Working principle and characteristics of varistor
Time：2020.08.27
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Varistor is a kind of voltage sensitive element made by using the nonlinear volt-ampere characteristics of semiconductor materials.
The figure below shows the volt-ampere characteristic curve of the varistor. It can be seen that it is a symmetrical non-linear curve. When the applied voltage is low, the current flowing through the resistor is very small, and the varistor is high resistance. State; when the applied voltage reaches or exceeds the varistor voltage Uc, the resistance of the varistor drops sharply and quickly turns on, and its working current will increase by several orders of magnitude, thus effectively protecting other components in the circuit from being damaged. Damaged by overvoltage.
Volt-ampere characteristic curve of varistor
The figure below shows the microstructure of a zinc oxide varistor, which includes zinc oxide (ZnO) grains and a grain boundary layer around the grains. The resistivity of zinc oxide crystal grains is very low, while the resistivity of the grain boundary layer is very high. A varistor cell is formed between two adjacent crystal grains. The breakdown voltage of each cell is about 3.5V. In the varistor, many such units are connected in series and parallel to form the base of the varistor. The more units connected in series, the higher the breakdown voltage; the larger the cross-sectional area of the matrix, the larger the current capacity.
Microstructure of zinc oxide varistor
The varistor is usually connected in parallel to the input end of the protected appliance in the circuit, as shown in the figure below.
Protection circuit composed of varistor
It can be seen from the figure that the impedance Zv of the varistor and the total impedance of the circuit (including the surge impedance Zs) form a voltage divider, so the limit voltage of the varistor can be determined by the following formula:
Vc = VsZv/(Zs+ Zv)
Where: Vc limit voltage;
Vs-surge voltage;
Zv----The impedance of the varistor, which can be reduced from a few megohms of normal value to a few ohms, or even less than 1Ω;
Zs----The total resistance of the circuit.
It can be seen from the above formula that when Zv flows a large current instantaneously, most of the instantaneous overvoltage falls on Zs, and the voltage obtained by the protected electrical appliance is below its withstand voltage, so it can play a protective role.
Operating characteristic curve of varistor
The working characteristic curve of the varistor is shown in the figure above, through which the protective effect of the varistor on overvoltage can be seen more clearly. The straight line is the load line determined by the total impedance Zs of the circuit. The curve is the volt-ampere characteristic curve of the varistor. The intersection point Q of the two is the protection operating point, and its corresponding limit voltage is, it uses a varistor The working voltage applied to electrical appliances after the device. Vs is the surge voltage, which has exceeded the withstand voltage Vl of the electrical appliance. After adding the varistor, the working voltage V is less than Vl, which effectively protects the electrical appliances.
