“varistor” is a type of resistor with nonlinear voltage-current characteristics, primarily used for voltage clamping in circuits when overvoltage occurs. It absorbs excess current to protect sensitive components. The English name for this device is “Voltage Dependent Resistor,” often abbreviated as “VDR,” or simply called a “Varistor.” The resistive material in a varistor is a semiconductor, making it a type of semiconductor resistor. Currently, zinc oxide (ZnO) varistors are widely used, and their main constituent materials are the divalent element zinc (Zn) and the hexavalent element oxygen (O). From a material perspective, zinc oxide varistors are classified as “II-VI group oxide semiconductors.” In Taiwan, varistors are referred to as “surge absorbers” or sometimes as “electrical surge suppressors.”
Characteristics and Applications of Varistors
Nonlinear Voltage-Current Characteristics: Varistors utilize their nonlinear properties to clamp the voltage to a relatively fixed value when an overvoltage appears across their terminals, thereby protecting the downstream circuit.
Key Parameters:
Varistor Voltage (V1mA): The voltage at which the varistor begins to conduct significantly.
Surge Current Rating: The maximum current the varistor can handle without damage.
Junction Capacitance: Typically in the range of hundreds to thousands of picofarads (pF).
Response Time: On the order of nanoseconds (ns), faster than gas discharge tubes but slightly slower than TVS diodes. This response time is generally sufficient for overvoltage protection in electronic circuits.
Applications and Considerations:
High-Frequency Signal Lines: Due to their relatively high junction capacitance, varistors are not typically suitable for direct use in high-frequency signal line protection. The capacitance can introduce additional leakage currents.
AC Circuit Protection: When used in AC circuit protection, the higher junction capacitance can increase leakage currents. This must be carefully considered during the design of protective circuits.
Surge Current Capacity: Varistors have a significant surge current capacity, though it is generally lower than that of gas discharge tubes.
Summary:
Boarden varistors, also known as VDRs, are voltage-sensitive, nonlinear overvoltage protection semiconductor devices. They are widely used to protect sensitive electronic components from overvoltage conditions by clamping the voltage to a safe level. With their fast response times and robust surge current ratings, varistors are essential in a variety of applications, including consumer electronics, industrial control systems, and communication equipment. However, their use in high-frequency and AC circuits requires careful consideration due to their junction capacitance.
