Metal oxide varistor
Due to the low cost, varistor is widely used in transient interference absorption devices. The main parameters describing the performance of the varistor are the nominal voltage and the flow capacity of the varistor, ie the surge current absorption capability. The former is a user often easy to get confused with a parameter. Varistor nominal voltage refers to the constant current conditions (outside the diameter of 7mm below the varistor to take 0.1mA; 7mm above the take 1mA) appears in the varistor voltage drop across. Since the varistor has a large dynamic resistance, the voltage at both ends of the varistor (also known as the maximum limiting voltage) at the specified impulse current (usually 8 / 20μs of the standard impulse current) is about the varistor Nominal voltage of 1.8 to 2 times (this value is also known as residual voltage ratio). This requires the user to select the varistor in advance to estimate, it is possible to encounter a large impact current occasions, should choose to use the larger size of the device (varistor current absorption capacity is proportional to the device Flow area, the withstand voltage is proportional to the device thickness, and the absorbed energy is proportional to the device volume). Use a varistor to pay attention to its inherent capacitance. Depending on the dimensions and the nominal voltage, the capacitance is between thousands and hundreds of pF, which means that the varistor is not suitable for use at high frequencies, and is suitable for use in frequency applications such as thyristors and power supplies Into the line for protection purposes. It is important to note that the varistor has a high-speed performance (up to ns) level for transient interference absorption. Therefore, it is necessary to pay attention to the inductance of the lead, and too long leads will introduce inductance due to lead inductance Voltage (in the oscilloscope, the induction voltage was spiked). The longer the lead, the greater the induced voltage. In order to achieve satisfactory interference suppression effect, should try to shorten its lead. On the varistor voltage selection, to be considered the protection of the line may have voltage fluctuations (generally take 1.2 to 1.4 times). If it is an AC circuit, pay attention to the relationship between the voltage rms and the peak. So for the 220V line, the nominal voltage of the selected varistor should be 220 × 1.4 × 1.4 ≈ 430V. In addition, the current absorption capacity of the varistor, 1kA (8 / 20μs of the current wave) used in thyristor protection, 3kA for electrical equipment in the surge absorption; 5kA used in lightning and electronic equipment overvoltage Absorbed on; 10kA used in lightning protection. Varistor voltage grade more suitable for the equipment of the primary or secondary protection.