U.S. patent number 3,912,966 [Application Number 05/355,898] was granted by the patent office on 1975-10-14 for incandescent lamp series string having protection against voltage surges.
This patent grant is currently assigned to General Electric Company. Invention is credited to John D. Harnden, Jr..
United States Patent |
3,912,966 |
Harnden, Jr. |
October 14, 1975 |
Incandescent lamp series string having protection against voltage
surges
Abstract
A body of sintered polycrystalline varistor material connected
across the filament of an incandescent lamp shunts a transient
voltage surge thereby protecting the lamp filament. Additionally, a
plurality of such varistor shunt protected incandescent lamps may
be serially interconnected in a series string. Upon the opening of
a lamp filament in the series string, conduction continues through
the varistor element so that the remaining lamps in the string
continue to function.
Inventors: |
Harnden, Jr.; John D.
(Schenectady, NY) |
Assignee: |
General Electric Company
(Schenectady, NY)
|
Family
ID: |
23399257 |
Appl.
No.: |
05/355,898 |
Filed: |
April 30, 1973 |
Current U.S.
Class: |
315/46; 315/71;
315/126; 315/122 |
Current CPC
Class: |
H05B
39/105 (20130101); H02H 9/044 (20130101) |
Current International
Class: |
H02H
9/04 (20060101); H05B 39/10 (20060101); H05B
39/00 (20060101); H01J 017/34 () |
Field of
Search: |
;315/46,48,71,125,205,92,58,72,122,126,123,124 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Primary Examiner: Kaufman; Nathan
Attorney, Agent or Firm: Haken; Jack E. Cohen; Joseph T.
Squillaro; Jerome C.
Claims
The invention claimed is:
1. In a series string of incandescent lamps having a plurality of
incandescent lamps therein, each said incandescent lamp having a
filament, the improvement which comprises:
a plurality of bodies of polycrystalline varistor material, each
said body being connected across a corresponding one of said
filaments and electrically in parallel therewith for protecting
said filaments against voltage surges by instantaneously decreasing
impedance in response to increased voltage and by instantaneously
increasing impedance in response to decreased voltage.
2. The improvement of claim 1 wherein said polycrystalline varistor
material comprises zinc oxide as a major constituent and a minor
constituent selected from the group consisting of other metal
oxides and halides.
3. The improvement of claim 2 wherein said bodies have a varistor
.alpha. exponent in excess of 10 in the current density range of
10.sup.-.sup.3 to 10.sup.2 amperes per square centimeter.
4. The improvement of claim 1 wherein said bodies have a varistor
voltage slightly higher than the voltage drop across said
corresponding filaments for preventing leakage current through said
bodies when said corresponding filaments are electrically
conducting and for providing a low impedance current path through
said bodies when said corresponding filaments are electrically
open.
5. The improvement of claim 4 wherein said varistor voltage is
approximately equal to 125 percent of the voltage rating of said
lamps.
Description
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 further relates to series string arrays of incandescent
lamps and more particularly relates to the inclusion of a
polycrystalline varistor electrically in parallel with each lamp
filament in the series string to protect the lamps of the string
and to permit the remainder of the string to continue to operate
upon the failure of some of the lamps in the string.
This invention is related to my concurrently filed application,
Ser. No. 355,897. This related application is assigned to the
assignee of this invention and is incorporated herein by reference
thereto.
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 filaments 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 lifetime of an 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 ratio 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 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.
It is frequently desirable to interconnect a plurality of
incandescent lamps in such fashion that their filaments are
connected electrically in series with each other. Obviously, in
such a series string, the opening of the filament of any one lamp
will extinguish all of the lamps in the string.
Accordingly, another object of this invention is to provide an
alternative current path in parallel with each filament of a series
string of incandescent lamps so that upon the opening of any
filament in the series string, current conduction continues through
the alternative path and the remaining lamps in the string continue
to function.
It is another object of this invention to provide such a series
string wherein the alternative paths comprise polycrystalline
varistors.
It is another object of this invention to provide such protective
polycrystalline varistors which have negligible switching times and
negligible steady state leakage currents.
Briefly, and in accordance with one embodiment of this invention,
there is provided a series string of incandescent lamps wherein
each element in the string is shunted by a polycrystalline metal
oxide varistor element which protects its associated filament from
damage by voltage surges, and which switches from a very high
impedance to a very low impedance state when its associated lamp
element opens thereby permitting continued operation of the
remainder of the lamps in the series string.
The novel features of this invention sought to be patented are set
forth with particularity in the appended claims. The 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 varistor
used in practicing this invention.
FIG. 2 is an electrical schematic diagram of a portion of a series
string of protected incandescent lamps in accordance with this
invention.
FIG. 3 is a graphical representation of the operating
characteristics of the lamp and varistor elements of the series
string of this invention illustrating the mode of operation
thereof.
There are a few known materials which exhibit nonlinear resistance
characteristics and which require resort to the following equation
to relate quantitatively current and voltage: ##EQU1## 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 .alpha. is an exponent greater than 1.
Both C and .alpha. are functions 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 whereas .alpha. 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
(.alpha.) 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 dissipation 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 value.
A new family of varistor materials having alphas in excess of 10
within the current density range of 10.sup.-.sup.3 to 10.sup.2
amperes per square centimeter has recently been produced from metal
oxides. The metal oxide varistor 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 polycrystalline 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 nonlinear 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 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 (.alpha.) wherein the
top line .alpha. = 4 is typical for silicon carbide varistors and
the three lines .alpha. = 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 obtain even greater
enhanced voltage clamping action than that exhibited for the
.alpha. = 40 line.
FIG. 2 illustrates a series string of incandescent lamps in
accordance with this invention comprising lamps 11, 12, and 13
having, respectively, filaments 14, 15, and 16. Filaments 14, 15,
and 16 are connected in series by power line 17. Additionally,
power line 17 connects polycrystalline varistor members 20, 21, and
22 electrically in series. Polycrystalline varistor members 20, 21,
and 22 are connected respectively electrically in parallel with
filaments 14, 15, and 16. During normal steady state operating
conditions, varistors 20, 21, and 22 behave essentially as open
circuits in power line 17 and almost all of the current flowing in
line 17 flows through filaments 14, 15, and 16. For example, in a
typical steady state operating condition, 1 ampere flows through
the filaments and less than 10 microamperes of leakage current
flows through varistor members 20, 21, and 22. When a voltage surge
exceeding the varistor voltage of varistors 20, 21, and 22 appears
on power line 17, on the other hand, the varistor members switch to
an extremely low impedance state and more than 99 percent of the
surge current flows through the varistor members and less than one
percent of the surge current flows through filaments 14, 15, and
16. Accordingly, the operating lives of lamps 11, 12, and 13 is
greatly extended.
For large multi-lamp lighting installations, the use of a series
string lamp arrangement in preference to parallel interconnected
lamps is advantageous for a number of reasons. Among these reasons
are: a saving of up to 50 percent of the interconnection wiring
required may be obtained depending upon the configuration of the
string; a series string comprises a plurality of low voltage lamps,
the sum of whose voltage ratings equal the supply voltage, as
opposed to the plurality of supply voltage rated lamps employed in
a parallel interconnection, and low voltage lamps exhibit a longer
life and higher efficiency in terms of lumens per watt than high
voltage lamps; low voltage lamps are less expensive than high
voltage lamps since, for a given wattage, the length of filament in
a low voltage lamp is substantially less than in a high voltage
lamp and filament support required in the lamp is accordingly
greatly simplified.
Those skilled in the art will readily appreciate that varistors
such as silicon carbide cannot be employed as heretofore described
partly because the low alpha of silicon carbide, for example,
varistors would allow too much surge current to pass through the
filaments to be acceptably effective in preserving lamp life, but
principally because of the excessive leakage currents through such
varistors in the steady state condition which would waste an
unacceptable amount of energy and would overly decrease the
illumination provided by the series string.
In another facet of this invention, which takes advantage of the
precise controllability of varistor characteristics of
polycrystalline metal oxide varistors, in a preferred embodiment of
this invention, the varistor voltage of varistors 20, 21, and 22 is
made to be only slightly higher than the operating voltages of
lamps 11, 12, and 13. The advantage of this feature of this
invention may be appreciated with reference to FIG. 3 which
indicates the resistance characteristics of the lamp filaments and
of the varistor members. In FIG. 3, line 31 represents the
resistance characteristic of a lamp filament and line 32 represents
the resistance characteristic of a varistor member. Point 33 is the
normal, steady state, operating point of a lamp filament. Should a
filament, for example, filament 15 in FIG. 2, open, the voltage
across the open filament and its associated varistor 21 begins to
rise toward the supply voltage on line 17. When the voltage reaches
the value at point 34, however, varistor 21 commences to conduct
substantial current. Accordingly, the opening of filament 15 does
not cause the extinguishment of lamps 11 and 13. Since the rise of
potential across varistor 21 to point 34 and the firing of varistor
21 into conduction occur in approximately 1 microsecond, the
provision of illumination by the remaining lamps in the string
other than lamp 12, continues without operational interruption.
In fact, one could, in accordance with this invention clamp lamp
voltages to rated voltage by using varistors having varistor
voltages equal to the lamp voltage. This, however, would result in
a substantial increase in the average current flowing through the
varistors over time as they switched responsively to minor supply
voltage fluctuations and would consequently require heat-sinking of
the varistors. Accordingly, this invention contemplates varistor
voltages equal to approximately 125 percent of rated lamp voltage
as preferred.
In addition to providing for continuation of illumination from the
remaining lamps in a series string upon failure of some of the
lamps thereof, this invention provides two other notable
advantages. First, maintainence procedures are greatly simplified
because a failed lamp announces itself by being unilluminated.
Second, since conduction continues through a varistor element
having a slightly greater voltage drop across it than the failed
lamp had when operating, a lamp failure in a string in accordance
with this invention results in a decreased stress on the remaining
lamps of the string. This is a distinct advantage in cases in which
the initial lamp failure is the result of a power system anomally
which may have a duration of several seconds. In such cases, the
failure of the first lamp provides a real protection to the
remaining lamps, some of which would otherwise also fail during the
duration of such power system anomally.
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.
* * * * *