormal overvoltage can be suppressed in the circuit, which can protect the circuit from excessive voltage.
Nominal parameters of the pressure sensitive resistance
Varistor with letters "my" said that if J is home, behind the letter W, G, P, l, h, Z, B, C, N, K were used for voltage regulation, overvoltage protection, high frequency circuit, lightning protection, arc, eliminating noise, compensation, degaussing, high energy and high reliable. 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.
The main parameters of the pressure sensitive resistor
The main parameters of the varistor has nominal voltage, voltage ratio, maximum control voltage and residual voltage ratio, through flow capacity, leakage current, voltage temperature coefficient, current temperature coefficient, nonlinear coefficient of voltage, insulation resistance, static capacitance and so on.
Type selection of pressure sensitive resistance
Before the selection of the pressure sensitive resistor, we should first understand the following technical parameters: the nominal voltage is defined as the temperature and the DC current, the voltage value of the voltage at both ends of the voltage. The leakage current is the current value flowing through the pressure sensitive resistor at 25 C, when the maximum continuous DC voltage is applied. Voltage is the voltage peak value that is present at both ends of the voltage sensitive resistor through the 8/20 grade current pulse. The flux is represented by the peak current applied to the pulse current (8/20 s) waveform. Surge environment parameters include maximum surge current IPM (or maximum surge voltage VPM and inrush source impedance Zo), surge pulse width TT, two adjacent surges the minimum time interval TM and within the varistor predetermined working life, of the surge pulse total number N and.
In general, varistors often with protected device or devices in parallel, under normal circumstances, the DC or AC voltage at both ends of the pressure sensitive resistor should be lower than the nominal voltage, even in power fluctuations and in the worst case, also should not is higher than the rated values of maximum continuous operating voltage, the maximum continuous operating voltage value corresponding to the nominal voltage value is the value to choose. For overvoltage protection application, the varistor voltage value should be greater than the actual circuit voltage value, the general should use selection: VmA = av/bc type: a circuit voltage fluctuation coefficient, generally take 1.2; V DC operation voltage (AC for valid values); B for varistor voltage error, generally take 0.85; C as components of the aging coefficient, generally take 0.9. This calculated VmA actual numerical is 1.5 times of the DC voltage, in a state of communion also considering the peak, so the calculation results should be extended 1.414 times. In addition, the selection must also pay attention to:
(1) it is necessary to ensure that the continuous operating voltage will not exceed the maximum allowable value in the voltage fluctuation, otherwise it will shorten the service life of the pressure sensitive resistance;
(2) in the power supply line and the earth between the use of pressure sensitive resistance, and sometimes due to the bad grounding between the line and the ground voltage rise, so the use of more than the line between the line and the use of higher voltage of the voltage sensitive resistor. The surge current absorbed by the pressure sensitive resistor should be less than the maximum flow rate of the product.