Current surge eliminator
United States Patent [191 Kahn [ CURRENT SURGE ELIMINATOR [75] Inventor: David Kahn, Harrisburg, Pa.  
 [73] Assignee: AMP Incorporated, Harrisburg, Pa.  
 [22] Filed: Apr. 16, 1973 [21] Appl. No.: 351,717  
 Primary E.ruminer.lames D. Trammell Assistant -E.\&#39;aminerPatrick R. Salce Attorney, Agent, or Firm-Russell J. Egan, Esq.  
 [451 Apr. 1, 1975 [57] ABSTRACT A device for eliminating current surges in electrical circuits at the initiation of operation of the circuit includes a polyconductive resistance element formed of a polyconductor material having the characteristic that its electrical resistance value at temperatures below a predetermined transition temperature is at least several orders of magnitude larger than its resistance value at temperatures above the transition temperature. Thus, when electrical current is initially supplied to the circuit, through the polyconductor, it passes through the polyconductor in the latters high resistance state until the current heats the polyconductor to its transition temperature, at which time the polyconductor switches to its low resistance state. During the heat up time period the current also heats the electrical elements in the circuit. raising them to their normal operating values, thereby preventing a current surge in the circuit.  
 11 Claims, 2 Drawing Figures 1 CURRENT SURGE ELIMINATOR The present invention relates to a device for eliminating current surge in electrical circuits and in particular. to a device for eliminating current surge when a cold incandescent lamp is energized.  
  It is well known that the operative life expectancy of conventional incandescent lamps or light bulbs is severely effected by current surge through the filament of the lamp when the lamp is energized. This current surge occurs when a cold incandescent lamp is turned on because of the fact that the resistance value of the cold filament within the lamp is more than 10 times smaller than its resistance value when heated. i.e., its value after the lamp has been on for a short period of time. Since the resistance value of the filament in the incandescent lamp is relatively low when the bulb is cold, there is a relatively large current flow through the filament when the lamp is initially turned on, resulting in a thermal shock being applied to the filament. This thermal shock causes deterioration of the filament and thus lessens its operative life expectancy.  
  Accordingly. it is an object of the present invention to substantially eliminate current surge in an incandescent lamp when the latter is energized.  
  Another object of the present invention is to reduce current surge in electrical circuits.  
  Yet another object of the present invention is to automatically delay the surge of current passing through the filament of an incandescent lamp when the lamp is first energized, and then. after a predetermined time delay. to allow a full predetermined current to flow through the filament.  
  Yet another object of the present invention is to provide a device for eliminating current surge in electrical circuits and in incandescent lamps. which is relatively inexpensive in manufacture and is durable in operation.  
  A still further object of the invention is to provide a current surge eliminator in which the time delay for controlling current surge is easily adjustable during manufacture.  
  The current surge eliminator of the present invention comprises a polyconductor electrically connected to an electrical circuit and/or to an incandescent lamp. The polyconductor used in the present invention is composed of recently developed materials which can be made in a variety of compositions and which have the peculiar feature of exhibiting a negative temperatureresistance characteristic such that the material exhibits metallic electrical conductivity above. and little or no electrical conductivity below a predetermined transition temperature. That is, these materials have a relatively high resistance value below their critical transition temperature. and a resistance which is several orders of magnitude less when the material is at a temperature above the critical transition temperature. for example, to one thousandth of its high resistance value. These materials again resume their high resistance state when the temperature of the element is reduced below its critical transition temperature.  
  A number of such polyconductors and processes for doping the conductors in order to adjust the desired transition temperatures thereof are disclosed in US. Pat. Nos. 3,402,131 to Futaki et a1 and 3,532,641 to Chamberland. In addition to the polyconductors disclosed in those patents, numerous other types of polyconductors are available, each of which has a transition temperature at which the element abruptly changes its electrical property, being relatively non-conductive below the transition temperature, T and being electrically conductive above the transition temperature.  
  In a preferred embodiment of the invention, a polyconductor resistance element exhibiting this negative temperature-resistance characteristic is positioned in the base of a socket for an incandescent lamp in contact with the electrical contact at the base of the socket. In this position the polyconductor is connected in series between the source of current and the filament of an incandescent bulb placed in the lamp socket. As a result. when the lamp is turned on by operating the appropriate control switch, a relatively low electric current is initially passed through the polyconductor resistance element and gradually heats the same to its transition temperature, in accordance with Joules law. This low electric current passing through the polyconductor is supplied to the filament of the incandescent lamp bulb and raises the temperature of the latter.  
  As is well known, the resistance value of filaments in incandescent lamps increases in proportion to its temperature. Thus, the low electric current supplied to the filament causes the temperature thereof to increase and thus increases its resistance value. Ultimately. (within fractions of a second) the polyconductor reaches its transition temperature wherein its conductivity is increased by several orders of magnitude so that the intended current load is then supplied to the bulb filament. Because the filament has been preheated. its resistance value has been substantially increased. and the current surge normally evident when cold incandescent lamps are energized is eliminated.  
  The time delay required for the polyconductor element to reach its critical transition temperature, due to Joule heating, is selected to be equal to or greater than the time required for the filament in the incandescent lamp to reach a temperature at which its resistance value is such as to avoid current surge therethrough. This can be accomplished by doping the polyconductor with a material having a high resistivity and a low temperature coefficient characteristic, e.g. a carbon compound, so as to vary the Joule effect heating of the polyconductor when current is initially applied from the source. The carbon compound thus provides a simply way of controlling the time delay of the polyconductor, i.e., the speed at which the polyconductor reaches its critical temperature, but does not effect the critical temperature of the polyconductor.  
  The above, and other objects, features and advantages of the present invention will be apparent in the following detailed description of an illustrative embodiment thereof which is to be read in connection with the accompanying drawings, wherein:  
  FIG. 1 is a partial elevational view, in section. of an incandescent lamp assembly using a polyconductor element in accordance with the present invention; and  
  FIG. 2 is a perspective view of the polyconductor element shown in FIG. 1.  
  Referring to the drawing in detail. and initially to FIG. 1 thereof, an incandescent lamp or bulb 10 is shown mounted in a conventional lamp socket 12 connected to a schematic electrical circuit 14 through which current is selectively supplied to the lamp. Electrical circuit 14 includes a source of current 16, which I can be a conventional AC source or a battery, as would be apparent to those skilled in the art, and a switch 18 by the source of current is selectively connected to the lamp socket 12 in order to supply current to the bulb 10 mounted therein.  
  The bulb 10 is a conventional incandescent lamp which includes a resistance filament element 20 (formed of tungsten or the like) mounted therein in the conventional manner for producing light upon the supply of current thereto. As mentioned, resistance elements or filaments used in incandescent lamp bulbs have the characteristic that their resistance value increases substantially as the temperature of the filament increases. Accordingly, when the lamp is initially energized, in a conventional circuit, there is a current surge through the cold filament which produces thermal shock in the filament and deteriorates the latter, thereby reducing its useful life expectancy. This current surge continues until the filament reaches its normal operating temperature at which its resistance is increased as much as ten times, so that only a relatively low current flow passes therethrough.  
  Socket 12 is of conventional construction and includes a threaded collar portion 22 which threadedly receives the end 24 of bulb l and functions in the conventional manner as an electrical contact connected to the circuit 14. The base 26 of socket 12 includes a central contact member 28 which is also connected to a circuit 14 and which is electrically isolated from cylindrical wall 22 by an insulator disk 30 or the like, in the conventional manner. In a conventional incandescent lamp assembly, the central contact 28 of socket 12 is directly engaged with the center contact 32 on the bottom of base 24 of incandescent lamp 10.  
  In accordance with the present invention a polyconductor resistance element 34 is positioned between bulb contact 32 and the socket contact 28 in order to reduce and/or eliminate the current surge in the filament 20 of bulb when the switch 18 is initially closed. Polyconductor 34, as illustrated in FIG. 2 of the drawing, includes a central&#39;disk member 36 formed of a polyconductive material having a negative temperature-resistance characteristic such that its electrical resistance value at temperatures below a predetermined transition or critical temperature is at least several orders of magnitude larger than its resistance value at temperatures above the transition temperature, while exhibiting an abrupt transition between its high resistance value and low resistance value at the transition temperature. The polyconductive disk 36 is surrounded by a peripheral ring of insulation material 38 whose diameter corresponds substantially to the diameter of the socket 12. By this construction, the polyconductive disk 36 is located in the center of socket l2 and is electrically isolated therefrom so that it is operati vely connected in series between the socket and the filament of incandescent lamp l0, and acts as a series resistance element in the circuit.  
  Polyconductive disk 36 can be formed from any of the known polyconductive materials having a transition or Curie temperature at which the material abruptly transforms from a relatively non-conductive to a conductive material. In the preferred embodiment of the invention a vanadium dioxide (V0 material is used to form polyconductor disk 36. However, it is contemplated that other types of materials can be utilized such as 2 3, 4 1, 5 9 6 11 V8015, 2 4 V6018, z s, Ti O Ti O NbO Fe O NiS, CrS, FeS, Fesi CrN. Each of these polyconductor materials exhibits the desirable negative temperature-resistance characteristic at different transition temperatures. For example, the V0 material has a transition temperature of 65C.  
  Preferably, the material selected for use in forming the polyconductive disk 36 of the present invention has a transition temperature which is above the temperature to which the device is normally subjected, so that its temperature can vary above and below its transition temperature, in order to make use of the desirable characteristics of these materials. It will be appreciated that, for example, if the lamp 10 is to be used in high temperature applications, i.e., where the ambient temperature is above 65C, then a material having a transition temperature higher than 65C would have to be used in lieu of vanadium dioxide. In that case a different material, for example Ti O having a transition temperature of 187C would be selected.  
  With the proper polyconductive material selected to form disk 36 inserted in socket 12, as illustrated in FIG. 1, it will be seen that when switch 18 is closed to energize lamp 10, the polyconductive disk 36, whose initial temperature is below its transition temperature, will have a relatively high resistance value and will permit only a relatively low current to pass therethrough to filament 20. This low current flow will raise the temperature of the polyconductor element in accordance with Joules law, and will simultaneously raise the temperature of the filament 20. Ultimately, the polyconductor reaches its transition temperature and it abruptly switches to its relatively high conductive state, permitting the full current flow to pass to filament 20. By the time the polyconductor reaches its transition temperature, the resistance element 20 has been heated by the initial low current flow to a sufficiently high temperature to raise its resistance value to a desirable level so as to prevent a current surge therethrough when the full current flow is applied to the lamp in order to light the same. It is noted that this high current flow through the polyconductor element, once it has reached its transition temperature, will be sufficient to maintain the polyconductor at this critical temperature or higher by Joule effect heating until the switch 18 is opened. Thus, the polyconductor is, in effect, self latching in that once it reaches its conductive state it will stay in that state until current is removed therefrom. Thus, it will not return to its relatively non-conductive state as long as the lamp stays on.  
 The time delay between the closing of switch 18 and the supply of full current to filament 20, as provided bythe polyconductor element 36, can be adjusted in accordance with the value of current supplied from source 16 and the characteristics of filament 20, in order to provide a sufficiently long time period to enable the filament 20 to reach the desired temperature. This time period is normally only a matter of fractions of a second or even milliseconds. The adjustment of this time period can be effected by mixing with the polyconductive material forming disk 36 a material having a high resistance value and a low temperature coefficient characteristic. This will increase the effect of Joule heating and thereby decrease the time required for the polyconductor to reach its critical transition temperature. Such materials can, for example, constitute carbon compounds of various types which can be conveniently mixed with the crystals of polyconductive material used to form the disk. Such carbon compounds will cause the polyconductor to heat more rapidly under Joule effect heating so that its transition temperature is reached in a shorter time period, but they do not effect the transition temperature of the polyconductor itself. As the temperature of the disk 36, and more particularly of the polyconductor crystals, reaches the critical temperature the polyconductor crystals become substantially conductive and essentially short out the high resistance carbon components, so that those components have substantially no effect in the device above the critical temperature thereof. Of course, other high resistivity, low temperature coefficient compounds may be substituted for the carbon in other embodiments. An advantage of this form of polyconductive disk 36 is that the time delay of the device is easily and selectively controlled during manufacture without altering the critical temperature of the polyconductor. Thus, the critical temperature may be selected to be well above the ambient temperature of the operating environment and yet the time delay may still be selectively controlled by the proportion of added carbon material.  
  It is also noted that the transition temperature of the polyconductor materials themselves can be varied as desired in order to adjust the characteristics of the polyconductor disk to meet the requirements of the operating conditions of the lamp. This is achieved by doping the polyconductive material with minor amounts of fluoride or optionally with metals, as described, for example, in the above-identified patents.  
  It has also been found that a greater degree of control of the time delay and of the rate of Joule heating in the polyconductor element 36 can be achieved by forming the polyconductors 36 of a mechanical mixture of two or more polyconductor materials. That is, the transition of polyconductor disk element 36 from its relatively non-conductive state to its full conductive state can be staged by mechanically mixing two or more polyconductive materials having different transition temperatures. As these materials are normally provided in crystal form there will be no chemical reaction therebetween but the crystals will rather simply be located adjacent each other in the mechanical mixture to form the disk 36. Thus, for example, if crystals of V having a-transition temperature of 65C, FeS, having a transition temperature of 138C, and Ti O having a transition temperature of 187C, were mechanically mixed together in equal proportions to form the disk 36, the conductivity of the entire disk would increase in three stages as each of the three polyconductor materials reach their respective temperatures according to Joule heating. Thus, it is possible to control the rate of conductive transformation of the polyconductive element and thus the rate of heating of filament to its desired operating temperature.  
  Of course it is to be understood that although the present invention has been described herein particularly for use in preventing current surges in incandescent lamps, the invention is suitable for use in any type of electrical circuit or with other electrical devices or appliances, where it is necessary to limit large current surges. Thus, the device is not limited to simply protecting incandescent lamps from deterioration due to the effects of current surges, but is also suitable for use in other types of devices wherein current surges present a problem. Moreover, it is noted that by the use of the polyconductor element in a circuit such as shown in FIG. 1, arcing at the switchcontacts of switch 18 will be avoided since only a low current flow will pass through the contacts during the time taken for the polyconductor to reach its critical temperature. During that time period the contacts will reach their stable configuration, so that arcing due to bouncing of the contacts when they are closed is avoided.  
  Although an illustrative embodiment of the present invention has been described herein with reference to the accompanying drawings, it is to be understood that the invention is not limited to that precise embodiment and that various changes and modifications may be effected therein by one skilled in the art without departing from the scope or spirit of this invention.  
 What is claimed is:  
  1. A device for eliminating current surge between a source of current and an electrical appliance using said current, said device being adapted to be connected in series between said current source and appliance and comprising, a polyconductive material having the characteristic that its electrical resistance value at temperatures below a predetermined transition temperature is at least several orders of magnitude larger than its resistance value at temperatures above said transition temperature, said transition temperature being above the temperature to which said device is normally subjected whereby said polyconductive resistance element initially passes a relatively low predetermined magnitude of electrical current to said appliance until the temperature of the element is raised to said transition temperature by the Joule effect heating whereupon a higher magnitude of electrical current is passed through said device to operate said appliance; said polyconductive material comprising a mechanical mixture of at least two different polyconductive compositions having different transition temperatures, whereby said resistance element is transformed from its highest resistance state to its lowest resistance state in stages.  
  2. The device as defined in claim 1 wherein said polyconductive material includes V0 3. The device as defined in claim 1 wherein material having high resistivity and low temperature coefficient characteristics is mixed with the polyconductive material to aid in the Joule heating of the polyconductive material when current is applied thereto from said source.  
  4. The device as defined in claim 3 wherein said high resistivity material comprises a carbon compound.  
  5. The device as defined in claim 1 wherein said electrical appliance is an incandescent light bulb.  
  6. The device as defined in claim 1 wherein said at least two different polyconductive compositions are selected from the group consisting of V0 V 0 V 0 5 9 V6011 V8015, 2 4 V6013 z a, 3 5a s a NbO FE O NiS, CrS, FeS, FeSi and/or CrN.  
  7. A device for protecting an incandescent lamp from current surge comprising, a socket for an incandescent lamp, said socket having a base including an electrical contact member adapted to be electrically connected to a source of current, an incandescent lamp operatively engaged in said socket and having a base contact member adapted to be engaged with the base contact element of said socket, and a polyconductor resistance element positioned between said base contact elements, said polyconductive resistance element being formed of a polyconductive material having the characteristic that its electrical resistance value at temperatures below a predetermined transition temperature is at least several orders of magnitude larger than its resistance value at temperatures above said transition temperature; said transition temperature being selected to be above the temperature to which said device is normally subjected whereby said resistance element initially passes a relatively low predetermined magnitude of electrical current to said incandescent lamps to heat the filament in the lamp and increase its electrical resistance value at a controlled rate until the temperature of the resistance element is raised to its transition temperature by Joule effect heating whereupon a higher magnitude of electrical current is passed through said resistance element to the heated filament of said incandescent lamp; said polyconductive material comprising a mechanical mixture of at least two different polyconductive compositions having different transition temperatures, whereby said resistance element is transformed from its highest resistance state to its lowest resistance state in stages.  
  8. The device as defined in claim 7 wherein said resistance element comprises a relatively thin disk of said polyconductive material and a peripheral border of insulating material operatively connected to the periph-- high resistivity material comprises a carbon compound. =l=