Selection scheme of pressure sensitive resistance:
Prior to use of the pressure sensitive resistor, we should first understand the following relevant technical parameters: nominal voltage is defined as the temperature and the DC current, the voltage value of the two ends of the voltage sensitive resistor. The leakage current is the current value flowing through the pressure sensitive resistor at 25 C, when the maximum continuous DC voltage is applied. Grade refers to the varistor voltage through a voltage peak appeared in the process of two of the 8 / 20 grade when the current pulse. The peak current of the pulse current (8 / 20 s) waveform is shown by the performance of the flow. Surge parameter contains the maximum surge current Ipm (or the maximum surge voltage and surge impedance of Vpm source Zo), surge pulse width of Tt, the two adjacent surge minimum time distance Tm and the varistor appointment things in the life of the total number of N surge pulse.

1, the nominal voltage
As usual, and often cover varistor device or assembly using in parallel, arrhythmia, varistors or exchange two DC voltage should be lower than the nominal voltage, even in the worst case power bumps, nor the additional value is higher than the maximum continuous voltage in the choice of things, the maximum continuous voltage value of things the nominal voltage value corresponding to the selected value is. To deal with the use of overvoltage protection, pressure sensitive voltage value should be greater than the actual value of the circuit, the same as usual should be used to stop the choice:
VmA = AV / BC
Type A: the circuit voltage turbulence coefficient, usually 1.2; V for DC voltage (the exchange of useful things for the b value); breakdown voltage deviation, usually from 0.85 to C; the aging coefficient of components, usually take 0.9;
This calculated VmA realistic value is 1.5 times the DC voltage, in the exchange situation also consider the peak, the results should be extended 1.414 times. In addition, the election should also have to pay attention to:
(1) it is necessary to ensure that the maximum allowable value of the voltage is not the maximum allowable value, otherwise the use of extended pressure sensitive resistance will be extended;
(2) in the power supply line and the earth between the use of pressure sensitive resistance, and occasionally because of the ground to make the line between the voltage rise, in order to adopt more than the line between the lines and the use of a higher voltage between the nominal voltage.
The surge current received by the pressure sensitive resistor should be less than the maximum flow rate of the product. Application scheme of pressure sensitive resistance:
Circuit for circuit surge and transient protection. The varistor against cohesion, probably can be divided into four types:
The first example is the connection between the power line or between the power line and the earth, as the varistor, most representative is the place with the help of the lightning hit in the power line and signal line long distance transmission which leads the surge pulse condition of the electronic products from the cover of sense. As usual, the voltage between the lines can be used to feel the pulse, and the voltage between the ground and the ground is used to connect the transmission line and the earth. If the line between the two forms of convergence and convergence of the line together, you can have a better effect on the surge pulse.
Second examples for the load in the convergence, it is important for the rational load suddenly open and close to cause the impulse to stop receiving, in order to avoid damage to the components. As usual, just in parallel to the rational load on the can, but based on the type of current and energy differences, can be considered with C - R serial connection to the circuit application.
The third example is the connection between the contacts, this kind of connection weight if in order to avoid the charge generated by arc switch contacts are burned, as usual and contact varistors can be connected in parallel.
Fourth important examples for covering the cohesion of semiconductor devices, this kind of method for controllable silicon, an important cohesive power transistor and semiconductor device, method usually adopted and cover devices in parallel, to limit the voltage is lower than the voltage grade devices are protected, the semi conductor device is a useful.

2, Zinc Oxide, the results of the existence of the pressure sensitive resistance
The existing pressure sensitive resistance is divided into two categories, which can not be replaced by each other in the formula and the function:
2.1, low voltage type pressure sensitive resistance
Pressure sensitive resistance, its advantage is the high voltage gradient (100 ~ 250V/ sister) and high current characteristics (V10kA/V1mA < 1.4) but only for the narrow pulse width (2 ms) overvoltage and surge protection to aspiring, energy density is low (50 ~ 300) J/cm3.
2.2, high energy type pressure sensitive resistance
High energy varistors, its advantages are the large energy density (300J/cm3 ~ 750J/cm3), suffered a long pulse surge ability, but the voltage gradient is low (20V/ ~ 500V/ sister sister), high current feature difference (V10kA/V1mA>2.0).

The function of the differences between the two formulations of the formation of a lot of the utilization of the "dead zone" in the transmission and distribution system 10kV voltage grade in universal adoption of vacuum switch, because it moves fast, small arc, in the form of high control Shashi overvoltage and surge energy, if the selection of low-voltage varistors to cover (such as lightning of course, it is) high voltage gradient, low cost, but the energy capacity is small, easily damaged; if selecting high pressure sensitive resistor, while its energy capacity, long service life, but the voltage gradient is low, the cost is too high, the former is 5 ~ 13 times.
In the medium and small power frequency conversion power supply, the object of the overvoltage protection is the power semiconductor device, which is very strict to the request of the high current characteristic and the energy capacity of the pressure sensitive resistance, and to achieve the miniaturization of the components simultaneously. High energy varistors can satisfy the request in energy capacity, but the current function is not ideal, the residual pressure of small diameter element is relatively high, often not up to the pressure limiting request; high current characteristics of pressure sensitive resistance is good, easy miniaturization, but energy capacity is not enough, not up to the request of energy absorption. Medium and small power frequency conversion power supply in this category of the use of the pressure sensitive resistance almost vacant.