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          Selection of pressure sensitive resistance: selection of nominal voltage and flow rate of the pressure sensitive resistance
          Publish:Shenzhen Tergy Technology Co., Ltd.  Time:2016-10-20
          Selection of the pressure sensitive resistance
          Prior to the use of the pressure sensitive resistor, we should first understand the following technical parameters: the nominal voltage (i.e., voltage sensitive 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 at which the voltage is present at both ends of the voltage sensitive resistor through the 8 / 20 grade current pulse. The flux is expressed as the peak current applied to the pulse current (8 / 20 s). 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.
          Selection of the pressure sensitive resistance
          1, the nominal voltage selection
          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 voltage value of the voltage sensitive value should be greater than the actual value of the circuit:
          VmA = AV / BC
          Type: A for the circuit voltage fluctuation coefficient, generally take 1.2; V for the circuit DC operating voltage (AC) for the effective value; B for the voltage sensitive voltage error, generally take 0.85; C for the component of the aging coefficient, generally take 0.9;
          In this way, the actual value of VmA is 1.5 times of the DC working voltage, and the peak value is considered in the AC state, so the calculation results should be expanded 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.
          2, through the selection of flow
          Usually the product is given through the flow is given by the product standard waveform, shock times and the gap time pulse test can withstand the maximum current value of the product. The impact of number of products can bear is a function of waveform, amplitude and time, when the current waveform amplitude decreased 50% times of shock can be doubled, so in practical application, the maximum flow rate surge current absorption pressure sensitive resistance should be less than the product.
          3, the application of pressure sensitive resistance
          Application of pressure sensitive resistance
          Figure 1 shows the circuit connection diagram of the circuit surge and transient protection using the voltage sensitive voltage regulator. For the application of the pressure sensitive resistance, can be divided into four types:
          The first type is the connection between the power supply line or the power line and the earth, as shown in Figure 1 (a). As a voltage sensitive resistor, the most representative of the use of the power line and the long distance transmission of the signal line to meet the lightning strike and so that there is a surge in the case of electronic products, such as the protection of the protection of electronic products. General online access between the pressure sensitive resistor can be effective between the lines, and online and to access the pressure sensitive resistance between the transmission line and the ground between the induction pulse effective. If the connection between the wire and the wire connection is combined in two ways, it can absorb the surge pulse better.
          Second types of connections in the load, see Figure 1 (B). It is mainly used to absorb the inductive pulse caused by the sudden opening and closing of the inductive load in order to prevent the component from being damaged. Generally speaking, as long as in parallel with the inductive load on it, but according to the current type and energy of different size, and can be considered RC series absorption circuit combination.
          The third type is the connection between the contacts, see Figure 1 (C). This connection is mainly to prevent the induced charge switch contact arc is burned, and the general contact varistor can be connected in parallel.
          Fourth types are mainly used for the protection of semiconductor devices connected, see Figure 1 (D). This connection is mainly used for thyristor, power transistor and other semiconductor devices, commonly used methods and the protection devices in parallel, to limit the voltage protection device is lower than the rated voltage, which is an effective protection of semiconductor devices.
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