Use of varistor

Varistors are commonly used in parallel within circuits to provide protection against voltage surges. When a sudden voltage change occurs, the varistor conducts, causing a short circuit that trips the fuse and protects the circuit. They are widely applied in power supply systems for overvoltage protection and regulation. The performance and safety of a power surge protector largely depend on the correct application of the varistor. Several key principles should be considered during design. These include factors such as the quality of the power supply, the frequency and intensity of lightning strikes, the installation conditions of the equipment, and its tolerance to electrical stress. It's essential that lightning protection devices are tested under real-world conditions or simulated environments as close as possible to actual usage. 1. **Calculation of Varistor Voltage** The varistor voltage (U1mA) can typically be calculated using the formula: **U1mA = K × Uac**, where K is a coefficient related to power quality. Generally, K ranges from 2 to 3. In areas with stable power quality, K may be smaller, while in rural or mountainous regions with less stable power, K might be higher. For a 220V–240V AC power supply, a varistor with a clamping voltage between 470V and 620V is recommended. Choosing a slightly higher clamping voltage can reduce failure rates and extend service life, although it may result in a slightly higher residual voltage. 2. **Calculation of Nominal Discharge Current** The nominal discharge current of the varistor should exceed the expected surge current or the maximum surge that could occur annually. It’s advisable to calculate this based on the surge resistance life cycle, ensuring it’s at least 10 times greater than the rated value. This corresponds to about 30% of the maximum surge current (0.3 × IP). 3. **Parallel Connection of Varistors** If a single varistor does not meet the required discharge current, multiple varistors should be connected in parallel. Sometimes, even when the discharge current is sufficient, multiple varistors are used in parallel to lower the clamping voltage. However, it is crucial to select varistors with matching parameters (e.g., ΔU1mA ≤ 3V, Δα ≤ 3%) to ensure even current distribution and prevent uneven stress on individual components.

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