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Type MOV A variable-resistance resistor made from metal oxides and ceramic powders sandwiched between electrodes; resistance changes when a specific voltage is applied, opening a conductive path between electrodes and clamping line voltage at that level Fast response times (typically 10-20 ns), with good power dissipation characteristics; MOVs connected in parallel increase current-handling capability. When connected in a series, they increase voltage rating. MOV clamping voltage rises with current. Additionally, MOV performance degrades from repeated exposure to power surges, which means that an MOV will become less effective with age. Finally, MOV capacitance typically ranges from 100 picofarads (pF) to several nanofarads (nF) (depending on sizes and voltages). A gas-filled tube with electrodes at each end; at a specified voltage, a high current arcs between the electrodes to provide a conductive path, shunting the surge to ground Slower response time than other surge-suppression technologies but able to conduct very high current and dissipate considerable power levels. Once a GDT is activated, it continues to conduct even when voltage drops below its activation voltage, which is an important consideration in some applications. Because of slower response times, GDTs are often used in conjunction with other surge-suppression technologies. GDTs add a small capacitance (typically < 2 pF) to a circuit. GDT Description Operational Characteristics Transient voltage suppression (TVS) diode A particular type of junction diode with larger- than-normal junction cross-sections capable of conducting large currents; at a specific voltage, an avalanche breakdown occurs that creates a path for current across the junction between two electrodes and clamping line voltage at that level TVS diodes have fast response times (measured in picoseconds) and can protect against high voltages, but they have lower power dissipation capabilities than other technologies. TVS diodes add moderate capacitance to a circuit, and they do not degrade over time, which gives them a long service life if operated within their specified ratings. These devices combine a GDT and an MOV in one hybrid component; MOVs and GDTs are often used together to take advantage of their respective strengths These devices have a moderate response time of 1 to 2 µs. A hybrid GDT-MOV component provides more effective, long-lasting surge protection in a smaller package. The GDT prevents leakage current so the MOV is protected against age degradation. The MOV provides a minimum voltage breakdown, which can prevent the GDT from entering a holdover current event and failing from excessive heat. A holdover current event occurs when enough current is available after the surge event to keep the GDT in the arc mode indefinitely. GDT-MOV GMOV™ and IsoMOV™ hybrid components 18 Understanding Surge-Suppression Technologies Table 2: Common Surge-Suppression Technologies