HIGH PRESSURE DISCHARGE LAMP WITH THERMAL SWITCH

Abstract: An electric discharge lamp (5) has an arc tube within an outer gas filled glass envelope (7), and a thermal switching means is located within the outer envelope. The thermal switching means (47) has a bimetal strip (49) and a spring-like member (51) each affixed to at least one of the electrical conductors, and upon application of heat the bimetal strip (49) and spring-like member (51) are flexed to short-circuit the electrical conductors and remove any DC potential between the main electrode (21) and the starting electrode (25) minimizing electrolysis.

Description: The invention relates to an electric discharge lamp having thermal switching means according to the preamble of claim 1. Such an electrical discharge lamp is known from US-A-3 965 387. Electric discharge lamps such as metal halide lamps which include mercury, the halides of various metals and particularly sodium iodide undesirably are subject to electrolysis between the usual starter electrodes and the electrode immediately adjacent thereto. Moreover, any DC potential existing between the above-mentioned electrodes undesirably increases electrolytic activity and especially so as operational temperatures increase. Generally, the metal halide lamps include a sealed glass envelope with a fused silica arc tube disposed within the sealed glass envelope. For some time, the outer glass envelope was evacuated whereupon a thermal switch was subjected to a temperature which was determined by lamp wattage and substantially independent of lamp orientation or position of operation. Thus, a simple thermal switch could be incorporated into the outer glass envelope and accommodate temperature variations encountered while employing commercially available bimetal materials. As an example of the employment of a relatively simple thermal switch in a metal halide discharge lamp, reference is made to US-A-3 226 597. Therein, a substantially U-shaped bimetal strip expands, in response to heat, to contact an inlead and short-circuit the inleads. However, better performance and other desirable features and considerations led to the use of a gas fill within the outer glass envelope. Accordingly, the variable of convective heating was added to lamp wattage in determining the operating temperatures of a thermal or bimetal type switch. Moreover, lighting fixture manufacturers, in recent years, are designing systems to widen the beam spread of available lamps and have utilized the concept of moving the lamp further into the reflector cavity of the fixture. As a result, the temperature of the lamp components and of the thermal switch associated with the lamp has been increased. Unfortunately, such increased temperatures tend to stress the bimetallic material of available thermal switches beyond the elastic limit whereupon permanent deformation of the thermal switch undesirably results. Thus, thermal switch failure permits the previously-mentioned electrolysis to take place and results in failure of the discharge lamp. Additionally, the above-mentioned overheating of the thermal switch presents no problem with regard to deformation when a switch is used which is normally closed and opens as the temperature is increased. However, for normally open switches which close as the temperature increases such permanent deformation of the switch is a problem. Therein, contact closure imposes a restraint in movement of the bimetal material, and this restraint may cause a permanent deformation which prevents return of the switch to a normally open condition and eventually results in lamp failure. One known attempt to overcome the above-mentioned permanent deformation of the bimetal material in a discharge lamp is set forth in US-A-3 965 387. Therein, a spring-like member is affixed to one end of a bimetal strip. The bimetal strip is fastened at the opposite end to an electrical conductor of the discharge lamp. Upon activation due to heat, the bimetal flexes which, in turn, causes the spring-like member to contact a second electrical conductor. Thus, short-circuiting of a pair of electrical conductors is effected. Moreover, the spring-like member deflects which prevents distortion of the bimetal strip. Although the above-described structure has been and still is employed in various forms of discharge lamps, it has been found that this particular structure does leave a sufficient control capability to be desired. The technical problem to be solved by the invention is to improve an electric discharge lamp having thermal switching means according to the preamble of claim 1 such that a more effective control capability is achieved. This problem is solved by the features comprised by claim 1. According to the invention it has been found that individual attachment of a bimetal strip and of a spring-like member permits a precise setting of the gap therebetween which, in turn, enhances control over the operation of the structure. Moreover, this enhanced operational control is achieved without increased cost and with a reduction in complexity of the structure. Further, the response capability to increased temperature variations in an electric discharge lamp is enhanced. Advantageous embodiments are claimed by the depending claims. One way of carrying out the invention is described in detail below with reference to a drawing which illustrates only one specific embodiment, in which: FIG. 1 is an elevational view of a metal halide lamp having one embodiment of a thermal switch of the invention therein; and FIG. 2 is an enlarged perspective view of a thermal switch of the invention. Referring to FIG. 1 of the drawings, a metal halide lamp 5 includes an outer glass envelope 7 having a bulged substantially tubular configuration with a neck portion 9 closed by a stem member 11 hermetically sealed thereto. A pair of electrically conductive leads 13 and 15 are sealed into and pass through the stem member 11 and are electrically connected to the contacts of a screw-in base member 17. Disposed within the outer glass envelope 7 is a fused silica arc tube 19. The arc tube 19 has electrodes 21 and 23 sealed into opposite ends thereof, and a starting electrode 25 is sealed into one end of the arc tube 19 and positioned adjacent the electrodes 21. The electrodes 21 and 23 and the starting electrode 25 each have an electrically conductive member, 27, 29 and 31 respectively, electrically connected thereto and passing through the sealed arc tube 19. A first cage-like mounting member 33 is affixed to one end of the arc tube 19 and includes a pair of spring-clips 35 and 37 thereon which contact the outer glass envelope 7 and serve to support the arc tube 19 therein. The electrically conductive member 29 connected to the electrode 23 is also connected by a curved wire 39 to one of the electrically conductive leads 13 sealed into the stem member 11 affixed to the outer glass envelope 7. A second cage-like mounting member 40 is affixed to the opposite end of the arc tube 19 and also includes a pair of spring-clips 41 and 43 thereon which contact the outer glass envelope 7 and serve to more rigidly support the arc tube 19 therein. The second cage-like mounting member 40 is affixed to the other electrically conductive lead 15 sealed into and passing through the stem member 11 affixed to the other electrically conductive lead 15 sealed into and passing through the stem member 11 affixed to the outer glass envelope 7. The electrical conductive member 27 connected to the electrode 21 is also connected to the other electrically conductive lead 15 by way of the second cage-like mounting member 40. Also, the starting electrode 25 is connected by way of an electrical conductive member 31 to a resistor 45 which is, in turn, connected to the electrically conductive lead 13 sealed into the stem member 11. Additionally and importantly, a thermal switching means 47 is spaced from the arc tube 19 and affixed to the electrical conductors 27 and 31. As can be more clearly seen in FIG. 2, a preferred form of switching means 47 includes a bimetal strip 49 affixed at one end to the electrical conductor 31 and a spring-like member 51 attached to the adjacent electrical conductor 27. The bimetal strip 49 extends outwardly in a direction substantially normal to the longitudinal axis of the conductor 31 and the arc tube 19 and includes a notch 55 in the opposite end thereof. Similarly, the spring-like member 51 is fastened at one end to the adjacent electrical conductor 27. This spring-like member 51 extends in a direction substantially normal to the electrical conductor 27 and the arc tube 19 and parallel to the bimetal strip 49. Importantly, the bimetal strip 49 and the spring-like member 51 are spaced from one another and mounted on separate electrical conductors 27 and 31 which permit precise adjustment of the gap therebetween. Also, the notch 55 in the end of the bimetal strip 49 and the spring-like member 51 are formed for enhanced contact therebetween. Moreover, the spring-like member 51 is positioned to permit deflection thereof in response to pressure exerted thereon by the bimetal strip 49. Thus, distortion of the bimetal strip 47, when exposed to a relatively large increase in heat, is substantially eliminated by the form, location and physical characteristics of the spring-like member 51. As to operation, the arc tube 19 of a metal halide lamp, for example, normally has a fill which includes mercury and halides of various metals including sodium. Also, the outer glass envelope 7 includes a fill of an inactive gas, such as nitrogen, for example. The thermal switching means 47 is in the form of a normally-open (N/O) switch at room temperature. Moreover, the closure temperature of the thermal switching means 47 is dependent upon numerous factors, such as the placement of the switching means 47 within the envelope 7, the gas fill and the design of the switching means 47. Common thermostats practical for lamp use have been found to be satisfactory to the present application. Initially current flow to the electrode 21 and starting electrode 25 is effected with the thermal switching means 47 at room temperature. As the temperature increases, the bimetal strip flexes causing the pair of electrical conductors 49 and 51 to short-circuit. Thereupon, any electrolysis inducing DC potential between electrode 21 and starting electrode 25 is eliminated. Also, it can be seen that the notch 55 in the end portion of the bimetal strip 49 is formed to accommodate the spring-like member 51 whereupon improved electrical connection therebetween is effected. Moreover, the flexible spring-like member 51 readily flexes whenever the bimetal strip 49 flexes. In this manner, distortion of the bimetal strip 49 due to an excessive increase in temperature is prohibited. Thus, it can readily be seen that the bimetal strip 49 and spring-like member 51 in the form of individual members have numerous advantages over other known structures. Such features as individual adjustment of the gap therebetween and, in turn, the capability to select the location of the thermal switch with respect to the heat developed within the envelope 7 are advantages unavailable in other known structures. Also, the flexing of the spring-like member 51 whereby permanent deformation of the bimetal strip 51 is prevented even when subjected to excessive temperature excursions is still another advantage of the above-described structure.

Claims: An electric discharge lamp having thermal switching means comprising: an outer glass envelope (7) having a pair of electrically conductive leads (13,15) sealed therein and passing therethrough, an arc tube (19) within said outer glass envelope (7), said arc tube (19) having an electrode (21,23) within each end electrically connected to an electrical conductor (27,29) sealed into and passing therethrough and connected to one of said electrically conductive leads (13,15) with a starting electrode (25) within one end of said arc tube (19) adjacent one (21) of said electrode (21,23) and electrically coupled to an electrical conductor (31) passing through said arc tube (19) and coupled to an electrically conductive lead, said arc tube (19) having a fill of ionizable gas and subject to electrolysis at increased temperatures, and thermal switching means (47) internal of said outer glass envelope (7) and external of said arc tube (19), said thermal switching means (47) including a bimetal strip (49) and spring-like member (51) extending in a common plane substantially normal to said adjacent electrically conductors (27,31), said switching means having in its open state said bimetal strip (49) in permanent contact with one (31) of said electrical conductors and at raised temperatures in its closed state both said bimetal strips (49) and said spring-like member (51) in contact with the respective electrical conductors (31,27) and with each other in order to short-circuit said electrical conductors (27,31) of the starting electrode (25) and of the adjacent one (21) of said electrodes (21,23), to thereby maintain a common potential on both the starting and adjacent electrodes (25,21) during operation, characterized in that in said open state of said switching means said spring-like member (51) is out of contact with said bimetal strip (49) but in contact with the other (27) of said electrically conductors, and in that said bimetal strip (49) has a notch (55) formed to receive said spring-like member (51). The discharge lamp of claim 1 wherein said spring-like member (51) is formed for movement upon application of pressure thereto by said bimetal strip (49). The discharge lamp of claim 1 wherein said arc tube (19) has a gas fill including mercury and halides of various metals including sodium.

Inventors: BONAZOLI, ROBERT P.
Applicants: GTE PRODUCTS CORPORATION
Application year: 1984
Main classification: H01J61
IPC codes: H05B41, H01J61