Multiple discharge device HID lamp with preferential starting

Abstract: A high intensity discharge lamp having a plurality of discharge device, and a starting aid for preferantially starting one of the discharge devices. Upon initial application of power to the lamp the preferentially started discharge device will start first, and the other discharge devices will not start. A power interruption during lamp operation will cause an interruption in the operation of the discharge device started first, upon reapplication of lamp power one of the other discharge devices that has not been operating will start without a hot restart delay.

Description: The present invention relates to a high intensity discharge lamp, according to the preamble of claim 1. A lamp of the type described in the preamble is known from US-A-4.287.454 = EP-A-0 30 730. High intensity discharge (HID) lamps generally include a discharge tube or vessel having a translucent or transparent wall and containing an ionizable material. Additionally, the discharge tube may include a pair of discharge electrodes. In operation, an electrical discharge is developed within the ionizable material and emits light. For example, high pressure sodium discharge lamps have a mercury-sodium amalgam within the discharge tube, and an inert starting gas. The starting gas is ionized to vaporize some of the amalgam, the vaporized mercury and sodium are ionized, and an intense light-emitting electrical discharge is formed. The difficulty in restarting an HID lamp after the discharge has been interrupted is well-known. During operation of a typical HID lamp the gas pressure within the discharge tube attains several atmospheres. When the discharge is interrupted the high internal pressure makes it difficult to reinitiate the discharge, i.e. restart the lamp. Consequently, the discharge tube must cool until its internal pressure has dropped sufficiently to allow the discharge to be reestablished. The time required for the discharge tube to cool sufficiently to reestablish the discharge is called the hot restart delay. Because the time required for a hot discharge vessel to cool and the internal pressure to drop sufficiently to allow the lamp to start may be several minutes, or even longer, an element of unreliability is present in lighting systems that rely solely on HID lamps. A momentary power interruption can result in an interruption in lighting service that lasts much longer than the power interruption. In applications where an interruption in lighting service cannot be tolerated, it is necessary to provide auxiliary lighting to furnish light until the HID lamps cool sufficiently to restart. The problem of providing light during the hot restart delay has been addressed in different ways. One approach is to provide auxiliary incandescent lamps; however, such auxiliary lamps also require control circuitry for operation during periods of hot restart delay. This auxiliary equipment creates additional expense and creates a more elaborate system than that required for just operating the HID lamps. Another approach to the hot restart problem is the use of more than one discharge tube within the same lamp. U.S. Patent No. 4,287,454 to Feuersager et al discloses a high pressure discharge lamp in which a pair of lamp discharge tubes are contained within the same lamp and connected in parallel. When a starting voltage is applied to the lamp one of the discharge tubes operates first, and effectively shunts the second discharge tube with a low impedance thereby preventing it from operating. Only the operative discharge tube heats sufficiently to elevate its internal pressure and increase its starting voltage. Consequently, if the operating voltage applied to the lamp is interrupted, a reapplied voltage will start the previously inoperative discharge tube which will operate without experiencing a hot restart delay. The hot restart problem has also been addressed by the use of an auxiliary discharge gap exterior to an HID lamp discharge tube but within the lamp outer envelope. A lamp having this structure is disclosed in U.S. Patent No. 4,377,772 issued to Tsuchihshi et al. This lamp is started in the usual fashion when an operating voltage is applied. After the normal temperature rise in the discharge tube, the discharge tube will exhibit the usual hot restart delay, if the lamp voltage is interrupted. In this case, reapplication of the lamp voltage during the period of hot restart delay will cause a discharge across the auxiliary discharge gap within the lamp envelope. The discharge across the auxiliary gap will continue until the discharge tube cools sufficiently to restart, at which time the voltage drop across the discharge tube will decrease and the voltage across the auxiliary discharge gap will be insufficient to continue the auxiliary discharge. Optical shading caused by one discharge tube blocking the light from another does not occur in the prior art lamp having a single discharge tube and an auxiliary discharge gap within the lamp envelope. For lamps having more than one discharge tube, optical shading is avoided by positioning the discharge tubes aligned axially. This arrangement of discharge tubes is not practicable for high wattage lamps. For example, a 400 watt high pressure sodium HID lamp typically has a discharge vessel about 11.25 (4.5 inches) long and a lamp outer envelope almost 20 cm (8 inches) long. In such a lamp having multiple discharge tubes it would not be practicable to axially align the discharge tubes. Optical shading of one discharge tube by another is unavoidable when multiple discharge tubes are arranged side by side. According to the invention a high intensity discharge lamp for the kind as described in the preamble is provided with means by which the one discharge tube will be always started first when the lamp is started from a cool condition in which said discharge tubes are unheated and exhibit their normal starting voltage, and the other tube remains inoperative after the first one is started. An advantage to luminaire design of HID lamps according to the invention having a pair of discharge tubes is made such that the same discharge tube always starts first, when the lamp is started from a cool condition. In this case the luminaire can be designed to minimize the effect of optical shading by the inoperative discharge tube and maximize the use of the light emitted by the first tube. In a preferred embodiment of the invention the discharge tubes are comprised of a discharge vessel, a pair of discharge electrodes within the discharge vessel, and a discharge vessel filling of an inert starting gas an ionizable material. The means for preferentially starting is defined by the inert starting gas within the one discharge tube having a pressure less than the inert starting gas pressure of the other discharge tube. In another preferred embodiment the means for preferentially starting is comprised of a metallic starting aid positioned proximate one of the discharge tubes for imparting to the one discharge tube a lower starting voltage than that of the other discharge tube. The starting aid may advantageously be a metal wire wrapped around the discharge tube. Fig. 1 is a front elevation of the high intensity discharge lamp according to the invention having two discharge devices; Fig. 2 is a side elevation of the lamp shown in Fig. 1; Fig. 3 is a plan view of the lamp shown in Fig. 1; Fig. 4 is a vertical partial section of the lamp shown in Fig. 1; Fig. 5 is a schematic circuit diagram of the high intensity discharge lamp according to the invention; and Fig. 6 is a front elevation of another embodiment of the high intensity discharge lamp according to the invention. DETAILED DESCRIPTION OF THE INVENTION The lamp according to the invention, shown in Figs.1-4, is comprised of an outer glass envelope 1 having a screw base 2 at one end thereof. A pair of discharge devices 3 and 4 are housed within the transparent envelope 1 and are energizable for emitting light. In the illustrated embodiment the discharge devices 3 and 4 are high pressure discharge lamp discharge tube assemblies. Each of the discharge tube assemblies are comprised of a discharge tube, a pair of internal discharge electrodes within the discharge tube and a fill comprised of an inert starting gas and an ionizable material. Connection to the internal discharge electrodes of the discharge device 3 is made by conductive lead-throughs 5 and 6, and to the internal discharge electrodes of the discharge device 4 by conductive lead-throughs 7 and 8. When a sufficiently high voltage is applied across a pair of the conductive lead-throughs an electric discharge occurs within the discharge device and high intensity visible light is emitted. The discharge devices 3 and 4 are electrically connected in parallel by the metallic support structure which supports the discharge devices within the lamp envelope 1. Upstanding support rod 9 extends upwardly and supports horizontal member 10 and in anchored at its upper end 11. The horizontal member 10 has downwardly extending end portions which extend to respective tubular lead-throughs 5 and 7 for supporting the upper ends of the discharge devices 3 and 4. Electrical connection between the lead-throughs 5 and 7 and the horizontal member 10 is provided by metallic ribbons 12 and 13, each of which is connected between a respective electrode lead-throughs 5 and 7 and the horizontal member 10. The support rod 9 extends downwardly to the press 14 and makes connection with a wire lead-through 15. The lead-through 15 in turn extends through the press 14 and connects to the screw base 2 to complete the electrical circuit from the screw base 2 to the upper electrodes of the discharge assemblies 3 and 4. The lower electrode lead-throughs 6 and 8 are connected together mechanically and electrically by the metallic strap 16. This strap is solidly attached to the vertical wire 17 which extends through the press 14 and makes connection with the screw base 2. The discharge devices 3 and 4 are spaced sufficiently far apart so that operation of discharge device 3 does not heat discharge device 4 too much and raise its starting voltage above that supplied by the lamp starter circuit. Thus, it will be possible to start the discharge device 4 even if discharge device 3 is heated to the point of exhibiting hot restart delay. The electrical circuit defined by the metallic support structure is illustrated schematically in Fig. 5 in which the conductive paths are identified with the same reference numerals that are used in Figs. 1-4 to identify the mechanical elements defining those conductive paths. The discharge devices 3 and 4 are connected electrically in parallel by element 10 and straps 12 and 13, and by strap 16. Consequently, when a potential difference is applied across the screw base 2 the same potential difference is applied across the discharge device 3 and the discharge device 4. If the two discharge devices have substantially the same starting voltage, which discharge device starts first is unpredictable. In order to preferentially start one of the discharge devices, for example, discharge device 3, a starting aid, shown schematically in Fig. 5, coacts with the discharge device 3 to start it first, even when the same potential differences are applied across both discharge devices. The starting aid shown schematically in Fig. 5 can be realized by manufacturing the discharge device 3 with a lower inert fill gas pressure than that of the discharge device 4 to impart a lower starting voltages to it. The respective starting voltages must be sufficiently different so that the discharge device 3 will preferentially start even after the respective starting voltages undergo the inevitable voltage rise that occurs over the life of some HID lamps. At the same time, the starting voltage of both discharge devices 3 and 4 must be within the range of conventional starting voltages. Also, the actual starting voltage must be sufficiently close to its design value so that for large numbers of discharge devices those intended to have the lower starting voltage will always have a starting voltage that is lower by a certain value than those intended to have the higher starting voltage. In order to test the practically of defining preferential starting means by choice of inert fill gas pressure, 400 watt high pressure sodium HID lamps were fabricated having discharge devices with different fill gas pressures. These lamps had discharge devices containing a typical sodium-mercury amalgam fill comprised of 18.4 weight % of sodium and 81.6 weight % of mercury. The inert fill gas was pure xenon. The discharge devices were identical except that nineteen of them had a xenon fill pressure of 2128 Pa (16 torr) for a nominal starting voltage of 2100 volts, and forty-five of them had a xenon fill pressure of 2660 Pa (20 torr) for a nominal starting voltage of 2600 volts. Two of the higher starting voltage discharge devices filled at 2660 Pa (20 torr) had abnormally high starting voltages and are omitted from the following data. Summarized in the table below are the number of discharge devices in each group, the average starting voltage, and the range over which the starting voltage varied within each group. number of arc tubes nominal voltage average voltage voltage range 19 2100 V 2142 V 2000-2350 V 43 2600 V 2728 V 2550-2900 V The data shows that the starting voltages of the individual discharge devices within each group fell within a range of 350 volts, and the difference between the maximum starting voltage among the discharge devices having the lower nominal starting voltage and the minimum starting voltage among the discharge devices having the higher nominal starting voltage was 200 volts. The data shows that if randomly selected discharge devices were selected from the two groups and included in the same lamp, in the worst case the starting voltage of one discharge device would be 200 volts lower than the higher starting voltage. The starting voltage of a high pressure sodium discharge device can be expected to rise by about 100 volts over its life, which is just one half the worst case starting voltage difference between the two discharge devices. Thus, the discharge device with the lower starting voltage will preferentially start over the entire life of the lamp, and the use of different inert gas fill pressures to create preferential starting is practical for commercial lamps. Lamps were made having two discharge devices, with different starting voltages, from the discharge devices just described. In each case the discharge device having the lower starting voltage started first. During lamp operation, when power was interrupted resulting in interruption of the operating and heated discharge device, the second discharge device having the higher starting voltage would, upon reapplication of lamp power, start without hot restart delay. In another embodiment of the invention shown in Fig. 6, structure defining the preferential starting aid is defined by a metallic helical coil 18 wound around one of the discharge devices. The operation of this type of starting aid is described in detail in U.S. Patent 4,491,766 which is incorporated herein by reference. The metal spiral 18 is electrically connected to the support rod 9 through the diode 19. The diode has a polarity effective to impart a positive potential to the metal spiral 18 during each alternating half cycle of the applied voltage. For at least a half of each alternating current cycle the potential which is applied to the metal spiral 18 will be positive and opposite that of one of the discharge electrodes. This positive voltage causes local ionization within the discharge device and an incipient discharge. Electrons from the discharge are accelerated by the field between the discharge electrodes and enhance the formation of the main discharge. The main discharge occurs at a lower voltage than if the metal spiral 18 were not present and biased. The invention disclosed herein is not limited to the particular embodiments shown. For example, discharge devices of the electrodeless type can be used.

Claims: A high intensity discharge lamp, having a pair of high pressure discharge tubes (3, 4) each responsive to an applied voltage for developing an electrical discharge to emit highly intense light, said discharge tubes heating during operation and exhibiting a substantially higher starting voltage when hot than when cold; mounting means for mounting said discharge tubes within the lamp relative to each other such that heating of one discharge tube during operation does not heat the other discharge tube sufficiently to substantially increase the starting voltage of the other discharge tube; characterized in that the lamp is provided with means by which the one discharge tube will be always started first when the lamp is started from a cool condition in which said discharge tubes are unheated and exhibit their normal starting voltage, and the other discharge tube remains inoperative, after the first one is started. A high intensity discharge lamp according to Claim 1, wherein said discharge tubes have a pair of discharge electrodes within said discharge tube, an inert starting gas, and an ionizable material, and said means by which the one discharge tube will be always started first is defined by the inert starting gas within said one discharge tube having a pressure less than the inert starting gas pressure of the other discharge tube. A high intensity discharge lamp according to Claim 2, wherein the inert starting gas within the one discharge tube has a pressure of about 665 Pa (5 torr) less than the inert starting gas pressure of the other discharge tube. A high intensity discharge lamp according to Claim 1, wherein said means by which the one discharge tube (3) will always start first is comprised of a metallic starting aid (18, 19) positioned proximate said one discharge tube for imparting to said one discharge tube a lower starting voltage than that of the other discharge tube (4). A high intensity discharge lamp according to Claim 4, wherein said metallic starting aid is a metal wire (18) wrapped around said one discharge tube (3).

Inventors: JACOBS CORNELIS ADRIANUS JOANN, JACOBS, CORNELIS ADRIANUS JOANNES
Applicants: PHILIPS CORP, NORTH AMERICAN PHILIPS CORPORATION
Application year: 1987
Main classification: H01J61
IPC codes: H01J61