GB2060244A

GB2060244A - High Pressure Sodium Vapour Lamp

Info

Publication number: GB2060244A
Application number: GB7934954A
Authority: GB
Original assignee: Matsushita Electronics Corp
Current assignee: Panasonic Holdings Corp
Priority date: 1979-10-09
Filing date: 1979-10-09
Publication date: 1981-04-29
Also published as: GB2060244B

Abstract

In a high pressure sodium vapor discharge lamp having a discharge tube which contains xenon gas at a high pressure of, e.g. over 150 torr, one of its discharge electrodes 6 has a projection conductor 9 which is disposed extending therefrom towards an inside surface of the wall 1 of the discharge tube. There is a predetermined gap, e.g. 0.3 mm to 0.7 mm, between the projection 9 and wall 1. Moreover, a starting aid conductor 8 is disposed lengthwise on the outside surface of the side wall, and may be connected to the other discharge electrode 5. <IMAGE>

Description

SPECIFICATION A High Pressure Sodium Vapor Discharge Lamp The present invention relates to a high pressure sodium vapor discharge lamp which comprises a translucent gas-discharge tube having therein sodium, buffer gas and xenon gas.

Generally speaking, high pressure sodium vapor discharge lamps have about a twice high efficiency in comparison with the conventional high pressure mercury lamp and have a comfortable color appearance, and therefore are attracting attention as energy saving light sources which may replace high pressure mercury lamps in near future. In a discharge lamp, xenon gas is sealed to have a pressure of about 20 Torr, as a starting gas having a low thermal conductivity. As a result of the presence of xenon gas, starting voltage of the high pressure sodium vapor discharge lamp is as high as about 2KV, which is very high in comparison with less than 200V of the mercury lamp.Recently, there has been proposed a high efficiency high pressure sodium vapor discharge lamp, wherein xenon is filled with a pressure of over 1 50 Torr, for example, 350 Torr, thereby raising lamp efficiency to be about 10% higher than those having xenon gas filled with a pressure of about 20 Torr. In such a lamp with high pressure xenon gas, the starting voltage of the lamp is further raised, for example, to about 8-9KV for a high pressure sodium vapor discharge lamp having xenon gas filled with the pressure of 350 Torr. In such a lamp, the starting voltage can be lowered to about 3.5KV when utilizing a starting aid wire, but a voltage of 3.5KV is too high for most practical use and is liable to induce breakdown of insulation in the ballast and the related lamp circuit.

Hence it would be desirable to provide a high pressure xenon gas having a low starting voltage.

According to the invention, we provide a high pressure sodium vapor discharge lamp comprising an outer bulb enclosing a discharge tube in an evacuated space therein, the discharge tube comprising a translucent and chemically stable tube envelope containing therein sodium, xenon gas, a buffer gas which is mercury and/or cadmium, and a discharge electrode sealed in each end part of the tube envelope, in which one of said discharge electrodes has a projection conductor which extends from the electrode towards and in proximity with an inside surface of a side wall of said tube envelope, with a predetermined gap between the distal end of the conductor and the tube envelope, and in which a starting aid conductor is disposed lengthwise on an outside surface of said side wall in such a manner as to partly overlap the distal end of the projection conductor, the starting aid conductor being connectable to the other discharge electrode so that its potential is substantially identical to that of the other discharge electrode.

It has been found that the starting voltage of a high pressure sodium vapor discharge lamp having a high pressure xenon gas of 1 50 to 350 Torr can be reduced to such a low voltage of 3KV or lower.

The invention will be better understood from the following non-limiting description of examples thereof given with reference to the accompanying drawings in which: Fig. 1 is a plan view of a discharge tube of a high pressure sodium vapor discharge lamp without a projection of conductor at its electrodes.

Fig. 2 is a sectional elevation view of the lamp shown in Fig. 1.

Fig. 3 is a graph showing relations between distances "f" from the end tip of an electrode to the end tip of a starting aid conductor and starting voltages for D.C. operations of the discharge tube of Figs. 1 and 2.

Fig. 4 is an enlarged fragmental elevation view of a discharge tube of a high pressure sodium vapor discharge lamp embodying the present invention.

Fig. 5 is a side view of a high pressure sodium vapor discharge lamp in accordance with the present invention, wherein the discharge tube 100 shown in Fig. 4 is sealed in an evacuated glass bulb 21.

Fig. 6 is a graph showing relations between gap "d" from the end tip of a conductor protrusion 9 of an electrode 6 to the inside surface of a tube wall of the discharge tube of Fig, 4 and starting voltages for D.C. operations of the lamp of Figs. 4 and 5.

Fig. 7 is a circuit diagram of the lighting apparatus utilizing the lamp of Fig. 5.

In the course of many experiments concerning starting characteristics of high pressure sodium vapor discharge lamps having xenon gas filled therein as starting gas, the inventors found that the starting voltage of the lamp can be lowered by a large margin, by providing a protrusion of a conductor (metal) extending from either one of the electrodes towards the inside surface of the side wall of the tube envelope, providing a starting aid conductor stripe (wire) on the outside surface of said side wall and applying a potential which is substantially identical to that of the other electrode, on the starting aid conductor. The present invention arises out of the abovementioned finding.

The experiments leading to the present invention and the details of the example of the present invention are elucidated referring to the accompanying drawings. As shown in Fig. 1 and Fig. 2, the discharge tube, which is to be disposed in an evacuated outer glass bulb, has a translucent tube envelope 1 of a polycrystalline alumina tube or a single crystal aluminum tube and a pair of electrodes 6, 7. The electrodes 6, 7 are made of tungsten coils, carry electron-emitting substance thereon and are disposed and supported in and near the ends of the tube envelope 1 by lead-in wires 4, 5 which are niobium tubes, which penetrate and are fixed to end caps 2, 3. In the tube envelope 1 is sealed sodium as light emitting substance, mercury as buffer gas and the rare gas xenon as starting gas.A starting aid conductor stripe 8 made of a molybdenum wire of 0.8 mm diameter is disposed on the outer surface of the wall of the tube envelope 1 and is connected to one electrode terminal 5.

The inventors made experiments to study how the starting voltage of the discharge tube changes as one end "b" of the starting aid conductor 8 changes its position with respect to one electrode 6, while the other end "a" is fixed substantially to the end of the tube envelope 1 and electrically connected to the other electrode 6, so that potential of the other electrode 7 is impressed on to the starting aid conductor 8. Fig. 3 shows curves showing the relation between a distance "f" from the end tip of the electrode 6 to the end tip of the starting aid conductor 8 and various starting voltages (D.C.

voltages were used for the experiment). The parameters 350 Torr, 250 Torr and 20 Torr of these curves are pressures of xenon gas filled in the tube envelope 1. As shown by Fig. 3, it is found that the starting voltages becomes smallest when end tip of the starting aid conductor 8 is near to the electrode 6 and further when it overlaps the electrode, however being separated therefrom by the side wall 1. As a conclusion, it is found that the starting voltage is dependent of the distance "f" between the end tip of the starting aid conductor and the end tip of the electrode 6, and the starting voltage becomes lower as the distance "f" becomes shorter.

By using an alumina discharge tube, which has an inner diameter of 7.4 mm, tube thickness of 0.7 mm and a distance of 83 mm between the electrodes and is for a 360W high pressure sodium vapor discharge lamp, further experiments were made. A projection conductor 9 was formed to extend from the electrode 6 as shown in Fig. 4, and the starting voltages were measured for various gaps "d" between the end tip of the projection conductor 9 and the inside surface "e" of the tube wall 1. A molybdenum rod of 0.7 mm diameter was used as a projection conductor 9 and was fixed to a stem part of the tungsten coil of the electrode 6 by spot welding. Fig. 6 shows results of the experiments, which, in comparison with the curve of Fig. 3, show a drastic improvement of the starting voltage by the provision of the projection conductor 9.

Similar experiments were made for lamps of various wattages, and similar results were obtained.

Especially for high power lamps of 700W and 1000W, the starting voltages were remarkably lowered.

It is believed that, in such high power lamps, wherein the gap "d" between the end surface of the electrode and the inside surface "e" of the tube wall 1 is large, the provision of a starting aid conductor 8 only on the outside surface of the tube wall 1 still leaves a relatively low potential gradient between the end of the starting aid conductor 8 and the electrode 7. However, provision of the projection conductor 9 extending towards the starting aid conductor 8 effectively raises the potential gradient in the gap.

An example of a practical lamp manufactured in accordance with the present invention is as follows: Table I tube input power 360W discharge tube 1: inner diameter 7.4 mm thickness of tube 0.7 mm inter-electrode distance 83 mm material single crystal alumina electrode 6,7: outer diameter of coil 3.6 mm starting aid conductor 8: molybdenum wire of 0.8 mm projection conductor 9: molybdenum wire of 0.7 mm distance "d" 0.5 mm sealed gas: sodium 4.5 mg mercury 20 mg xenon gas 350 Torr The starting aid conductor 8 is connected to the electrode 7 through a bimetal switch 10 as shown in Fig. 7, which is opened by means of a high temperature of the discharge tube 1, in order to disconnect the starting aid conductor 8 from the opposite electrode 7 after ignition to prevent loss of sodium from the discharge tube due to unnecessary connection of the starting aid conductor 8 to the opposite electrode 7.

The circuit of the lamp of this example is shown in Fig. 7, wherein commercial A.C. current is fed through a single choke ballast 14 for conventional 400W mercury lamp into a high pressure sodium vapor discharge lamp 15. In the bulb 21 or in the stem 22 of the lamp 1 5 are provided a bimetal starter switch 12, a series resistor 11, a 3000 pF ceramic capacitor 1 3 and the bimetal switch 10. As shown in Fig. 7, the capacitor 13 and a series connection of the resistor 11 and the switch 12 are connected in parallel across the terminals 4 and 5 of the discharge tube, so that the lamp 1 5 can be started by producing of a kick voltage by the choke coil 14, in a known manner.The series resistor 11 is to limit the cut-off current of the bimetal starter switch 12 to the value of 0.5A and also serves to make the bimetal starter switch 12 open by Joule heating thereof. The capacitor 1 3 is for suppressing the production of an extraordinary high kick voltage and also to shape the kick voltage pulse to have an appropriate pulse width. The capacitor 13 serves to protect the lamp,apparatus from insulation breakdown damage and also improves certainty of starting. Tests show that the starting voltage for the 360W lamp of the abovementioned example is about 2KV, and on the other hand, average value of induced high voltages across the terminals of the discharge tube 100 is about 2.5KV, which is sufficiently higher than the starting voltage of the lamp. Hence the lamp is started with a high certainty.

Furthermore, on account of lowness of the induced high voltage in the lamp apparatus is comparison with the conventional high pressure sodium vapor discharge lamps, there have been no insulation breakdowns of ballast and circuit during burning tests of the lamps.

Another example according to the present invention is a 630W lamp having similar details of design as in Table I except the following changes Table II The inner diameter of polycrystalline alumina discharge tube 9.7 mm the inter-electrode distance 130 mm distanced" 0.5 mm The abovementioned 630W lamp example has a low starting voltage; it is almost equal to that of the abovementioned example of 360W lamp. The starting voltage of a 630W lamp having a similar construction but no projection conductor 9 is however 4.5KV, that is higher by about 1.OKV than that of the above mentioned 360W lamp. The abovementioned 630W lamp can be certainly and stably started by utilizing a conventional single choke ballast for a 700W mercury lamp.

It has been confirmed that the projection conductor 9 need not necessarily be of a rod shape, but can be shaped as a ring or a disk. An advantageous feature of the projection conductor 9 is that the distance "d" between the inner surface "e" of the tube wall 1 and the outer end of the projection conductor 9 is within a certain range. The distance "d" preferably should be from 0.3 mm to 0.7 mm.

When the distance "d" is longer than 0.7 mm, the lowering of starting voltage is not satisfactory. On the other hand, when the distance "d" is shorter than 0.3 mm, due to smallness of the gap, every igniting discharge strikes in the same small area, and hence that small area is likely to suffer, giving rise to a high possibility of making cracks of the tube envelope. Tests show that the optimum distance is between 0.4 mm and 0.6 mm.

Claims

1. A high pressure sodium vapor discharge lamp comprising an outer bulb enclosing a discharge tube in an evacuated space therein, the discharge tube comprising a translucent and chemically stable tube envelope containing therein sodium, xenon gas, a buffer gas which is mercury and/or cadmium, and a discharge electrode sealed in each end part of the tube envelope, in which one of said discharge electrodes has a projection conductor which extends from the electrode towards and in proximity with an inside surface of a side wall of said tube envelope, with a predetermined gap between the distal end of the conductor and the tube envelope, and in which a starting aid conductor is disposed lengthwise on an outside surface of said side wall in a manner as to partly overlap the distal end of the projection conductor, the starting aid conductor being connectable to the other discharge electrode so that its' potential is substantially identical to that of the other discharge electrode.

2. A lamp in accordance with claim 1, in which the xenon gas has a pressure of 1 50 Torr or more.

3. A lamp in accordance with claim 1 or 2 in which the predetermined gap is between 0.3 mm to 0.7 mm.

4. A lamp in accordance with claim 1,2 or 3 including a switch which is arranged to open after the start of discharge of the discharge tube.

5. A lamp in accordance with claim 4, in which the switch is a bimetal switch which opens when exposed to heat from the discharge tube.

6. A lamp in accordance with any preceding claim, which further comprises a series connection of a starting switch and a current adjusting resistor, this series connection and capacitor being connected in parallel across the pair of discharge electrodes.

7. A lamp in accordance with any preceding claim in which the projection conductor and the starting aid conductor are of molybdenum wire.

8. A high pressure sodium vapor discharge lamp substantially as herein particularly described with reference to and as illustrated in Figs. 4, 5 and 7 of the accompanying drawings.