In the field of electronic circuit design and protection, varistors play a vital role. This component is widely used for its ability to protect circuits from overvoltage surges. To deeply understand the performance and application of varistor, we must first master several of its core parameters: varistor voltage, current capacity, junction capacitance and response time. These parameters jointly define the working characteristics and applicable scope of varistor.
The varistor voltage is the primary parameter for evaluating the performance of a varistor, which marks the voltage threshold at which the varistor switches from a high-resistance state to a low-resistance state. In practice, the determination of this parameter is not intuitive because of the technical challenges of directly measuring the varistor voltage. To solve this problem, the International Electrotechnical Commission developed standards that use specific DC reference currents (1mA and 0.1mA) to define the varistor voltage. Although this method introduces certain human factors, it greatly facilitates the performance evaluation and comparison of varistors.
The current capacity, or maximum peak current value, reflects the maximum current surge value that the varistor can safely withstand. This parameter is directly related to the protection ability of the varistor in practical applications. The industry generally uses the 8/20uS waveform (ie, the standard surge test waveform) to test the current capacity of the varistor. Generally, the manual of the varistor will provide the energy capacity parameters of one and two 8/20uS surge impacts, which provides an important reference for designers to select appropriate protection components.
Junction capacitance is an important electrical characteristic that describes the varistor in the non-conducting state. Since the resistance value of the varistor is extremely large when it is not conducting, its two ends can be regarded as a dielectric, forming a nanofarad-level capacitance. This characteristic means that special care is required when using varistors in high-frequency or digital circuits, as high junction capacitance may affect the proper operation of the circuit.
For a varistor, the definition of response time is slightly different from the general definition. It is not just the time difference between the device being triggered and starting to act. According to the IEEE standard, the response time of a varistor refers to the time difference from the voltage peak point to the 50% residual voltage point. This definition more accurately reflects the dynamic response performance of the varistor in practical applications.