ELECTRIC INCANDESCENT LAMP FOR OPERATION AT MAINS VOLTAGE

Abstract: The electric incandescent lamp for operation at mains voltage has a glass lamp vessel (1) provided with a pinched seal (5), in which a metal foil (9) is embedded, which is connected to an internal current conductor (14) extending to a filament (4). The major surfaces (7, 8) of the pinched seal (5) have above the internal current conductor (14) an embossed part (17, 18), as a result of which a space (23) is present in the pinched seal (5) on either side of said current conductor (14). Depressions (19-23) increase the resistance to pressure of the lamp vessel (1). A discharge arc that can be produced in the lamp extinguishes when the arc has reached the pinched seal (5) if the internal current conductor (14) has a wire diameter of at most 200µm.

Description: The invention relates to an electric incandescent lamp for operation at mains voltage comprising: a glass lamp vessel sealed in a vacuum-tight manner, having a part enclosing a cavity in which a helically wound filament is arranged and having at an end a pinched seal which has two major surfaces, the glass containing at least 95% by weight of SiO₂, a metal foil embedded in said pinched seal parallel to the major surfaces thereof and being connected at a first end to an external current conductor emanating from the pinched seal, an internal current conductor connected to said filament and extending from the cavity of the lamp vessel into the pinched seal and welded to the metal foil at a second end thereof, said internal current conductor being in the pinched seal partially free from the glass of the pinched seal. Such a lamp is known from GB 2 072 416 (PHN 9719). The known lamp has within the pinched seal a second cavity in which the part of the internal current conductor located in said cavity is on all sides free from the glass of the pinched seal. This second cavity becomes manifest in that the surface of the pinched seal has an embossed part, i.e. a fold, which extends at right angles to the internal current conductor. The cavity of the lamp vessel in which the filament is situated communicates through a narrow duct with the second cavity in the pinched seal in that the internal current conductor, after the pinched seal has been provided at an elevated temperature, shrinks to a greater extent than the glass of the pinched seal. Due to the construction of its pinched seal, the known lamp has the property that a discharge arc, which may be formed when the filament breaks and which is capable of melting away the fragments of the filament and the internal current conductor, extinguishes in time. Although no additional components are used for assembling the lamp, the lamp therefore has nevertheless the function of an incorporated fuse. It is important that an incandescent lamp has an incorporated fuse because said lamp is often used in luminaires not provided with a fuse. In a lamp without an incorporated fuse, when a discharge arc is produced after the filament has broken at the end of the life of the lamp the current through the lamp can increase to such an extent that the lamp explodes or that the fuse of the equipment to which the lamp is connected melts. A disadvantage of the known lamp is that its construction causes its manufacture to be time-consuming because the second cavity in the pinch can be evacuated and cleaned only with great difficulty due to the fact that this space is accessible only through a narrow duct from the cavity in which the filament is arranged. Another disadvantage is that the manufacture of the lamp requires pinch blocks, which results in a distinctly different profile of the pinched seal. Consequently, for the manufacture of lamps which do not require a fuse function and which need not or must not have a strongly deviating pinch surface profile, other pinch blocks must be mounted on the production machine. The invention has for its object to obviate these disadvantages and to provide a lamp having a construction which not only fulfils the function of an incorporated fuse without the use of additional components, but can also be realized very simply and rapidly and can be manufactured with standard pinch blocks, which can be used also for other lamp types, such as low-voltage lamps. According to the invention, this object is achieved in a lamp of the kind described in the opening paragraph in that the major surfaces of the pinched seal have above the internal current conductor an embossed part extending from the part of the lamp vessel enclosing its cavity beyond the second end of the metal foil and having a width transverse to the internal current-conductor which is at least the quintuple of the diameter of the internal current conductor, and in that the internal current conductor has a wire thickness of at most approximately 200µm. Due to the embossed part on the major surfaces of the pinched seal, a space is present in a plane parallel to said major surfaces laterally of said internal current conductor on either side thereof, as a result of which said current conductor is free on said two sides from the glass of the pinched seal. In a plane through the internal current conductor at right angles to the major surfaces, the glass is generally in contact with the current conductor in spite of the embossed parts. It has been found that this geometry results in that a discharge arc extinguishes when said arc, after having molten away the fragments of the filament and the internal current conductor, has reached the pinched seal. The lamp according to the invention has a fuse function which is very reliable, although no additional components are required to realize said function. The modification of the pinched seal required for this function is moreover very small so that the pinch blocks can be used for many types of lamps. The lamp has a simple construction, which can be rapidly and readily realized. The lamp vessel can be readily evacuated and cleaned during the manufacture of the lamp, i.e. more readily than in the case of a conventional lamp without a fuse function and more readily than in the case of a lamp according to the Patent Specification mentioned in the opening paragraph. This can be explained as follows. Due to the very small expansion coefficient of the glass of the lamp vessel, an internal current conductor, which in fact has a considerably higher expansion coefficient, cannot be embedded in a pinched seal in a vacuum-tight manner. A narrow duct extending beyond the weld of said current conductor to the metal foil is always present along said current conductor. However, this duct is so narrow that said current conductor in fact touches the glass of the pinched seal. In conventional lamps during its manufacture, volatile constituents have also to be removed from this narrow duct. During the manufacture of a lamp according to the aforementioned British Patent Specification, even the second cavity in the pinched seal has to be evacuated via such a duct. In the lamp according to the invention, however, such a duct is comparatively wide in the plane through the metal foil due to the fact that there is a space in said plane on either side of the internal current conductor. Said space can be observed very clearly by means of a magnifyingglass having a magnification of only six times. It has been found that the measure according to the invention results in a reduction of the gas pressure in the lamp vessel, at which the pinched seal is destroyed. Fatal gas pressures of 30 bar have been measured. However, during operation of the lamp under normal conditions, such pressures are not reached. Nevertheless, because of a larger safety margin and a larger permissible spread, it is of importance that a measure has been found by which the gas pressure that can cause the pinched seal to be destroyed is increased. Said measure consists in that depressions are provided in the major surfaces of the pinched seal, these depressions adjoining the part of the lamp vessel enclosing its cavity and the embossed part of said major surfaces. With depressions of 0.2 mm under the surface of the major surfaces, an increase of the fatal gas pressure to at least 50 bar was found. The measure according to the invention is especially of importance for mains voltage lamps having a power of less than 1000 W. Lamps of higher powers have filaments of wire having such a thickness that a discharge arc does not melt away said wire. The fragments of the filament thus remain connected in series with the discharge arc which may be formed and consequently limit the arc current. In such lamps, an incorporated fuse is therefore not required. The embossed part on the major surfaces of the pinched seal must have a considerable width in order to achieve that a space is formed on either side of the internal current conductor. This width is related to the diameter of the internal current conductor. When this internal current conductor is a helically wound (s.c.) end of a coiled coil (c.c.) filament, the minimum width of the embossed part is larger than when the internal current conductor is a straight wire end of the wire from which the filament is wound. The width of the embossed part should be at least the quintuple of the diameter of the internal current conductor. The embossed part generally projects above the surface of the major surfaces by a few tenths of a millimetre, for example 0.4 or 0.5 mm. The internal current conductor may be an end of the filament that may be helically wound. On the other hand, the internal current conductor may be a separate body fitting into or arranged to surround the filament. In order to prevent that the internal current conductor will act as an electrode on which a discharge arc can be maintained, said current conductor has a wire thickness of at most approximately 200µm. The lamp according to the invention may have two pinched seals with a metal foil embedded in each of them or one pinched seal with two metal foils embedded therein. In these two cases, respective embossed parts can be present only above one of the two or above each of the two internal current conductors on the major surfaces of the pinched seal(s). The lamp may have a halogen/containing gas filling. It should be noted that in DE GM 1952883 an incandescent lamp is described, in which the major surfaces of the pinched seal have above the internal current conductor an embossed part which has a width of twice the diameter of said conductor. A similar embossed part is provided above the external current conductor. These embossed parts together aim at preventing that the metal foil cracks when the seal is formed. The embossed parts do not result in that a space is formed on either side of the internal current conductor, the embossed part is too narrow to achieve this. Just because of this, a fuse function is not obtained for this lamp. Furthermore, the internal current conductor is a very thick wire on which a discharge arc is maintained like on an electrode. Due to the fact that the discharge arc is maintained on the internal current conductors and fragments of the filament then fail, the arc current can increase to a very high value and the lamp can explode. An embodiment of the mains voltage incandescent lamp according to the invention will be shown in the drawing. In the drawing: Figure 1 is a side elevation of a lamp, Figure 2 is a side elevation of the lamp shown in Figure 1 rotated through 90°, Figure 3 shows a cross/section taken on III-III in Figure 1. In the Figures, the lamp has a lamp vessel 1 of glass, which is sealed in a vacuum-tight manner. The glass has an SiO₂ content of at least 95% by weight; it consists, for example, of quartz glass. The lamp vessel 1 has a part 2 enclosing a cavity 3, in which a helically wound filament 4 is arranged, and having at an end a pinched seal 5. The lamp shown has a second pinched seal 6. The pinched seal 5 has two major surfaces 7, 8. A metal foil 9 (10), for example of molybdenum, is embedded in the pinched seal 5 (6) parallel to the major surfaces 7, 8. At a first end 11, the foil 9 (10) is connected to an external current conductor 12 (13), for example of molybdenum, which emanates from the pinched seal 5 (6). An internal current conductor 14 (15), in the drawing a single-coiled (s.c.) limb of the coiled-coil (c.c.) tungsten filament 4 connected to said filament 4 and extending from the cavity 3 into the pinched seal 5 (6), is welded to the foil 9 (10) at the second end 16 thereof. The internal current conductor 14 is in the pinched seal 5 partially free from the glass of this seal. The major surfaces 7, 8 of the pinched seal 5 have above the internal current conductor 14 an embossed part 17, 18, which extends from the part 2 of the lamp vessel 1 enclosing the cavity 3 to beyond the second end 16 of the foil 9 and has a width w (Figure 1), i.e. a dimension transverse to the internal current conductor 14, which is at least the quintuple of the diameter of the internal current conductor 14. The internal current conductor 14 has a wire thickness of at most 200µm. The major surfaces 7, 8 of the pinched seal 5 have depressions 19, 20, 21, 22, which adjoin the part 2 of the lamp vessel 1 enclosing its cavity 3 and the embossed part 17, 18 of said major surfaces 7, 8. Figure 1 indicates that the internal current conductor 14 has in a plane parallel to the major surfaces 7, 8 on both sides a space 23, which narrows from the cavity 3 (cf. also Figure 3). A lamp having the shape shown in the Figures consumed at 220 V a power of 500 W. The lamp had a filling of 2.5 bar of Ar and 1.5% by volume of CH₂Br₂. The internal current conductors were constituted by the s.c. limbs of the c.c. filament and had a diameter of 433µm and a wire thickness of 121µm. An embossed part having a width w of 4 mm was situated above an internal current conductor on each of the two major surfaces of the pinch. The embossed part projected above the surface of the pinched seal by approximately 0.4 mm. On either side of the embossed parts, a depression was present having a depth of approximately 0.2 mm. In an experiment half of the filament was shortcircuited. The lamp was operated for 60 seconds at 150 V, after which the voltage was suddenly increased to 260 V. The non-shortcircuited part of the filament burned through and a discharge arc was obtained. In all cases (28 lamps), the arc extinguished within 10 ms. In lamps without the measure according to the invention, an explosion always followed under the same conditions. In the test lamps, the distance between the end 16 of the metal foil 9 and the cavity 3 of the lamp vessel 1 varied from 2.5 to 6.5 mm. Lamps of the kind shown in the drawing, but having an s.c. filament having internal current conductors constituted by straight limbs of the filament (wire diameter 164µm), were intended to consume a power of 500 W during operation at 120 V. During normal operation, either an internal current conductor or the filament was molten by means of a laser. The discharge arc then produced extinguished when the pinched seal was reached without explosion occurring.

Claims: An electric incandescent lamp for operation at mains voltage comprising: a glass lamp vessel (1) sealed in a vacuum-tight manner, having a part (2) enclosing a cavity (3) in which a helically wound filament (4) is arranged and having at an end a pinched seal (5) which has two major surfaces (7,8), the glass containing at least 95 % by weight of SiO₂, a metal foil (9) embedded in said pinched seal (5) parallel to the major surfaces (7,8) thereof and being connected at a first end (11) to an external current conductor (12) emanating from the pinched seal (5), an internal current conductor (14) connected to said filament (4) and extending from the cavity (3) of the lamp vessel (1) into the pinched seal (5) and welded to the metal foil (9) at a second end (16) thereof, said internal current conductor (14) being in the pinched seal (5) partially free from the glass of the pinched seal. characterized in that the major surfaces (7,8) of the pinched seal (5) have above the internal current conductor (14) an embossed part (17,18) extending from the part (2) of the lamp vessel (1) enclosing its cavity (3) beyond the second end (16) of the metal foil (9) and having a width transverse to the internal current conductor which is at least the quintuple of the diameter of the internal current conductor (14), and in that the internal current conductor (14) has a wire thickness of at most approximately 200 µm. An electric lamp as claimed in Claim 1, characterized in that the major surfaces (7,8) of the pinched seal (5) have depressions (19,20,21,22) which adjoin the part (2) of the lamp vessel (1) enclosing its cavity (3) and the embossed part (17,18) of said major surfaces (7,8).

Inventors: OOMS, LEO FRANS MARIA
Applicants: N.V. PHILIPS' GLOEILAMPENFABRIEKEN
Application year: 1987
Main classification: H01K1
IPC codes: H01K1, H01K5, H01J5