Discuss the behavior of varistors (Voltage Dependent Resistors) and their use in surge protection.
1 Answer
Varistors, also known as Voltage Dependent Resistors (VDRs), are electronic components that exhibit a nonlinear resistance characteristic. Their resistance changes significantly with the applied voltage, making them useful in various applications, especially in surge protection.
Behavior of Varistors:
Varistors are typically made from semiconductor materials, such as zinc oxide, and they have a ceramic-like structure. At low voltages, they behave as high-resistance elements, effectively blocking or limiting the flow of current. As the voltage across the varistor increases beyond a certain threshold, the material's resistance drops dramatically, allowing a significant amount of current to pass through.
This nonlinear behavior arises due to the presence of grain boundaries and impurities within the varistor material. Under normal operating conditions, with voltage levels below the threshold, the varistor has very little impact on the circuit. However, when a transient voltage surge or spike occurs, such as during a lightning strike or power surges, the varistor quickly switches to its low-resistance state, providing an alternative, low-impedance path for the excess current to flow through.
Use in Surge Protection:
One of the primary applications of varistors is in surge protection devices. When electronic devices are exposed to voltage spikes or transient overvoltages, they can be damaged or even destroyed. Varistors are placed in surge protectors and other protective circuits to shunt the excessive voltage away from sensitive components, thereby safeguarding them from damage.
Here's how varistors work in surge protection:
Normal Operation: Under normal operating conditions, the voltage is within the varistor's rated voltage range. At this point, the varistor's resistance is high, allowing only a small leakage current to pass through, which has little effect on the circuit.
Surge Occurs: When a voltage surge or spike exceeds the varistor's threshold voltage, the varistor's resistance rapidly decreases. This behavior is almost instantaneous, providing an effective response to voltage spikes.
Voltage Diversion: Once the varistor enters its low-resistance state, it diverts the majority of the excess current away from the protected circuit and towards ground, effectively limiting the voltage across the sensitive components.
Absorption of Energy: Varistors dissipate the energy of the transient voltage surge as heat while preventing it from reaching the protected components. The energy is absorbed by the varistor without causing significant harm to the circuit.
It's important to note that varistors have a limited capacity to absorb energy, so they can only handle a certain number of surges or a particular energy level. Once their maximum ratings are exceeded, they may fail and lose their protective ability. Therefore, surge protection devices often include additional protective components like fuses and/or resettable fuses (PTCs) to ensure the complete safety of the protected electronics.
Overall, varistors are essential components in surge protection devices, providing a cost-effective and reliable means of protecting sensitive electronic equipment from voltage surges and transients.
Behavior of Varistors:
Varistors are typically made from semiconductor materials, such as zinc oxide, and they have a ceramic-like structure. At low voltages, they behave as high-resistance elements, effectively blocking or limiting the flow of current. As the voltage across the varistor increases beyond a certain threshold, the material's resistance drops dramatically, allowing a significant amount of current to pass through.
This nonlinear behavior arises due to the presence of grain boundaries and impurities within the varistor material. Under normal operating conditions, with voltage levels below the threshold, the varistor has very little impact on the circuit. However, when a transient voltage surge or spike occurs, such as during a lightning strike or power surges, the varistor quickly switches to its low-resistance state, providing an alternative, low-impedance path for the excess current to flow through.
Use in Surge Protection:
One of the primary applications of varistors is in surge protection devices. When electronic devices are exposed to voltage spikes or transient overvoltages, they can be damaged or even destroyed. Varistors are placed in surge protectors and other protective circuits to shunt the excessive voltage away from sensitive components, thereby safeguarding them from damage.
Here's how varistors work in surge protection:
Normal Operation: Under normal operating conditions, the voltage is within the varistor's rated voltage range. At this point, the varistor's resistance is high, allowing only a small leakage current to pass through, which has little effect on the circuit.
Surge Occurs: When a voltage surge or spike exceeds the varistor's threshold voltage, the varistor's resistance rapidly decreases. This behavior is almost instantaneous, providing an effective response to voltage spikes.
Voltage Diversion: Once the varistor enters its low-resistance state, it diverts the majority of the excess current away from the protected circuit and towards ground, effectively limiting the voltage across the sensitive components.
Absorption of Energy: Varistors dissipate the energy of the transient voltage surge as heat while preventing it from reaching the protected components. The energy is absorbed by the varistor without causing significant harm to the circuit.
It's important to note that varistors have a limited capacity to absorb energy, so they can only handle a certain number of surges or a particular energy level. Once their maximum ratings are exceeded, they may fail and lose their protective ability. Therefore, surge protection devices often include additional protective components like fuses and/or resettable fuses (PTCs) to ensure the complete safety of the protected electronics.
Overall, varistors are essential components in surge protection devices, providing a cost-effective and reliable means of protecting sensitive electronic equipment from voltage surges and transients.