Low-pressure mercury vapour discharge lamp

Abstract: A low-pressure mercury vapour discharge lamp comprising an electrode (6), first and second lead-in conductors (7,8) connected to respective ends of the electrode (6) and a metallic guard (9) partially surrounding the electrode (6) and supported by a conductive guard support (10). A bimetal strip (11) arranged for electrically connecting a lead-in conductor (8) with the guard (9) or guard support (10) upon heating by the discharge arc maintains the electrode (6) and the guard (9) at the same electrical potential during lamp operation to reduce the anode fall and the lamp power dissipation.

Description: The invention relates to a low-pressure mercury vapour discharge lamp comprising an electrode, first and second lead-in conductors connected to respective ends of the electrode, a metallic guard partially surrounding the electrode and supported by a conductive guard support, a discharge arc being maintained in the lamp during operation. Low-pressure mercury vapour discharge lamps generally have electrodes made of coiled tungsten wire. These electrodes are coated with a material for enhancing the thermionic emission of electrons. During lamp operation tungsten and emitter material can evaporate or sputter from the electrodes and be deposited in the area of the electrodes on the lamp wall in the form of tungsten and tungsten products. This deposition is evident as visible blackening and is a detrimental consequence of lamp operation. One technique for suppressing the blackening from electrode materials is to partially surround each electrode with a guard. The guard is typically in the form of a closed ring made of a conductive metal strip and positioned surrounding the sides of the electrode. Examples of this structure are shown in U.S. Patents 4,032,813 and 4,032,814. In these patents the conductive metal strip carries a getter such as a mixture of zirconium and aluminium for reducing the quantity of unwanted impurity gasses. Additionally, the metal strip supports a small capsule of mercury needed for normal lamp operation. Finally, the conductive metal strip reduces the lamp's energy consumption. Both of the patents just mentioned show lamp structure in which the electrical potential of the electrode guard is floating relative to the electrode potential. This is achieved by mechanically supporting it on a mount that is electrically insulated from the supports of the electrode. The floating arrangement of the electrode guard is also mentioned in the book "Fluorescent Lamps And Lighting", W. Elenbaas, ed., Sec. 5.3 (1962). In fluorescent lamps that operate on alternating current the electrodes operate alternately as a cathode and an anode. As discussed in the book "Electric Discharge Lamps", John Waymouth, Chapter 4 (MIT 1971), the alternating function of the electrodes requires compromises in the electrode design. Ideally, a fluorescent lamp anode would have a large area to reduce the potential difference between the anode and the plasma within the lamp, known as the "anode fall". The large area, however, would be detrimental to cathode operation of the electrode which requires rapid heating to thermionic emitting temperatures and to avoid sputtering during glow discharge. It would be desirable to use the prior art cathode guard as part of the electrode when the electrode is operating as an anode. This would increase the effective area of the anode and thereby reduce the anode fall; however, care would have to be taken to avoid degrading this cathode operation of the electrode. A lamp employing an electrode shield as part of the anode is disclosed in German Democratic Republic Patent Specification 221,881. In that patent a cold-starting, self-heating electrode is connected to a cylindrical concentric auxiliary electrode, having the structure of a shield, through a diode. When the electrode is biased positive to operate as the anode, the diode polarity is effective to establish a conductive path between the self-heating electrode and the auxiliary electrode. Consequently, the effective electrode total area includes the auxiliary electrode area and the anode fall is reduced. When the electrode is biased negative, that is, as a cathode, the diode polarity presents a high impedance path to the auxiliary electrode which is effectively disconnected from the self-heating electrode. It would be desirable to use the same technique in lamps with other types of electrodes. However, there are practical reasons for not doing so. The use of a diode within the lamp discharge envelope presents severe quality requirements for the component. The diode must be able to withstand an intense ultraviolet flux, and elevated temperatures for the life of the lamp which may exceed 20.000 operating hours. The diode also contributes to the lamp cost and consequently the avoidance of using a diode would be advantageous. It is an object of the invention to provide an electrode assembly which exhibits a reduced anode fall without the use of rectifying elements or similar components. According to the invention a low-pressure mercury vapour discharge lamp of the kind described in the opening paragraph is characterized in that a bimetal strip is provided being arranged such that upon starting of the lamp heat from the discharge arc causes the strip to bend and electrically connect one of the lead-in conductors and the metallic guard. During lamp operation, heat from the discharge arc keeps the bimetal strip in contact with the metallic guard and the lead-in conductor, maintaining the metallic guard and the electrode at the same potential irrespective of polarity of the electrode voltage. A favourable embodiment of the low-pressure mercury vapour discharge lamp according to the invention is characterized in that the bimetal strip has one end fixed to the lead-in conductor and a free end disposed adjacent the metallic guard and contacting said metallic guard during operation of the lamp. A further favourable embodiment of the low-pressure mercury vapour discharge lamp according to the invention is characterized in that the bimetal strip has one end fixed to the lead-in conductor and a free end disposed adjacent the conductive guard support and contacting said guard support during operation of the lamp. Use of the bimetal strip in the above fashion may be readily automated since the bimetal strip can be supplied and cut with conventional ribbon feeding machines. Since the free end of the bimetal strip may be located within a large tolerance zone because of the ability of the bimetal to bend to a large extent, precise positioning of the strip is not critical. Additionally, the placement of welding electrodes for spot welding is easy since the width of the bimetal strip provides a large area for the weld location. Embodiments of the lamp according to the invention will now be described, by way of example, with reference to the drawings. Fig. 1 shows an end portion of a low-pressure mercury vapour discharge lamp in longitudinal section. Fig. 2 is an isometric view of an electrode assembly of another low-pressure mecury vapour discharge lamp. The lamp of Fig. 1 comprises an envelope having a tubular section 1 with a smaller diameter end 2 which is closed by a stem 3. The inner surface of the lamp envelope is coated with a fluorescent material 4 which fluoresces in response to ultraviolet radiation. Electrode assembly 5 within the lamp envelope 1 is energizable for sustaining an electrical discharge through a plasma of mercury atoms within the lamp. The atoms undergo excitation and emit the ultraviolet radiation which is incident on the coating of fluorescent material 4. The fluorescent material 4 fluoresces in the visible region and emits light. The electrode assembly 5 is comprised of a filament electrode 6 supported by a pair of lead-in conductors 7 and 8. This structure permits a heater current to flow through the filament electrode 6 during lamp operation and is the type of electrode structure used with rapid-start type lamp ballasts. A metallic cylindrical guard 9 partially surrounds the filament electrode 6 and is supported on a guard support 10. A bimetal strip 11 is welded to lead-in conductor 8 and has a free end 11a disposed adjacent the metallic guard 9. The stem 3 terminates at a press seal 12 in which the lead-in conductors 7, 8 and guard support 10 are embedded. Leads 13, 14 are connected to the lead-in conductors 7, 8 and emerge from the press seal 12. A lamp end cap 15 covers the end 2 of the lamp envelope 1 and carries two lamp pins 16, 17 that are insulated from each other in a conventional manner. The respective leads 13, 14 are connected to pins 16, 17. An external voltage is applied across the pair of pins 16, 17 and is consequently developed across the filament electrode 6 for heating the electrode and operating the lamp. In the inoperative condition of the lamp the free end 11a of the bimetal strip 11 does not touch the metallic guard 9. Upon starting of the lamp heat from the discharge arc causes the free end 11a to bend against and contact the metallic guard 9. During lamp operation, the free end 11a remains in contact with the metallic guard 9 and electrically connects the guard 9 to the lead-in conductor 8 because the bimetal is continuously heated by the discharge arc. The electrical connection between the guard 9 and the filament electrode 6 established by the bimetal strip 11 increases the effective area of the electrode when it is positively biased, and operating as an anode. This reduces the anode fall and consequently the power required for lamp operation. In Figure 2 the same reference numerals have been used for identical components as in Figure 1. One end portion of the bimetal strip 11 is welded to the lead-in conductor 8 with the free end 11a disposed adjacent the guard support 10. The position of the free end 11a in the inoperative condition of the lamp is shown in broken lines. The bimetal is oriented such that heat from the discharge arc causes the free end 11a to bend against and contact the guard support 10. During lamp operation, the free end remains in contact with the guard support and electrically connects the guard 9 to the lead-in conductor 8 because the bimetal is continuously heated by the discharge arc. As shown in Figure 2, only the lower edge of the bimetal contacts the sloping portion of the guard support 10. However, this provides sufficient electrical contact. A preferred bimetal strip has a low expansion side consisting of 36% nickel, 64% iron and a high expansion side of 75% nickel, 22% iron and 13% chrome. According to the preferred embodiment, the distance between the lead-in conductor 8 and the guard support 10 at the location of the bimetal strip is approximately 4 mm. A suitable length for the bimetal is 5 mm. Six 40 Watt cool white lamps were tested having an electrode assembly with the above dimensions. It was found that the free end 11a may be spaced up to 2 mm from the guard support 10 to provide reliable contact during lamp operation. With the free end of the bimetal spaced 2 mm from the guard support, the bimetal took an average of 20 seconds to electrically connect the guard support 10 to the lead-in conductor 8 after the heater current was first applied to the filament. The bimetal is chosen such that heat from the filament electrode 6 due to the application of the heater current during starting of the lamp is not sufficient to cause the free end of the bimetal to contact the guard support. Since any filament failure would normally occur when the heater current is initially applied and before the discharge arc is established, the danger of a short-circuit from a portion of the filament falling against the electrode guard is remote since the bimetal would not yet have contacted the guard support. If the electrode 6 were to fail and a portion of the filament contacted the guard 9 after a discharge arc has been established the danger of ballast damage is very low. If little emitter material remained on the failed filament, the lamp would cease normal operation and only a glow discharge would be maintained. Since heat from the glow discharge is not sufficient to keep the free end 11a of the bimetal against the guard support 10, electrical contact between the lead-in conductor 8 and the guard support 10 would cease soon after the normal discharge arc extinguished. Alternatively, if enough emitter material remained on the filament 6 connected to the guard 9, a discharge could still be maintained. While heat from the discharge would maintain the electrical connection between the guard support 10 and the bimetal 11, there would be enough resistance in the electrically connected filament portion such that ballast damage would not occur. To test the integrity of the ballast in such an event, a standard 40 watt cool white lamp was operated on a standard 2-way magnetic ballast with a direct short between the guard 9 and the lead-in conductor 8. Damage to the ballast did not occur until after 72 days of continuous operation. The bimetal may also be located closer to the stem (not shown) where the lead-in conductors and guard support are parallel. However, since the distance between the lead-in conductor 8 and the guard support 10 at this location is less than at locations closer to the guard 9, care must be taken to ensure that the free end of the bimetal does not contact the lead-in conductor 7 as it contacts guard support 10, causing an electrical short between the lead-in conductors 7 and 8. Those one of ordinary skill in the art will appreciate that many variations of the electrode assembly which would come within the scope of the invention are possible. For example, the bimetal could be welded to the guard support or to the guard with its free end arranged adjacent a lead-in conductor to electrically connect the metallic guard during lamp operation.

Claims: A low-pressure mercury vapour discharge lamp comprising an electrode (6), first (7) and second (8) lead-in conductors connected to respective ends of the electrode (6), a metallic guard (9) partially surrounding the electrode (6) and supported by a conductive guard support (10), a discharge arc being maintained in the lamp during operation, characterized in that a bimetal strip (11) is provided being arranged such that upon starting of the lamp heat from the discharge arc causes the strip (11) to bend and electrically connect one of the lead-in conductors (7, 8) and the metallic guard (9). A low-pressure mercury vapour discharge lamp as claimed in claim 1, characterized in that the bimetal strip (11) has one end fixed to one of the lead-in conductors (7, 8) and a free end disposed adjacent the metallic guard (9) and contacting said metallic guard during operation of the lamp. A low-pressure mercury vapour discharge lamp as claimed in claim 1, characterized in that the bimetal strip (11) has one end fixed to one of the lead-in conductors (7, 8) and a free end disposed adjacent the conductive guard support (10) and contacting said guard support during operation of the lamp.

Inventors: LATASSA FRANK, LEYH THOMAS, LATASSA, FRANK, LEYH, THOMAS
Applicants: PHILIPS ELECTRONICS NV, PHILIPS ELECTRONICS N.V.
Application year: 1990
Main classification: H01J61
IPC codes: H01J61