The varistor, also known as a Metal Oxide Varistor (MOV), is a nonlinear resistor component primarily made of zinc oxide (ZnO). It has high surge current tolerance and a large nonlinear coefficient. When the voltage across it is below the threshold, its resistance is very high, allowing almost no current to flow. However, once the voltage exceeds the threshold, the resistance drops significantly, enabling the surge current to be discharged safely. Due to this behavior, varistors are widely used as protective elements in electronic and electrical systems, playing a crucial role in absorbing abnormal voltages and lightning surges.
Varistors are typically connected in parallel within a circuit. When an abrupt voltage change occurs, the varistor can short-circuit, triggering a fuse to protect the system. They are commonly used for overvoltage protection in power supplies and other circuits.
**Varistor Parameters**
1. **Varistor Voltage UN (U1mA):** This is the voltage at which 1 mA of DC current flows through the varistor, indicating the conduction voltage. It is often referred to as U1mA, with a typical tolerance of ±10%. In testing, if the voltage drops by 10%, it may indicate failure.
2. **Maximum Continuous Working Voltage UC:** This refers to the highest AC or DC voltage the varistor can handle continuously. For AC, it is approximately 0.64 × U1mA, and for DC, around 0.83 × U1mA.
3. **Maximum Clamp Voltage VC:** This is the voltage that appears on the varistor when a specified 8/20 μs impulse current is applied.
4. **Leakage Current Il:** This is the current that flows when the maximum DC voltage is applied. Typically, 0.83 × U1mA or 0.75 × U1mA is used for measurement. Static leakage current is usually required to be ≤ 20μA (sometimes ≤ 10μA). Stability after impact or under high temperature is more important than the actual value.
5. **Impulse Current and Repeated Impact Times:** These define the varistor's ability to handle repeated surges without degradation.
**Usage Tips**
1. **Varistor Voltage Calculation:** The varistor voltage (U1mA) can be calculated using U1mA = K × Uac, where K depends on the power quality. For urban areas, K is around 2–3, while in rural or mountainous regions, it may be higher. For a 220–240V AC supply, a varistor with a voltage range of 470–620V is typically selected.
2. **Nominal Discharge Current:** The varistor’s nominal discharge current should exceed the expected surge current. It is recommended to use 30% of the maximum impact current (0.3 × IP) for safety.
3. **Parallel Connection:** If a single varistor cannot meet the current requirements, multiple units can be used in parallel. It’s essential to select varistors with similar parameters (e.g., ΔU1mA ≤ 3V, Δα ≤ 3) to ensure even current distribution.
**Safety Considerations**
A thermal fuse should be used alongside the varistor to isolate it in case of failure. If the varistor shorts due to a high-voltage surge, the thermal fuse will blow, protecting the system. To prevent overheating, a power frequency fuse can be added in series.
Due to its high parasitic capacitance, a varistor can generate significant leakage current in AC systems. Over time, this can cause the varistor to overheat and potentially explode. To reduce leakage, a gas discharge tube is often placed in series with the varistor. This setup minimizes total capacitance and isolates the varistor during normal operation. However, it increases response time. Adding a fast-acting varistor like R1 helps maintain quick response times.
Common surge protection devices include gas discharge tubes, MOVs, silicon transient voltage suppressors, and solid-state surge protectors. In AC power systems, a combination of gas discharge tubes and varistors is typically used for effective lightning protection.
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