IE83276B1 - A thermally protected metal oxide varistor - Google Patents
A thermally protected metal oxide varistorInfo
- Publication number
- IE83276B1 IE83276B1 IE2001/0410A IE20010410A IE83276B1 IE 83276 B1 IE83276 B1 IE 83276B1 IE 2001/0410 A IE2001/0410 A IE 2001/0410A IE 20010410 A IE20010410 A IE 20010410A IE 83276 B1 IE83276 B1 IE 83276B1
- Authority
- IE
- Ireland
- Prior art keywords
- link
- metal oxide
- hot melt
- varistor
- fused electrode
- Prior art date
Links
- 229910044991 metal oxide Inorganic materials 0.000 title claims description 16
- 150000004706 metal oxides Chemical class 0.000 title claims description 16
- 229910000679 solder Inorganic materials 0.000 claims description 23
- 239000000463 material Substances 0.000 claims description 21
- 239000000289 melt material Substances 0.000 claims description 20
- 238000002844 melting Methods 0.000 claims description 12
- 239000012212 insulator Substances 0.000 claims description 11
- 230000004907 flux Effects 0.000 claims description 9
- 239000000155 melt Substances 0.000 claims description 6
- 239000000615 nonconductor Substances 0.000 claims description 5
- 238000005538 encapsulation Methods 0.000 claims description 3
- 238000009413 insulation Methods 0.000 description 6
- 230000002159 abnormal effect Effects 0.000 description 4
- 230000002459 sustained Effects 0.000 description 4
- 239000004593 Epoxy Substances 0.000 description 2
- 239000008393 encapsulating agent Substances 0.000 description 2
- 125000003700 epoxy group Chemical group 0.000 description 2
- 238000002955 isolation Methods 0.000 description 2
- 230000001052 transient Effects 0.000 description 2
- XLOMVQKBTHCTTD-UHFFFAOYSA-N zinc monoxide Chemical compound [Zn]=O XLOMVQKBTHCTTD-UHFFFAOYSA-N 0.000 description 2
- PNEYBMLMFCGWSK-UHFFFAOYSA-N AI2O3 Inorganic materials [O-2].[O-2].[O-2].[Al+3].[Al+3] PNEYBMLMFCGWSK-UHFFFAOYSA-N 0.000 description 1
- 239000004952 Polyamide Substances 0.000 description 1
- 229910007116 SnPb Inorganic materials 0.000 description 1
- 230000001154 acute Effects 0.000 description 1
- 230000002411 adverse Effects 0.000 description 1
- 238000010276 construction Methods 0.000 description 1
- 230000005496 eutectics Effects 0.000 description 1
- 238000006062 fragmentation reaction Methods 0.000 description 1
- 238000010438 heat treatment Methods 0.000 description 1
- 238000004519 manufacturing process Methods 0.000 description 1
- 239000002184 metal Substances 0.000 description 1
- 229910052751 metal Inorganic materials 0.000 description 1
- 238000010943 off-gassing Methods 0.000 description 1
- 229920002647 polyamide Polymers 0.000 description 1
- 239000011528 polyamide (building material) Substances 0.000 description 1
- 230000001629 suppression Effects 0.000 description 1
- 230000000007 visual effect Effects 0.000 description 1
- 239000011787 zinc oxide Substances 0.000 description 1
Description
A thermally protected metal oxide varistor
INTRODUCTION
Field of the Invention
The invention relates to a metal oxide varistor (MOV) of the type having an integral
thermally activated fl.1S€ for protection. The fuse causes the varistor to go open
circuit in conditions of overheating due to sustained over-voltages.
Prior Art Discussion
One such device is described in United States Patent Serial No. 5901027 (Leviton),
in which a flat thermal fusible layer is deposited on a MOV element. United States
Patent Serial No. US5708553 (Hung) also describes such a varistor, in which a lead
is spaced-apart from an electrode and is connected to it by a column of solder
extending outwardly from the electrode.
While these varistors appear to be reasonably effective, there is scope for improving
characteristics in such a device. One such improvement is provision of an improved
insulation gap after fusing, without relying on properties such as outgassing in an
epoxy. Another desirable improvement is better handling of transient peak currents.
It is also desirable that manufacturing be simplified.
SUMMARY OF THE INVENTION
According to the invention, there is provided a thermally protected metal oxide
varistor comprising a varistor body, electrodes including a fused electrode, leads, and
a thermal fuse connecting a lead to the fused electrode, characterised in that, the fuse
comprises:-
an insulator overlying part of the fused electrode;
a link having a portion overlying the insulator and a portion electrically
connected to the fused electrode, the link being of a material having a melting
point at or below a thermal safety temperature threshold for the varistor; and
a body of hot melt material in contact with the link, the hot melt material
being an electrical insulator and having a melting point such that it melts and
flows to create an insulating gap between the fused electrode and the material
of the link when the link becomes molten.
In one embodiment, the link is of elongate wire shape.
In another embodiment, the link comprises a solder material and internal flux within
the solder material.
In one embodiment, the solder material is Sn/ Pb.
In another embodiment, the flux is located centrally within the link.
In a further embodiment, the hot melt material surrounds the link between the
insulator and the fused electrode.
In one embodiment, the hot melt material is in contact with the fused electrode.
In another embodiment, a portion of the hot melt material lies between the fiised
electrode and the link.
In another embodiment, the link is electrically connected to the fused electrode by a
low temperature solder fillet.
In one embodiment, the hot melt material acts to retain the link in position, so that
the link has a stable position before encapsulation.
According to another aspect, the invention provides a metal oxide varistor
comprising a varistor body, electrodes including a fused electrode, leads, and a
thermal fuse connecting a lead to the fused electrode, characterised in that the fuse
comprises:
an insulator overlying part of the fused electrode; and
a link of elongate shape and comprising flux surrounded by solder material
having a melting point at or below a thermal safety temperature threshold for
the varistor, the link comprising:
a first portion in Contact with the fused electrode,
a second portion surrounded by a body of hot melt material, said hot
melt material also being in Contact with the fused electrode, and
a third portion overlying the insulator and being connected to a lead;
wherein said hot melt material is an electrical insulator and has a melting
point such that it melts and flows to create an insulating gap between the
fused electrode and the material of the link when the link becomes molten.
DETAILED DESCRIPTION OF THE INVENTION
Brief Description of the Drawirm
The invention will be more clearly understood from the following description of
some embodiments thereof, given by way of example only with reference to the
accompanying drawings in which:-
Fig. l is a diagrammatic cross-sectional plan view of a varistor of the
invention, and Fig. 2 is a diagrammatic cross-sectional side view;
Fig. 3 is a plot of representative temperature of points on the external surface
of the varistor versus time; and
Fig. 4 illustrates four sets of times for fuse opening, one set for each of four
limited current values.
Description of the Embodiments
Referring to Figs. l and 2, a varistor 1 comprises a zinc oxide disc 2 for over-voltage
protection. A lead 3 is connected to an electrode 4 on one side of the disc 2, and a
lead 5 is connected to an electrode 6 on the other side via a thermal fuse 7. Thus, the
electrode 6 is referred to as a “fused” electrode for the purposes of clarity.
The fuse 7 comprises a wire link 10 of 60:40 w.w. Sn/ Pb material with a fluxed core,
having a relatively low melting point of c.l80°C. This is the primary active element
of the fuse 7. The link 10 has a diameter of 1.2 mm, sufficient to handle peak pulses
while also allowing effective disconnection under fuse conditions. The fluxed core
runs centrally in a symmetrical pattern through the link 10.
The link 10 is soldered at a first portion to the fused electrode 6 by a low temperature
solder fillet ll of non-eutectic solder having a melting point in the region of 165°C.
The fact that the solder fillet ll has a slightly lower melting point than the link 10
allows relatively simple assembly in which application of the fillet 11 does not
adversely affect the link 10.
A body of polyamide hot melt 12 surrounds the link l0 at a second portion where it
is sloped at a small acute angle away from the fused electrode 6 to lie over an
insulation disc 13 of alumina material. The hot melt 12 has a melting point of
approximately 150°C. The hot melt material 12 is in contact with the fused electrode
6 below the link 10. In this specification, the term “hot melt” means any material
which is an electrical insulator and which becomes molten at approximately the
fusing temperature.
The arrangement of the hot melt material l2 is such that it lies on part of the fused
electrode 6 as illustrated, and it surrounds the link 10 where it is ramped away from
the electrode 6.
At a third portion, the link 10 is soldered to the varistor lead 5 by a low temperature
solder fillet of the same material as the fillet 11. The link 10 may alternatively be
connected to the lead 5 by heating the end of the lead 5, causing the link 10 to melt
locally at its inner end and adhere to the lead 5. The lead 5 has a right angled bend
over the insulator 13.
Finally, the varistor 1 comprises an encapsulant of conventional epoxy material,
providing an external diameter dimension such as 14 mm or 20 mm.
The following are parameter values, with reference to Fig. 1.
All dimensions in mm
Fig. 1 Parameter 14mm Dia. 20mm Dia.
Notation Varistor Varistor
e Lead Spacing 7.5il.0 7.5il.O
B Bend Distance 5.3il .3 5.3il.3
C Insulation Diameter 7.0il .0 l0.0i1.0
Xl Hotmelt Overlap on Insulation 3mm 3mm
X2 Hotmelt Overlap on Electrode 3mm 3mm
Dl Solder Fillet Width 3.9451 .9 4.5i2.6
L2 Fusing Distance 3.5i2.l 4.li1.5
l>
The Varistor l operates as a surge suppressor meeting the requirements of the UL
and other standards and guidelines. The fuse 7 provides integrated thermal
protection which open-circuits the Varistor l in the event of overheating due to
sustained over-voltages. This prevents fire, fragmentation, and scorching when
abnormal sustained over-voltages occur. Referring to Fig. 3, plots for encapsulant
surface temperature during abnormal over—voltage limited currents of 0.125 A, 0.5 A,
.5 A, and 5 A are illustrated. It will be appreciated that the surface temperature
does not exceed c. 170°C.
The following are the ratings for the varistor 1.
Condition
Continuous:
Steady State Applied Voltage:
AC VOlt3.g€ Range (VM(_Ac)RMs) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..
Transient:
Value
to 420
Peak Pulse Current (ITM) For 8/20ps Current Wave,
single pulse ....................................................... ..
to 10000
Units
Single Pulse Energy Capability For 10/ l0O0us Current
Wave .............................................................. .. 50 to 273 J
Operating Ambient Temperature range (TA) .............. .. -55 to +85 °C
Storage Temperature (Tm) ................................... .. -55 to +125 °C
Temperature Coefficient (ocV) of Clamping Voltage (Vc)
at Specified Test Current ...................................... .. <0.01 °/o/°C
Hi-Pot Encapsulation (Isolation Voltage Capability) .... .. 2500 V
Thermal Protection Isolation Voltage Capability (when
operated) .......................................................... . . 600 V
Insulation Resistance ........................................... .. 100 M9
The thermal characteristics are shown in Fig. 4 which illustrates the time to open
circuit under abnormal over—voltage with limited current values as for Fig. 3.
The fuse 7 operates by the solder fillets ll and 14, the link 10, and the hot melt 12
becoming molten due to sustained abnormal over-voltages. However, the link 10 is
the primary active fuse element because it is of SnPb solder composition with a
fluxed core. The flux causes it to form into a ball, pulling away from the electrode 6.
The internal flux core causes the solder material of the link 10 to form into a sphere,
with the flux causing the solder to wet to itself. Surface tension is also an important
aspect of the action to withdraw into a sphere. It is allowed to do so as the solder
fillet ll also melts. At the same time, the hot melt 12 rapidly fills the emerging gap
between the material of the link 10 and the electrode 6. This action is particularly
quick because the hot melt 12 is already in contact with the electrode 6 and it is only
required to spread across the face of the electrode as the link 10 melts and retracts
away from the electrode surface. The insulative properties of the hot melt 12 ensure
a very effective and substantial insulation gap between the lead 5 and the electrode 6
in a short time period as illustrated in Fig. 4.
The invention is not limited to the embodiments described but may be varied in
construction and detail. For example, the varistor of the invention may additionally
comprise a third lead connected to the electrode 6 via the low temperature solder
fillet 11. If this solder flows, the third lead is electrically disconnected and a visual
and/ or audible indicator is activated. Also, the metal of the link 10 may have a
different composition such as SnPbAg or SnPbBi.
Claims (1)
- Claims A thermally protected metal oxide varistor (1) comprising a varistor body (2), electrodes (4, 6) including a fused electrode (6), leads (3, 5), and a thermal fuse (7) connecting a lead (5) to the fused electrode (6), characterised in that, the fuse (7) comprises:- an insulator (13) overlying part of the fused electrode (6); a link (10) having a portion overlying the insulator (13) and a portion electrically connected to the fused electrode (6), the link being of a material having a melting point at or below a thermal safety temperature threshold for the varistor; and a body of hot melt material (12) in contact with the link (10), the hot melt material being an electrical insulator and having a melting point such that it melts and flows to create an insulating gap between the fused electrode (6) and the material of the link (10) when the link (10) becomes molten. A metal oxide varistor as claimed in claim 1, wherein the link (10) is of elongate wire shape. A metal oxide varistor as claimed in claims 1 or 2, wherein the link (10) comprises a solder material and internal flux within the solder material. A metal oxide varistor as claimed in claim 3, wherein the solder material is Sn/ Pb. A metal oxide varistor as claimed in claim 3 or 4, wherein the flux is located centrally within the link (10). A metal oxide varistor as claimed in any preceding claim, wherein the hot melt material (12) surrounds the link (10) between the insulator (13) and the fused electrode (6). A metal oxide varistor as claimed in claim 6, wherein the hot melt material (12) is in contact with the fused electrode (6). A metal oxide varistor as claimed in claim 7, wherein a portion of the hot melt material (l2) lies between the fused electrode (6) and the link (10). A metal oxide Varistor as claimed in any preceding claim, wherein the link (l0) is electrically connected to the fused electrode (6) by a low temperature solder fillet (ll). A metal oxide varistor as claimed in any preceding claim, wherein the hot melt material (12) acts to retain the link (10) in position, so that the link (10) has a stable position before encapsulation. A metal oxide varistor (1) comprising a varistor body (2), electrodes (4, 6) including a fused electrode (6), leads (3, 5), and a thermal fuse (7) connecting a lead (5) to the fused electrode (6), characterised in that the fuse (7) comprises: an insulator (13) overlying part of the fused electrode (6); and a link (l0) of elongate shape and comprising flux surrounded by solder material having a melting point at or below a thermal safety temperature threshold for the varistor, the link comprising: a first portion in contact with the fused electrode (6), a second portion surrounded by a body of hot melt material (12), said hot melt material (12) also being in Contact with the fused electrode (6), and a third portion overlying the insulator (13) and being connected to a lead (5); wherein said hot melt material is an electrical insulator an has a melting point such that it melts and flows to create and insulating gap between the fused electrode (6) and the material of the link (10) when the link (10) becomes molten.
Priority Applications (1)
Application Number | Priority Date | Filing Date | Title |
---|---|---|---|
IE2001/0410A IE83276B1 (en) | 2001-04-25 | A thermally protected metal oxide varistor |
Applications Claiming Priority (4)
Application Number | Priority Date | Filing Date | Title |
---|---|---|---|
EPEUROPEANPATENTOFFICE(EPO)26/04/2 | |||
EP00650040 | 2000-04-26 | ||
IE20010227 | 2001-03-09 | ||
IE2001/0410A IE83276B1 (en) | 2001-04-25 | A thermally protected metal oxide varistor |
Publications (2)
Publication Number | Publication Date |
---|---|
IE20010410A1 IE20010410A1 (en) | 2001-11-28 |
IE83276B1 true IE83276B1 (en) | 2004-01-28 |
Family
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