Varistor Metal Oxide Varistor Products Page 10 Varistor Metal-Oxide Varistor Products

2017 Littelfuse, Inc. Specifications are subject to change without notice. Revised: 09/14/17 Metal-Oxide Varistors (MOVs) Varistor Construction The process of fabricating a Littelfuse Varistor is illustrated in the flow chart of Figure 7. The starting material may differ in the composition of the additive oxides, in order to cover the voltage range of product. Device characteristics are determined at the pressing operation. The powder is pressed into a form of predeter- mined thickness in order to obtain a desired value of nomi- nal voltage. To obtain the desired ratings of peak current and energy capability, the electrode area and mass of the device are varied. The range of diameters obtainable in disc product offerings is listed here: Nominal Disc Diameter-mm 3 5 7 8 10 14 16 20 22 25 32 34 40 60 62 Of course, other shapes, such as rectangles, are also possible by simply changing the press dies. Other ceramic fabrication techniques can be used to make different shapes. For example, rods or tubes are made by extruding and cutting to length. After forming, the green (i.e., unfired) parts are placed in a kiln and sintered at peak temperatures in excess of 1200C. The B ismuth oxide is molten above 825C, assisting in the initial densification of the polycrys- talline ceramic. At higher temperatures, grain growth oc- curs, forming a structure with controlled grain size. Electroding is accomplished, for radial and chip devices, by means of thick film silver fired onto the ceramic surface. Wire leads or strap terminals are then soldered in place. A conductive epoxy is used for connecting leads to the axial 3mm discs. For the larger industrial devices (40mm and 60mm diameter discs) the contact material is arc sprayed Aluminum, with an overspray of Copper if necessary to give a solderable surface. Many encapsulation techniques are used in the assembly of the various Littelfuse Varistor packages. Most radials and some industrial devices (HA Series) are epoxy coated in a fluidized bed, whereas epoxy is "spun" onto the axial device. Radials are also available with phenolic coatings applied using a wet process. The PA Series package consists of plastic molded around a 20mm disc subassembly. The RA, DA and DB Series devices are all similar in that they all are composed of discs or chips, with tabs or leads, encased in a molded plastic shell filled with epoxy. Different pack- age styles allow variation in energy ratings, as well as in mechanical mounting. TABLE 2. BY-TYPE CERAMIC DIMENSIONS PACKAGE TYPE SERIES CERAMIC DIMENSIONS Leadless Surface Mount CH, AUML, ML, MLE, MLN Series 5mm x 8mm Chip, 0603, 0805, 1206, 1210, 1812, 2220 Axial Leaded MA Series 3mm Diameter Disc Radial Leaded ZA, LA, C-III, TMOV , i TMOV ,UltraMOV, TMOV25S Series 5mm, 7mm, 10mm, 14mm, 20mm Diameter Discs Boxed, Low Profile RA Series 5mm x 8mm, 10mm x 16mm, 14 x 22 Chips Industrial Packages BA, BB Series DA, DB Series DHB Series HA, HB Series HC, HF Series HG Series 32mm, 40mm Diameter Disc, 34mm Square Disc, 40mm Diameter Disc, 60mm Diameter Disc Industrial Discs CA Series 60mm Diameter Discs MECHANICAL ASSEMBLY SINTER ELECTRODE POWDER MIXING PRESS ENCAPSULATE ADDITIVE OXIDES (MAINLY BL 2 0 3 ) ZnO POWDER PREPARATION FORM CERAMIC BODY PACKAGE AS/IF REQUIRED FINAL PRODUCT TO ELECTRICAL TEST FIGURE 7. SCHEMATIC FLOW DIAGRAM OF LITTELFUSE VARISTOR FABRICATION Figure 9A, 9B and 9C (next page) show construction details of some Littelfuse varistor packages. Dimensions of the ceramic, by package type, are above in Table 2. FIGURE 9A. CROSS-SECTION OF MA SERIES FIGURE 9B. CROSS-SECTION OF RADIAL LEAD PACKAGE Varistor Characteristics, Terms and Consideration Factors (continued)

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