What is a varistor
Varistor means "within a certain current-voltage range, the resistance value changes with voltage", or that "the resistance is sensitive to voltage". The corresponding English name is "Voltage Dependent Resistor", abbreviated as "SDE".
As the applied voltage increases, its resistance can change from the MΩ level to the mΩ level. At low voltages the varistor operates in a leakage current region, showing very high resistance and very small leakage current. When voltage rises into the nonlinear region, current changes over a large range while the voltage changes little, giving a good clamping characteristic. If the voltage increases further the varistor enters a saturation region and behaves like a small linear resistance; because the current is large, prolonged operation can overheat and burn out or even cause bursting. Under normal use the varistor stays in the leakage region and only enters the nonlinear region to discharge surge current during transient events; it should not enter the saturation region during normal surge protection.
The resistive material of a varistor is a semiconductor, so it is a type of semiconductor resistor. Commonly used zinc oxide varistors are composed of the divalent element Zn and oxygen O. From a materials perspective, zinc oxide varistors are II-VI oxide semiconductors.
Varistor failure symptoms
When a varistor is damaged or fails, the following common symptoms may appear:
- Insensitive voltage response: Normally a varistor responds to voltage and its resistance drops sharply when voltage exceeds a certain range to protect the circuit. A damaged varistor may not respond properly to voltage changes, resulting in insensitive voltage response and inability to protect the circuit.
- Voltage fluctuation or overvoltage: If a varistor fails, it may cause large voltage fluctuations or fail to effectively limit overvoltage conditions. This can damage other components in the circuit or lead to malfunction or power supply failure.
- Burning or short-circuit: When a varistor is damaged it may burn out or short. This is usually due to internal structural damage or failure of the thermally sensitive material. In such cases the varistor may emit noticeable smoke, odor, or burn marks, and may cause circuit interruption or short circuit.
- Resistance value change: A damaged varistor may show a changed resistance value. This means the resistance may become unstable or cannot be measured correctly under normal conditions.
- Physical damage: A damaged varistor may show visible physical damage such as cracks, deformation, or breakage. These signs, together with other symptoms, indicate the varistor has failed.
If a varistor is suspected to be damaged or failed, replacement or further testing and diagnosis is recommended to ensure correct circuit operation and protection.
Differences between varistor and thermistor
Varistors (Varistor) and thermistors (Thermistor) are two different types of resistive devices with different operating principles and applications. The main differences are:
- Principle: The varistor is described here as operating based on the piezoelectric effect, responding to and limiting voltage changes. When voltage exceeds a specific range its resistance drops sharply to protect the circuit. The thermistor operates based on the temperature dependence of resistance; its resistance changes with temperature.
- Characteristics: Varistors mainly respond to and limit voltage to protect other components from overvoltage damage and are commonly used as overvoltage protection devices. Thermistors are used for measuring and controlling temperature; their resistance change with temperature provides temperature information and is used in temperature compensation and control applications.
- Types: Varistors are typically nonlinear and, depending on the varistor material, include zinc oxide varistors and polymer varistors. Thermistors are divided into negative temperature coefficient (NTC) and positive temperature coefficient (PTC) types. NTC thermistors decrease resistance as temperature rises, while PTC thermistors increase resistance as temperature rises.
- Applications: Varistors are used in electronic circuits and power systems for overvoltage protection and voltage stabilization, commonly found in power supplies, measuring instruments, and communication equipment. Thermistors are mainly used for temperature measurement and control, such as thermometers, thermostats, and temperature compensation circuits.
