Note: Descriptions are shown in the official language in which they were submitted.
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This invention relates to incandescent lamps.
More particularly, this invention relates to protecting
incandescent lamps from over-voltage damage by including
a polycrystalline varistor electrically in parallel with
the filament of the lamp.
This invention is related to my Canadian
application, Serial No. ~o,~b~ filed ~y ~\, ~
This related application is assigned to the assignee of this
invention.
Incandescent lamps comprise a coiled tungsten
filament contained in an envelope from which oxidizing
agents are excluded. Tungsten metal is brittle and difficult
to draw and, therefore, incandescent lamp filaments produced
in an economically practical manner contain pinches, or thin
regions therein. Because these thin regions have increased
electrical resistance and decreased mechanical strength with
respect to the rest of the filament, they represent weak
spots at which filament failure is likely to occur when the
lamp is subjected to an over-voltage condition on its supply
line. Another filament failure mechanism results from the
fact that the filament is coiled. Current flowing through
the coiled filament sets up a magnetic field which tends
to pull the turns of the coil together. A voltage surge
causes an increased current to flow through the filament
which in turn increases the intensity of the magnetic field
which further draws the turns together and may cause turn-to-
turn shorting. Such shorting decreases the electrical
resistance of the filament causing more current to flow
further increasing the intensity of the magnetic field and
shorting more turns in a chain reaction fashion until the
filament burns out. As a result of these factors, it is
empirically known that the operating life-time of an
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incandescent lamp filament is inversely proportional to the
voltage applied across the filament raised to the thirteenth
power.
It is also known that incandescent lamp filaments
exhibit a positive temperature coefficient of resistance
such that the resistance of the filament of a lamp at
operating temperature is approximately 10-20 times the
resistance of the same filament when cold. Naturally, the
currents through the lamp exhibit the same 10:1 ~ 20:1 raio
inversely to the resistance. Therefore, a lamp is most
likely to fail at the moment of turn-on and is most susceptible
to voltage transients on its supply line at that time. It
has been shown that a typical residential electrical system
is statistically subjected to a voltage transient exceeding
500 volts once per day, a voltage transient exceeding 1000
volts once per week, and a voltage transient exceeding 10,000
volts once per year.
It is, accordingly, one object of this invention to
prevent supply line voltage surges from being impressed
across the filaments of incandescent lamps to thereby prolong
the operating life of the lamps.
Another object of this invention is to so protect
lamp filaments by means of a polycrystalline varistor
member connected electrically in parallel with the lamp
filaments and mechanically configured to be reliably and
inexpensively includable in lamp circuits of current manufacture.
Briefly, and in accordance with one embodiment of
this invention, a body of polycrystalline metal oxide
varistor material is mechanically included in the base
member of an incandescent lamp and is connected electrically
in parallel with the filament of the lamp to protect the
filament against voltage surges. In accordance with another
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embodiment of this invention, a body of polycrystalline
metal oxide varistor material having a spring contact member
is provided for insertion into an incandescent lamp socket so
that the body of varistor material is electrically in
parallel with the filament of the lamp when the lamp is
inserted into its operating position in the socket.
~ he novel features of this invention sought to be
patented are set forth with particularity in the appended
claims. ~he invention, together with further objects and
advantages thereof, may be understood from a reading of
the following specification and appended claims in view
of the accompanying drawings in which:
FIG. 1 is a log-log graphical representation of
the current density vs. voltage gradient characteristic
of the polycrystalline vasistor used in practicing this
invention.
FIG. 2 is a cross-sectional elevation view of a
protected incandescent lamp in accordance with this invention.
FIG. 3 is a cross-sectional elevation view of an
alternative embodiment of a protected incandescent lamp
in accordance with this invention.
FIG. 4 is a cross-sectional elevation view of a
varistor lamp protector adapted for insertion in a lamp
socket in accordance with this invention.
FIG. 5 is a cross-sectional elevation view of a
lamp socket having a lamp base inserted therein and including
a protective device to FIG. 4 illustrating the operational
interaction between the device of FIG. 4 and the protected
lamp and its socket.
There are a few known materials which exhibit
nonlinear resistance characteristics and which require resort
to the following equation to relate quantitatively current
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and voltage: CL
I = (V)
where V is the voltage between two points separated by
a body of the material under consideration, I is the
current flowing between the two points, C is a constant,
and d is an exponent greater than 1. soth C and OC are func-
tions of the geometry of the body formed from the material
and the composition thereof, and C is primarily a function
of the material grain size whereasC~ is primarily a
function of the grain boundary. Materials such as silicon
carbide exhibit nonlinear or exponential resistance
characteristics and have been utilized in commercial
silicon carbide varistors, however, such nonmetallic
varistors typically exhibit an alpha (C~) exponent of no
more than 6. This relatively low value of alpha represents
a nonlinear resistance relationship wherein the resistance
varies over only a moderate range. Due to this moderate
range of resistance variation, the silicon carbide varistor
is often connected in series with a gap when used in a
circuit for transient voltage suppression since continuous
connection of the varistor could exceed the power dissipa-
tion capabilities thereof unless a relatively bulky body of
such material is used in which case the steady state power
dissipation is a rather severe limitation. An additional
drawback is the ineffectiveness of the voltage clamping
action as a result of the limited value of silicon carbide
alpha exponent. The moderate range of resistance variation
results in voltage limitation which may be satisfactory for
some applications, but is generally not satisfactory when
the transient voltage has a high peak valueO
A new family of varistor materials having alphas in
-- 4 --
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excess of 10 ~ithin the current density range of 10-3 to 102
amperes per square centimeter has recently been produced
from metal oxides. The metal oxide varistor material is a
polycrystalline ceramic material formed of a particular metal
oxide with small quantities of one or more other metal oxides
or halides being added. As one example, the predominant
metal oxide is zinc oxide with small quantities of bismuth
oxide being added. Other additives may be aluminum oxide,
iron oxide, magnesium oxide, and calcium oxide for example.
The predominant metal oxide is sintered with the additive
oxide(s) to form a sintered ceramic metal oxide body. Since
the varistor is fabricated as a ceramic powder, the material
can be pressed into a variety of shapes of various sizes.
Being polycrystalline, the characteristics of the metal oxide
varistor are determined by the grain (crystal) size, grain
composition, grain boundary composition, and grain boundary
thickness, all of which can be controlled in the ceramic
fabrication process.
The nonlinear resistance relationship of polycrystal-
line metal oxide varistors is such that the resistance is
very high (10,000 megohms has been measured) at very low
current levels in the microampere range and progresses in a
nonlinear manner to an extremely low value (tenths of an ohm)
at high current levels. The resistance is also more non-
linear with increasing values of alpha. These nonlinear
resistance characteristics result in voltage versus current
characteristics wherein the voltage is effectively limited,
the voltage limiting or clamping action being more enhanced
at the higher values of the alpha exponent as shown in FIG. 1.
Thus, the voltage versus current characteristics of the
polycrystalline metal oxide varistor is similar to that of
the zener diode with the added characteristics of being
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bidirectional and of operating over more decades of current.
The voltage versus current characteristics plotted in
FIG. 1 of the drawings illustrate the nonlinear or exponential
resistance characteristics exhibited by varistor material,
and in particular, the increasing nonlinearity and enhanced
voltage limiting obtained with increased values of the
exponent alpha (~ ) wherein the top line ~ = 4 is typical
for silicon carbide varistors and the three linesC~ = 10, 25,
and 40 apply to varistors fabricated of polycrystalline metal
oxide material. It should be understood that metal oxide
materials are available having alpha exponents even greater
than 40 which thereby obtains even greater enhanced voltage
clamping action than that exhibited for the OC = 40 line.
FIG. 2 is a cross-sectional elevation view of a
protected incandescent lamp in accordance with one embodiment
of this invention. A lamp indicated generally at 20 comprises
an envelope 21 of a light transmissive fluid impervious
material such as glass containing a filament 22, leads 23 and
24, and support member 25 in an atmosphere excluding reagents
capable of reaction with tungstenO Lamp 20 also includes a
base member indicated generally at 30 comprising a metallic
shell 31 which is configured with a plurality of ridges to adapt
lamp 20 to be screwed into a socket and which serves as an
electrical contact between the socket and lead 24 which
fluid-sealingly penetrates support member 25, a second
electrical contact 32 which provides for connection between a
corresponding second contact in the socket and filament 22
through lead 23 which also fluid-sealingly penetrates support
member 25, and a body of polycrystalline metal oxide varistor
material 33 disposed electrically and mechanically between
contacts 31 and 32. In steady state operation of lamp 20,
varistor 33 functions as an insulator between contacts 31
and 32. When on the other hand, a voltage surge occurs on
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the power lines supplying power to lamp 20, varistor member
33 becomes conductive and the surge current flows between
contacts 31 and 32 almost entirely through varistor member
33 and only insubstantially flows in filament 22, thereby
protecting the filament.
FIG. 3 illustrates another version of a protected
incandescent lamp in accordance with this invention.
Lamp 26 has envelope 21, filament 22, leads 23 and 24, and
support member 25 which are similar to the corresponding
elements of lamp 20 of FIG. 2. Lamp 26 also includes
base member 34 comprising shell member 31 and contact
member 32 corresponding to the similarly numbered elements
of FIG. 2. In lamp 26, shell member 31 and contact
member 32 are spaced and electrically isolated from each
other by conventional insulator member 35. Base member 34
further includes a body of polycrystalline metal oxide
varistor material 36 having electrodes 39 and 40 on
opposite faces thereof. Conductive tab 37 connects shell
member 31 to electrode 40 of varistor 36, electrically
in parallel with the connection of filament 22 to shell
member 31 by lead 24. Similarly, contact 32 is connected
by lead 38 to electrode 39 of varistor member 36, electri-
cally in parallel with the connection of filament 22 to
contact 32 by lead 23. ~hus, under normal steady state
operating conditions, varistor member 36 exhibits a very
high resistance and substantially all current flowing
between contact members 31 and 32 flows through filament 22.
Upon the occurrence of a.voltage spike exceeding the
varistor voltage of varistor member 36, member 36 becomes
substantially conductive and substantially all of the
surge current flowing between contact members 31 and 32
flows through varistor member 36 and almost no surge
current flows through filament 22.
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In accordance with another embodiment of this
invention, surge protection of conventional incandescent
lamps is provided by inserting the protective device of
FIG. 4 in a conventional lamp socket as illustrated in
FIG. 5. The protective device of FIG. 4 comprises a disk
41 of polycrystalline metal oxide varistor material having
a cylindrical hole through the approximate center thereof.
Because of the mechanical properties of the polycrystalline
varistor material, the central hole may be formed either
in the initial molding of the disk or by subsequent drilling
of a solid molded disk. The cylindrical hole through
varistor disk 41 contains a cylindrical sleeve 43 of
electrically insulating material. An electrically conductive
member 42 provides electrical contact to a first face 44 of
varistor disk 41. Member 42 includes conductive spring
member projections 46 and 47 which traverse the hole in
varistor member 41 interiorily to insulating sleeve 43 to
provide for electrical contact between member 42 and a
contact member of an incandescent lamp base. At least one
of members 46 and 47 is adapted to apply downward pressure
upon conductive spring finger member 49 through insulator
block 48 when member, for example, 47 as shown, is
compressed by a lamp base. Conductive spring member 49 forms
an electrical contact to a second face 45 of varistor
member 41 and upon compression through insulator block 48 is
urged outwardly to contact the shell member of a lamp base.
FIG. 5 illustrates the installation of the protective
device of FIG. 4 in a lamp socket. A lamp socket as shown
in FIG. 5 comprises a shell contact member 51 and a center
contact member 52 electrically isolated from each other by
insulator members 53 and 54. Shell member 51 is adapted
to receive a shell contact member 31 of a lamp base and
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contact 52 is positioned to provide electrically connection
to a contact member 32 of a lamp base when the lamp is
screwed into the socket. In accordance with this invention
the protective device of FIG.4 is positioned in the lamp
socket of FIG. 5 with conductive member 42 resting upon and
in electrical contact with contact member 52 of the socket.
Electrical conduction is provided by spring members 46
and 47 between contact member 52 through contact 42 and
the spring members to contact 32 of the lamp base. When
the lamp is securely screwed into the socket, lamp contact
member 32 compresses spring member 47 which through
insulating member 48 exerts a compressive force on spring
contact member 49 thereby urging it outwardly and into
electrical contact with socket shell member 51. Accordingly,
as hereinbefore discussed, the lamp filament is protected
against surges in that steady state current flows from
contact 52 tnrough conductive member 42 and spring contact
members 4~ and 47 to lamp contact 32, through the filament
of the lamp, lamp base shell contact 31 and socket shell 51.
Surge currents, on the other hand, flow from socket contact
52 through conductive member 42, varistor 41, and spring
contact 49 directly to socket shell member 51 without going
through the lamp. In the drawing and in the foregoing
discussion, this invention has been illustrated in connection
with screw-type lamp bases. Other lamp base configurations,
however, such as bayonet, may be substituted in practicing
this invention without departing from the scope thereof.
While this invention has been described with reference
to particular embodiments and examples, other modifications
and variations will appear to those skilled in the art in
view of the above teachings. Accordingly, it should be
understood that within the scope of the appended claims, the
invention may be practiced otherwise than is specifically
described.
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