US 3930183 A
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. The body of varistor material is alternatively included in the lamp base member or comprises a pill of varistor material having contact members for insertion in a lamp socket.
Description (OCR text may contain errors)
'  Inventor:
United States Patent 11 1 Harnden, Jr.
[ Dec. 30, 1975 INCANDESCENT LAMPS HAVING PROTECTION AGAINST VOLTAGE SURGES John D. Harnden, Jr., Schenectady, NY.
 Assignee: General Electric Company,
22 Filed: Apr. 30, 1973 21 Appl. No.: 355,897
 US. Cl. 315/71; 315/75; 315/126; 315/311; 338/219  Int. C l. 1101K 1/62  Field of Search 315/46, 48, 71, 125, 205, 315/92, 58, 72, 75,126, 311; 313/315; 338/219  References Cited UNITED STATES PATENTS 2,324,212 7/1943 lnsley 315/71 2,356,296 8/1944 Zinn 315/125 X 2,878,422 3/1959 Stoelting 315/125 X 3,222,567 12/1965 Smith 1 I 315/71 3,467,937 9/1969 Norton.. 338/219 3,818,263 6/1974 Belko 315/71 X Primary Examiner siegfried H. Grimm Attorney, Agent, or Firm.lack E. Haken; Joseph T Cohen; Jerome C. Squillaro  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. The body of varistor material is alternatively included in the lamp base member or comprises a pill of varistor material having contact members for insertion in a lamp socket.
11 Claims, 5 Drawing Figures Patent Dec. 30, 1975 3,930,183
El IIIIIII I lllllll I lllllll llllllll ll ||I l|| 5 G4 oc=25 0640 i llllllll llllllll llllllll llllllll |||l|l|| INCANDESCENT LAMPS HAVING PROTECTION AGAINST VOLTAGE SURGES 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 concurrently filed application Ser. No. 355,898. 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 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 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 -20 times the resistance of the same filament when cold. Naturally, the currents through the lamp exhibit the same 10:1 :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 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 objectof this invention is to be 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 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.
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 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 alter native embodiment of a protected incandescent lamp in accordance with this invention.
FIG. 4 is a cross-sectional elevation view of a varistorlamp 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 of FIG. 4 illustrating the operational interaction between the device of FIG. 4 and the protected lamp and itssocket.
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:
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 a is an exponent greater than 1. Both C and a 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 a is primarily a function of the grain boundary. Materials such as silicon carbide exhibit nonlinear or exponential resistance characteristics and have been utilied in commercial silicon carbide varistors, however, such nonmetallic varistors typically exhibit an alpha (0:) 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 within the current density range of 10 to 10 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 becontrolled in the ceramic fabrication process. The nonlinear resistance relationship of polycrystalline metal oxide varistors is such that the resistance is very high (l0,000 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. l
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 (or) wherein the top line a 4 is typical for silicon carbide varistors and the three lines a 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 a 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 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 tungsten. Lamp 20 also includes a base member indicated generally at 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 fluidsealingly 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 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 isolted 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, electrically in parallel with the connection of filament 22 to contact 32 by lead 23. Thus, 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 exeeding 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.
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 contactto a second face 45 of varistor member 41 and upon compres sion through insulator block 48 is urged outwardly to contact the shell member of a lamp base.
FIG. 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 contact 52 is positioned to provide electrical 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 tocontact 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 through conductive member 42 and spring contact members 46 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.
The invention claimed is: 1. A voltage surge protected incandescent lamp comprising:
a filament; an envelope for containing said filament in an oxidizing agent excluding atmosphere; and
polycrystalline varistor means connected electrically in parallel with said filament for shunting voltage surges to protect said filament therefrom.
2. The surge protected lamp of claim 1 wherein:
said lamp further comprises a base member including two conductive members for connecting said filament to a source of electrical energy; and
said polycrystalline varistor means is disposed between said conductive members in electrical and mechanical contact therewith.
3. The surge protected lamp of claim 1 wherein:
said lamp further comprises a base member including two conductive members for connecting said filament to a source of electrical energy and an'insulating member disposed between said conductive members; and
said polycrystalline varistor means comprises a body of polycrystalline varistor material having a pair of opposed faces, each of said opposed faces being electrically connected to a corresponding one of said pair of conductive members.
4. The surge protected lamp of claim 1 wherein said polycrystalline varistor means is a body of polycrystalline varistor material comprising zinc oxide as a major constituent and a minor constituent selected from the group consisting of other metal oxides and halides.
5. The surge protected lamp of claim 4 wherein said body has a varistor a exponent in excess of l0 in the current density range of 10' to 10 amperes per square centimeter.
6. The surge protected lamp of claim 1 wherein said polycrystalline varistor means is adapted to be inserted in a lamp socket and comprises:
a body of polycrystalline varistor material having a pair of substantially parallel opposed faces and a void in said body extending from a first of said faces to a second of said faces;
first conductor means connected to said first face for providing electrical connection between said first face and a center contact of a lamp socket;
second conductor means connected to said first conductor means and traversing said void for providing electrical connection between said first conductor means and a center contact of a lamp base; and
third conductor means connected to said second face for providing electrical connection between said second face and a shell contact of a lamp socket.
7. The surge protected lamp of claim 6 wherein said third conductor means is a conductive spring member and further including:
means responsive to compressive forces provided by a lamp base secured in said lamp socket for urging said conductive spring member into contact with said shell contact.
8. The surge protected lamp of claim 7 wherein said means responsive to compressive forces comprises:
an extension of said second conductor means depending toward said third conductive means; and
insulator means disposed between said extension and said third conductor means for transmitting mechanical forces and preventing electrical contact therebetween.
9. The surge protected lamp of claim 6 wherein said body of 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.
10. The surge protected lamp of claim 9 wherein said body has a varistor a exponent in excess of 10 in the current density range of 10 to 10 amperes per square centimeter.
11. A surge protective device adapted to be inserted in a lamp socket comprising:
a body of polycrystalline varistor material having a pair of substantially parallel opposed faces and a void in said body extending from a first of said faces to a second of said faces;
3,930,l 83 7 8 first conductor means connected to said first face for ductor means and a protected d i d grovldng i zgg gg betweel f first third conductor means connected to said second face ace an a cen er a amp soc e a Second conductor means connected to Said first for providing electrical connection between said conductor means and traversing said void f second face and a shell contact of said lamp socket.
viding electrical connection between said first con-