How does a varistor protect against voltage surges?
1 Answer
A varistor, also known as a voltage-dependent resistor (VDR) or metal-oxide varistor (MOV), is an electronic component used to protect electrical and electronic devices against voltage surges and transient overvoltages. It works based on its nonlinear voltage-current characteristic.
When a varistor is exposed to a normal operating voltage, it behaves as an insulator with high resistance, allowing only a tiny leakage current to flow through it. However, when a voltage surge or transient overvoltage occurs, the varistor's resistance decreases significantly, allowing a much larger current to flow through it. This effect is often referred to as "clamping" or "crowbar" action.
Here's how a varistor protects against voltage surges:
Normal operation: Under normal operating conditions, the voltage across the varistor remains within its specified range. The varistor maintains a high resistance, offering little impedance to the normal current flow, and thereby ensuring minimal impact on the protected circuit.
Voltage surge detection: When a voltage surge occurs (e.g., lightning strike, power spikes, switching transients), the voltage across the varistor increases beyond its rated threshold level. At this point, the varistor starts conducting heavily.
Voltage clamping: As the voltage increases, the varistor enters its nonlinear region and begins to conduct significantly. The varistor's resistance drops dramatically, allowing a surge current to flow through it. By conducting this excess current, the varistor effectively "clamps" the voltage to a safe level, thereby protecting the connected circuit.
Energy absorption: The varistor dissipates the energy from the surge as heat. This energy absorption capability prevents the excessive voltage from reaching and damaging sensitive components within the protected circuit.
Recovery: Once the transient voltage subsides or the surge event ends, the varistor returns to its high-resistance state, effectively disconnecting itself from the circuit. This allows normal operation to resume without affecting the protected devices.
Varistors are commonly used in surge protection devices, such as surge protectors and power strips, to safeguard electronic equipment and appliances from voltage spikes that could otherwise lead to damage or malfunction. They are designed to have a specific voltage rating, and their protective action is dependent on the magnitude and duration of the surge they encounter.
When a varistor is exposed to a normal operating voltage, it behaves as an insulator with high resistance, allowing only a tiny leakage current to flow through it. However, when a voltage surge or transient overvoltage occurs, the varistor's resistance decreases significantly, allowing a much larger current to flow through it. This effect is often referred to as "clamping" or "crowbar" action.
Here's how a varistor protects against voltage surges:
Normal operation: Under normal operating conditions, the voltage across the varistor remains within its specified range. The varistor maintains a high resistance, offering little impedance to the normal current flow, and thereby ensuring minimal impact on the protected circuit.
Voltage surge detection: When a voltage surge occurs (e.g., lightning strike, power spikes, switching transients), the voltage across the varistor increases beyond its rated threshold level. At this point, the varistor starts conducting heavily.
Voltage clamping: As the voltage increases, the varistor enters its nonlinear region and begins to conduct significantly. The varistor's resistance drops dramatically, allowing a surge current to flow through it. By conducting this excess current, the varistor effectively "clamps" the voltage to a safe level, thereby protecting the connected circuit.
Energy absorption: The varistor dissipates the energy from the surge as heat. This energy absorption capability prevents the excessive voltage from reaching and damaging sensitive components within the protected circuit.
Recovery: Once the transient voltage subsides or the surge event ends, the varistor returns to its high-resistance state, effectively disconnecting itself from the circuit. This allows normal operation to resume without affecting the protected devices.
Varistors are commonly used in surge protection devices, such as surge protectors and power strips, to safeguard electronic equipment and appliances from voltage spikes that could otherwise lead to damage or malfunction. They are designed to have a specific voltage rating, and their protective action is dependent on the magnitude and duration of the surge they encounter.