I. Introduction: The PolySwitch self-recovery fuse is a new component manufactured by Raychem, USA, made of a polymer-doped conductor. Polymer-based conductive composites tend to have a positive temperature coefficient (PTC) effect, that is, the electrical resistance of the material increases with temperature, and increases sharply near the melting point of the polymer, with thermal switching characteristics. Second, the main content of the program: 1. PolySwitch component action protection features 2. Establishment of protection mathematical model 3. Determine the critical point position 4. PolySwitch component action protection features Third, the establishment of mathematical models: In the coordinate system shown in Figure 1, the PolySwitch component action protection characteristic curve is formed by connecting the curved portion and the straight line portion, and the mathematical model can be established separately. Taking the curve of M=RXE160 as an example, the specific steps of establishing a mathematical model are as follows. Table 3 Experimental data:
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The PolySwitch component rises sharply under overcurrent conditions, the resistance increases rapidly, the loop current is reduced to a small enough size, and the circuit acts as an open circuit, thus protecting the circuit. When the overcurrent disappears, the temperature of the PolySwitch component decreases and the resistance returns to normal to allow the circuit to operate normally. PolySwitch components are widely used in the civil and industrial fields due to their automatic recovery characteristics and the need to replace them in general. They have many advantages over conventional fuses.
PolySwitch components are non-linear components and are sensitive to ambient temperature. Different components are used for different applications. The usual method of selecting a PolySwitch component is to use a look-up table method, which has complicated procedures and low precision. This paper analyzes the action protection characteristic curve given in the PolySwitch component manual, and establishes the mathematical model of the action protection characteristics of the PolySwitch component according to its characteristics. Through experimental verification and curve comparison, it is proved that the simulation curve drawn according to the mathematical model is consistent with the shape of the action protection characteristic curve given in the PolySwitch component manual, and is basically consistent with the measured data.
5.PolySwitch components are available in a variety of products with different product characteristics. Taking the commonly used RXE series components as an example, the electrical characteristics at 20 ° C are shown in Table 1.
Table 1. Electrical characteristics of the RXE series components at 20 °C: 
The meanings of the symbols in Table 1 are as follows:
IH: Maximum operating current at 20 ° C ambient temperature.
IT: The minimum current that the PolySwitch component initiates protection at 20 °C ambient temperature.
Vmax: Maximum operating voltage of the PolySwitch component.
Imax: The maximum current that the PolySwitch component can withstand.
PDMAx: The maximum power consumption of the PolySwitch component in its operating state.
Rmax: The initial maximum resistance of the PolySwitch component before it is activated.
Rmin: The initial minimum resistance of the PolySwitch component before it is activated.
It can be seen from Table 1 that the maximum current that the series of components can withstand is 40 amps, and the minimum current for starting protection during faults is twice the maximum operating current.
The action protection characteristic curve of the RXE series PolySwitch component at 20 °C is shown in the figure. The abscissa indicates the fault current in amps, and the ordinate indicates the action time in seconds. Both axes are logarithmic. Each curve in the figure corresponds to a component of a model, and constitutes a cluster of action protection characteristic curves of the series of components. Table 2 is a comparison table of the curve numbers and component models in Fig. 1.
Fig.1 Action protection characteristic curve of RXE series components at 20 °C In the logarithmic coordinate system shown in Fig. 1, each curve can be regarded as a connection between a curved part and a straight line part, and the critical point position of the two parts follows the component The model varies. Since the trend of the curves of the various types of components is basically the same, the method of establishing the mathematical model is also the same.
Table 2 Curve label and component model comparison table ,
Fourth, determine the critical point position:
By measuring the curve in Fig. 1, the critical point position of the curved portion and the straight portion of the action protection characteristic curve of the PolySwitch element can be obtained. For the curve of M = RXE160, the measured critical point position is about I = 4.3A.
The straight line is shown in Table 1. The maximum current that the RXE series PolySwitch components can withstand is 40A. The point between the point and the critical point is the straight line of the curve. Let the action time be t (seconds) and the fault current be I (an). Considering that the horizontal axis and the vertical axis are logarithmic coordinates, the following mathematical relationship should exist between them:
Where a and b are constants. Take a special point from the curve of the straight line in Figure 1, and perform mathematical processing by computer to get the specific values ​​of a and b:
a=813, b=22.54,
Substituting (1), you can get the mathematical formula of the straight line:
In the curve part, since the PolySwitch element has a PTC effect, the mathematical model of the action protection characteristic curve part can be established by referring to the temperature resistance characteristic of the PTC device. The mathematical model of this type of property can take the form of a power function or a logarithmic function. Let its mathematical formula have the following form;
Where t0, I0, and b are constants. Take some special points from the curve part curve of Fig. 1, and perform mathematical processing by computer to get the specific values ​​of t0, I0, b:
T0=-38.93, I0=4.42, b=-1.83,
Substituting (3), you can get the mathematical formula of the curve part:
(2) Equations and (4) form a mathematical model of the action protection characteristics of the RXE160 component. The two curves are connected smoothly at the critical point position.
V. Conclusion:
In order to verify the correctness of the mathematical model of the RXE160 component action protection characteristics, the RXE160 component is connected to the DC power supply protection circuit for actual testing, and the experimental data is shown in Table 3.
The experimental data is plotted in the same coordinate system as the mathematical model curve, as shown in Figure 2. The curves in the figure are simulation curves drawn according to equations (2) and (4), and the points in the graph are measured data points. For the convenience of comparison, the horizontal and vertical axes of the curve are logarithmic. The software used is Matlab6.0. Comparing Fig. 1 and Fig. 2, the simulation curve is consistent with the shape of the action protection characteristic curve given in the PolySwitch component manual, and is basically consistent with the measured data points. Mathematical models of other model curves can be established separately as described above.
Figure 2 RXE160 motion protection characteristics simulation curve (20 ° C):
Due to the accuracy of the component characteristic curve, it is inconvenient to select the PolySwitch component by using the look-up table method, and the precision is not high. According to the characteristics of the action protection characteristics of PolySwitch components, this paper establishes a mathematical model of the action protection characteristics of PolySwitch components, which solves this problem well. Experiments show that the simulation curve is consistent with the shape of the action protection characteristic curve given in the PolySwitch component manual, and the measured data is basically consistent with the curve drawn according to the mathematical model. 
