1.Voltage Dependent Resistor (VDR)
A voltage-dependent resistor, commonly known as a varistor, is a non-linear resistive device used to clamp voltage during overvoltage conditions and absorb excess current to protect sensitive components. The main material for modern VDRs is zinc oxide (ZnO), which is composed of divalent zinc and hexavalent oxygen. Therefore, from a material perspective, zinc oxide varistors can be classified as II-VI compound semiconductors, and are often referred to as Metal Oxide Varistors (MOV). VDRs respond in nanoseconds, faster than gas discharge tubes but slightly slower than Transient Voltage Suppressors (TVS). They effectively handle surge currents and are widely used for overvoltage protection in various electronic circuits, including ESD and lightning strikes.
2.Transient Voltage Suppressor (TVS)
A transient voltage suppressor (TVS) operates similarly to a Zener diode. When the voltage exceeds its breakdown voltage, the TVS diode conducts, providing a path for excess current. TVS diodes have a higher current-carrying capacity compared to Zener diodes. When subjected to reverse transient high-energy shocks, TVS diodes quickly transition from high impedance to low impedance, absorbing surge power of several kilowatts within a time scale of 10^{-12} seconds. This rapid response clamps the voltage across the device to a predetermined level, protecting downstream circuit components from transient high-voltage spikes. TVS diodes are commonly used due to their fast response times (sub-nanosecond) and significant surge absorption capabilities.
3. Gas Discharge Tube (GDT)
Gas discharge tubes (GDTs) consist of two or more metal electrodes with gaps, filled with inert gases like argon and neon. When the voltage across the electrodes reaches a level sufficient to ionize the gas, the GDT begins to conduct, transitioning from high to low impedance. This effectively short circuits the surge voltage to near zero, diverting excess current to the ground and providing protection for subsequent circuitry. While GDTs have a strong surge absorption capability under leakage current, their response time is slower, on the microsecond scale, which often necessitates pairing them with faster protective devices in high-speed applications.
4.Thyristor Surge Suppressor (TSS)
Thyristor surge suppressors (TSS) are solid-state surge protection devices resembling thyristors. They consist of a four-layer semiconductor structure with three PN junctions. When the breakdown current triggers the device, it conducts for a specific time, allowing significant surge or pulse current to flow while maintaining a low impedance state. Once the surge pulse has passed, the voltage must drop below a certain threshold to return to an open-circuit state. TSS devices are manufactured using ion implantation technology and offer precise triggering, fast response (nanosecond level), strong surge absorption, bidirectional symmetry, and high reliability.
5. Polymeric Resettable Fuse (PTC)
A polymeric resettable fuse, or PTC (Positive Temperature Coefficient) device, is an electronic overcurrent protection component made from high molecular organic polymers that incorporate conductive particles through a specialized process under high pressure and temperature. Traditional fuses provide one-time overcurrent protection and need replacement after tripping, whereas PTCs offer dual functionality: overcurrent and overheat protection with the ability to reset automatically. PTCs exhibit a significant increase in resistance over a narrow temperature range during high current flow, showcasing the nonlinear PTC effect. Various conductive polymers display this effect, making PTCs particularly suitable for current limiting and overcurrent protection, with quick response times and low power consumption.
