Use of varistor

Varistors are commonly used in parallel within circuits to provide protection against sudden voltage changes. When a surge occurs, the varistor conducts, creating a short circuit that triggers the fuse to cut off the power, thereby protecting sensitive components. These devices are widely applied in power supplies for overvoltage protection and regulation. The effectiveness of a power supply surge protector largely depends on the proper selection and application of the varistor. Several key principles should be considered during the design process. For example, factors such as power quality, frequency of lightning strikes, installation location, and equipment tolerance must all be taken into account. It is essential to test and verify the performance of the lightning protection system under real-world conditions or simulated environments as close as possible to actual usage scenarios. **1. Varistor Voltage Calculation:** A standard formula is often used to calculate the varistor voltage: **U1mA = K × Uac** Where: - **K** is a coefficient based on power quality. In areas with stable power supply, K can be around 2, while in regions with poor power quality (especially rural or mountainous areas), it may go up to 3. - **Uac** refers to the RMS value of the AC power supply. For a 220V–240V AC power supply, a varistor with a clamping voltage between 470V and 620V is typically recommended. Choosing a higher varistor voltage can reduce failure rates and extend service life, although it may slightly increase the residual voltage. **2. Nominal Discharge Current Calculation:** The nominal discharge current of the varistor should exceed the maximum expected surge current or the highest surge that could occur annually. To ensure reliability, it's advisable to calculate this based on the shock resistance life cycle from the manufacturer’s data sheet. A typical recommendation is to use at least 30% of the peak impulse current (i.e., 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. Even when the nominal current requirement is met, some engineers choose to connect them in parallel to further reduce the clamping voltage. However, it is crucial to select varistors with matching parameters—such as ΔU1mA ≤ 3V and Δα ≤ 3—to ensure even current distribution and avoid uneven stress that could lead to premature failure.

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