The varistor, also known as a Metal Oxide Varistor (MOV), is a nonlinear resistor made primarily of zinc oxide (ZnO). It has a high surge current capacity and a strong nonlinear coefficient. When the voltage across it is below its threshold, the resistance is very high, allowing almost no current to flow. However, once the voltage exceeds this threshold, the resistance drops dramatically, allowing large currents to be discharged safely. This unique property makes the varistor an essential component for protecting electronic and electrical equipment from overvoltage events, such as lightning surges or power line disturbances.
Varistors are typically connected in parallel within a circuit. If a sudden voltage spike occurs, the varistor conducts, causing a fuse to blow and protecting the rest of the circuit. They are widely used in power supply systems for overvoltage protection and regulation.
**Key Varistor Parameters:**
1. **Varistor Voltage UN (U1mA):** This is the voltage at which 1 mA of DC current flows through the varistor, indicating its conduction voltage. It's commonly referred to as U1mA, with an error margin of about ±10%. In testing, if the voltage drops by 10%, it may indicate failure.
2. **Maximum Continuous Working Voltage UC:** This is the maximum AC or DC voltage the varistor can handle continuously. Typically, Uac ≈ 0.64 × U1mA and Udc ≈ 0.83 × U1mA.
3. **Maximum Clamp Voltage VC:** This is the voltage across the varistor when a specific 8/20 μs impulse current is applied.
4. **Leakage Current Il:** This is the current that flows through the varistor under a maximum DC voltage. It’s usually measured at 0.83 × U1mA. A stable leakage current is more important than its absolute value, especially after stress tests or exposure to high temperatures.
5. **Impulse Current and Repeated Impact Times:** These define how much surge current the varistor can handle and how many times it can endure such events before degradation.
**Usage Tips:**
1. **Varistor Voltage Calculation:** The varistor voltage can be calculated using the formula: U1mA = K × Uac, where K depends on the quality of the power supply. For urban areas, K is around 2–3, while in rural or mountainous regions, it might be higher.
2. **Nominal Discharge Current:** The nominal discharge current should exceed the expected surge current. It’s recommended to use 30% of the maximum impact current for safety.
3. **Parallel Connection:** If a single varistor doesn’t meet the required discharge current, multiple units can be connected in parallel. Ensure they have similar parameters (e.g., ΔU1mA ≤ 3V) to ensure even current distribution.
A temperature fuse should be thermally coupled with the varistor. In case of failure, the heat generated will trigger the fuse, isolating the varistor from the circuit. However, in some cases, the fuse may not respond quickly enough. To prevent fire risks, a power frequency fuse can be added in series.
Due to its high parasitic capacitance, the varistor can generate significant leakage current in AC systems. Over time, this can cause overheating and even self-destruction. To reduce this effect, a gas discharge tube can be placed in series with the varistor. This setup minimizes the total capacitance and isolates the varistor during normal operation. However, it increases the response time. Adding a fast-acting varistor in parallel helps reduce this delay.
Common surge protection devices include gas discharge tubes, MOVs, silicon transient voltage suppressors, and solid-state protectors. In AC power systems, a combination of a gas discharge tube and a varistor is often used for effective lightning protection.
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