Varistor voltage and pressure ratio are affected by temperature

A varistor is a resistive device with nonlinear volt-ampere characteristics. It is mainly used to clamp the voltage when the circuit is under overvoltage, and to absorb excess current to protect sensitive devices. Nowadays, the varistor is widely used, and its main material is composed of a divalent element zinc and a hexavalent element oxygen. It is a voltage limiting type protection device, and utilizes the nonlinear characteristics of the varistor when an overvoltage occurs. Between the two poles of the varistor, the voltage is clamped to a relatively fixed voltage value to protect the latter circuit. In many cases, it should not be directly applied to the protection of high-frequency signal lines. When it is used in the protection of AC circuits, it will increase the leakage current because of its large junction capacitance. It needs to be fully considered when designing the protection circuit.

At a temperature of less than 180 ° C, the varistor voltage changes little with temperature, and the rate of change does not exceed five percent. The varistor voltage is almost unaffected by temperature from a room temperature to a current density of 188 ° C. This phenomenon can be very Conductive theory that fits well with the breakdown effect of the varistor. When the temperature exceeds 200 °C, the varistor voltage and nonlinearity are obviously affected by the temperature. As the temperature increases, the varistor voltage decreases parabolically, and the voltage ratio rises parabolically (non-linearly reduced), at about 280C. The varistor voltage change rate is 50% to 70%, the voltage ratio becomes about 10, the nonlinear index becomes 1, and the VA characteristic is nearly linear. After the temperature exceeds 300C, the varistor voltage changes faster, and the varistor voltage is reduced by half for every 10C rise.

It can be understood that below 180C, the thermal excitation current in the breakdown region is negligible relative to the current, and the conduction mechanism is dominated and dominated by the current, but above 200C, as the temperature increases, the thermal excitation current in the breakdown region increases and the current does not. Change, lead to the thermal excitation current varistor 1mA DC current can be compared with the current, the effect of temperature on the varistor voltage is caused by the thermal excitation current. As the temperature increases, the amount of thermally excited electrons in the depletion layer increases, which corresponds to an increase in the donor concentration, and accordingly the barrier height decreases at a high temperature, which also causes a decrease in the varistor voltage.