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MOV is widely used as components in lightning and surge protection in all kinds of electrical appliance power supply because of following characteristics : big surge capacity, quick response time and low cost. Meanwhile, MOV is an easy-degradation component. After withstanding surge strikes, all its parameters will decline and cause failure. The specific performances are increasing of leakage current, decreasing of resistance, rising of temperature. Finally, the failed MOV will burn or melt because of high temperature, even cause severe fire hazard.
Therefore in UL1449, as a surge protective component, MOV should not generate visible or audible electrical and mechanical damage when failed. UL1449 require protective components disconnect from main circuit in first time, so it’s necessary to leave a way out when MOV’s failure. To meet the requirements of UL1449, the traditional method is to series a thermal link in the branch circuit. When the MOV overheating, the thermal link will fuse and cut off the circuit to prevent MOV from burning.
This method is simple but exist some defects: it has good protective performance when encountering failure caused by leakage. But because there exist transmission medium between thermal link and MOV, the coupling is low. When MOV failed under big current, because the temperature rising rapidly, MOV may burn before thermal link fusing. Such situation often happen when conducting SPD temporary over-voltage testing, therefore the effect of thermal protection is not completely.
Based on the situation, some companies developed a new product which integrated thermal link into MOV, called Thermally protected MOV. From the structure, the Thermally protected MOVs are made by soldering a thermal link in MOV’s silvered electrode, then the thermal link connect with one of MOV’s lead, that realize the auto-cutoff function when MOV overheating. Such thermal link usually made by low melting point alloy. Currently Thermally protected MOV manufacturers are: SETfuse, Littlefuse, EPCOS, Brightking and Kestar.
Xiamen SETfuse is the top TFMOV manufacturer in China. Found in 2000, our company is specialized in circuit protection. We are a high and new tech enterprise in over temperature, over voltage and over current circuit protection component research and development, annual output 6 billion pcs. As a professional manufacturer, SET’s product have obtained UL, CUL, VDE, TUV, PSE, KTL security certification. Compared with other products, our TFMOV has its unique in structure and alert signal. From its shape, we can divide Thermally protected MOV into two types: one is product of Littlefuse, brightking and kestar, the feature is MOV closely integrated with thermal link, then pack them with epoxy, as following pics:
The other type is SET and EPCOS’ product. The MOV is being placed into a fire-retardant molded case, then encapsulate with epoxy. From the test report of Littlefuse, surface temperature will temporarily rise due to thermal inertia if only pack thermal link with epoxy one time. Under this circumstance, the epoxy sealing material sometime can bubble or even rupture. But for a fire-retardant molded TFMOV, because its sealing material is thicker than one time epoxy sealing, and the case is also hard, so the possibility of sealing rupture is rather small.
SET’s TFMOV have 2 pins, 3 pins and 5 pins specs. The schematics are following:
(a)is simplest TFMOV.
(b)is 3 pins TFMOV. MOV will fail after times of functions, in order to find out failed MOV from the circuit, TFMOV refer to the design of MOV SPD’s alert signal. As pic, the lead from MOV and thermal link is port of alert signal, the schematic is on the right pic, when the MOV is not failed, the thermal link does not cut off, the LED on alert signal port lights. If MOV fail, thermal link cut off, the LED extinguish and indicate that MOV need to be replaced.
(c)is SET’s unique TFMOV structure, this 5 pins TFMOV has fail alert signal and couple with a set of thermal link, we can use the thermal link extract another alert signal for protective power supply. SET’s TFMOV34S series 34mm has the biggest surge capacity, 40KA.
Because there is low melting point alloy inside TFMOV, so the soldering requirements is high. Too high soldering temperature will cause thermal link fused and unqualified. SET’s TFMOV solve the soldering problems by special process, which could be wave-soldered.
Here are two applications of TFMOV:
Application on anti-surge outlets
Application on SPD
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