PRODUCT CATALOG & DESIGN GUIDE - Circuit Protection Products
Document source : www.littelfuse.com
13
Varistor Products
©2009 Littelfuse, Inc.
Revision: November 5, 2009
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VERVIEW
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Specifications are subject to change without notice.
Please refer to www.littelfuse.com for current information.
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
Diametermm
3
5
7
10
14
20
32
34
40
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 1200ºC. The B ismuth oxide is molten above
825ºC, 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. BYTYPE 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, CIII,
TMOV
®
, i TMOV
®
,UltraMOVTM,
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
Summary :
BYTYPE 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, CIII, TMOV ® , i TMOV ® ,UltraMOVTM, 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.
Tags :
disc,ceramic,diameter,aristor,discs,figure,littelfuse,package,deices,epoxy,industrial,used,leads
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