Metal vapour discharge lamp

Abstract: A metal vapour discharge lamp, a starter connected in parallel a discharge tube (11), combines a coiled non-fusible metal wire (12) which is wound on or laid along the outer wall of the discharge tube (11) and a non-linear capacitor (15) which is series connected to the non-fusible metal wire (12). As a result the discharge lamp can be assuredly and stably lit wth a small size ballast. Furthermore, passage of large current through the starter can be prevented even if the incandescent tube leaks. Furthermore, if the evacuated outer envelope (16) leaks or is broken, the circuit of the starter can be opened in that the non-fusible metal wire (12), which is heated by the discharge tube (11) oxidises and thereby becomes incandescent and breaks.

Description: This invention relates to metal vapour discharge lamps having a starter connected in parallel with a luminous tube and accommodated in a light transmissible outer envelope, and is particularly applicable to a high pressure sodium lamp having a starter therein. Fig.1 of the accompanying drawings shows an example of the structure of a circuit for use in a known metal vapour discharge lamp having a starter therein. A metal vapour discharge lamp of the type described above is constructed in such a manner that a starter 5, formed by a series circuit comprising a bimetal switch 2, a heater 3 and a limiting current resistor 4, is connected in parallel with a luminous tube 1, and the thus connected components are accommodated in a light transmissible outer bulb or envelope 6. When an A.C. voltage is applied to the thus-constructed discharge lamp which has been connected to an A.C. power source 8 via an inductor 7, an electric current passes through the starter 5 formed by a series circuit comprising a bimetal switch 2, a heater 3 and a current limiting resistor 4. The bimetal switch 2 switches on and off repeatedly. This causes high voltage pulses to be generated in the inductor 7, and these pulses are added to the power source voltage and applied to the luminous tube 1, causing the discharge lamp to be lit. By using the above-described starter which utilizes switching on/off operation performed by the bimetal switch 2, high voltage pulses of substantially several thousands of volts can be generated. Therefore, such discharge lamp can be started by using a relatively small size ballast. However, since the mechanical switching on/off operation of the bimetal switch 2 is utilized, the heights and the intervals of the generated pulses cannot be made uniform, the starting characteristics of the discharge lamp are not stable, or excessively high voltage pulses are generated, causing the wiring instruments or the like to be damaged. In order to overcome the above-described problems and as shown in Fig.2 of the accompanying drawings, there has been devised a metal vapour discharge lamp in which a non-linear capacitor 9 is connected in parallel with the luminous tube 1 with a thermal response type switch 10, and the thus-connected conponents are accommodated in an evacuated outer light-transmissible bulb or envelope 6. The starter of this discharge lamp utilizes the non-linear capacitor 9 as a switching element depending upon the hysteresis characteristics of the charge of the non-linear capacitor 9 with respect to the voltage thereof. The thermal response type switch 10 is provided for the purpose of isolating the non-linear capacitor 9, which serves as a starter, from the luminous tube 1 after the discharge lamp has started. In this starter, since high voltage pulses are generated in the inductor 7 utilizing the electrical switching on off operation of the non-linear capacitor 9, the heights and intervals of the generated high voltage pulses can be made very regular and stable. However, the thus-generated high voltage pulses are within the range of substantially 1000 to 3000V, this level being slightly lower than that in a case of a starter comprising a bimetal switch. Therefore, in order to assuredly start the discharge lamp, the non-linear capacitor and its circuit elements need to be designed in such a manner that the high voltage pulses which are very close to the upper limit of the above-described pulses can be generated. However, it has been found that the starter and the like, designed as described above, will easily lead to the following failures: when the light-emitting life of the discharge lamp reaches its final stage, causing the rare gases in the luminous tube 1 to leak into the outer vacuum envelope 6 or causing the outer vacuum envelope 6 to generate a slow leak, the voltage which the non-linear capacitor 9 can withstand is reduced, as a result of which discharge occurs between its two electrodes. This leads to a large electric current being caused to be passed through the starter circuit. Consequently failure of circuit elements, such as the inductor 7, which are easily damaged, occurs. Although the non-linear capacitor 9 finally breaks down due to discharge between the two electrodes of the non-linear capacitor 9, causing the circuit of the starter to be shut off, a large current passes through over a short time until the breakdown occurs and this will cause the above-described failure. This invention is for the purpose of overcoming the above-described problems experienced in the conventional metal vapour discharge lamp having a starter therein. An object of the present invention is to provide a metal vapour discharge lamp having a starter therein which can assuredly and stably start light emission even with a small size ballast. Another object of the present invention is to provide a metal vapour discharge lamp having a starter therein in which passage of large electric current through the starter is prevented even if leakage failure occurs in the discharge tube, causing the non-linear capacitor to be short-circuited. A further object of the present invention is to provide a metal vapour discharge lamp having a starter therein in which the circuit of a starter thereof is arranged to be opened when leakage or breakage failure occurs in the evacuated outer envelope. According to the present invention as claimed in claim 1, a metal vapour discharge lamp comprises a starter which is connected in parallel with a luminous discharge tube and is accommodated together with the luminous discharge tube in an evacuated outer light-transmissible envelope, characterized in that the starter comprises a coiled non-fusible (hard to be fused) metal wire which is wound on or laid along the outer wall of the luminous tube in a contacting manner and a non-linear capacitor which is connected in series to one end of the non-fusible metal wire. As a result of the above-described structure, the coiled non-fusible metal wire acts as a so-called adjacent conductor for assisting starting, causing the discharge lamp to be started easily. Therefore, the discharge lamp can be assuredly and stably lit with only a relatively small size ballast. Furthermore, since the generated high voltage pulses are applied to the non-linear capacitor through the non-fusible metal wire, discharge is unlikely to occur between the two electrodes of the non-linear capacitor even if the rare gases leak from the discharge tube. Even if such discharge between the capacitor electrodes does occur, which would be capable of causing a large current to be passed through the non-linear capacitor, such large current does not actually pass since the non-fusible metal wire, heated by the discharge tube, serves as a current limiting resistor. Furthermore, if a large quantity of air is introduced into the outer evacuated envelope due to breakage of the envelope, the non-fusible metal wire, heated by the discharge tube, burns until it is broken, so opening the circuit of the starter and causing the discharge tube to leak and be rendered in a non-light emitting state. Also, generation of high voltage pulses is prevented. Consequently, assured and stable starting and safety in the event of failures can be simultaneously obtained. The invention is further described, by way of example, with reference to the accompanying drawings, in which:- Figs.1 and 2 are views illustrating the schematic structure and lighting circuits for use in two conventional metal vapour discharge lamps having a starter therein; Fig.3 is a cross-sectional view, from which a part is omitted, of a metal vapour discharge lamp according to a first embodiment of the present invention; Fig.4 illustrates a part of a modified example of the first embodiment in which a double coiled non-fusible metal wire is used; Fig.5 is a view illustrating the structure of a circuit for use in the first embodiment of the present invention; Fig.6 is a view illustrating the structure of a circuit for use in a second embodiment of the present invention; Fig.7 is a view illustrating the structure of a circuit for use in a third embodiment of the present invention; and Fig.8 is a view illustrating the structure of a circuit for use in a fourth embodiment of the present invention. Fig.3 illustrates a metal vapour discharge lamp according to a first embodiment of the present invention. A luminous discharge tube 11 has at least one pair of electrodes and encloses a substance, such as rare gas and other substances. In the case of a high pressure sodium lamp, this discharge tube 11 is made of a material, such as light-transmissible alumina ceramic, and a rare gas, such as xenon gas, is enclosed together with sodium and mercury. The electrodes of the discharge tube 11 are supported by tubes of niobium. A non-fusible metal wire 12 in the form of a coil is wound onto or laid along the outer wall of the luminous tube 11. As the non-fusible metal wire 12, that is, a wire which it is difficult to fuse, a tungsten wire is most preferable for use, and is formed in a single coil or, as shown in Fig.4, a double coil. One end of this non-fusible metal wire 12 is connected to a power supply part disposed at one end of the discharge tube 11 via a thermal response switch 13 such as a bimetal switch. The other end of the wire 12 is connected to one electrode of a non-linear capacitor 15 via another thermal response switch 14. The other electrode of the non-linear capacitor 15 is connected to a power supply part disposed at the other end of the discharge tube 11. Thus, a series circuit formed by the thermal response switch 13, the non-fusible metal wire 12, the thermal response switch 14 and the non-linear capacitor 15 constitutes a starter, this starter being connected in parallel with the discharge tube 11, as shown diagrammatically in Fig.5. A discharge lamp 20 is created by accommodating the discharge tube 11 and the starter in an evacuated outer light transmissible bulb or envelope 16. In the starter described above, the thermal response switches 13 and 14 act to isolate the starter from the discharge tube 11 by switching off by virtue of receiving heat from the discharge tube 11 after the discharge lamp has started and begun to warm up. The non-linear capacitor 15 is constituted in a manner in that metallized electrodes are formed on both sides of a disc-type body made of a material of the barium titanate type, these electrodes being connected with lead wires by a conductive adhesive. Furthermore, the surface of the electrode is coated over with an insulating material, such as glass. Since such a type of non-linear capacitor has a hysteresis type voltage-charge characteristic, it is made to operate similar to a switching element by using the above-described characteristics. When the thus-constituted discharge lamp 20 is connected to an AC power source 22 via an inductor 21 as shown in Fig.5, and the same is supplied with AC voltage, an electric current passes through the starter, and high voltage pulses are generated by the operation of the non-linear capacitor 15. Simultaneously, the non-fusible metal wire 12 serves as an adjacent conductor, as a result of which the discharge lamp 20 can easily start with a relative low pulse voltage. Fig.6 illustrates the structure of a circuit for use in a second embodiment. This embodiment is constituted in such a manner that a semiconductor switch 23 such as a TRIAC is connected in series with the non-linear capacitor 15 of the starter. Since the pulsed voltage generated due to the switching operation of the non-linear capacitor 15 can be further raised, it is preferable for use in a discharge lamp of a relatively large capacity. Fig.7 illustrates the structure of a circuit for use in a third embodiment. In this embodiment, a shunt resistor 24 is connected in parallel with the semiconductor switch 23 described with reference to the embodiment shown in Fig.6, this shunt resistor 24 acting to make the breakthrough voltage of this semiconductor switch 23 stable. A resistor 24 of 15 to 200 kΩ is generally used for this purpose. Fig.8 illustrates the structure of a circuit for use in a fourth embodiment, this embodiment being applicable to a case where a capacitive ballast having a capacitor is used to emit light. That is, a resistor 25, for discharging the electric charge which has been charged in the capacitance of the ballast and immediately returning the starter circuit, is connected in parallel with the starter. The thus-connected resistor 25 is, with the starter, accommodated in the outer envelope 16. A resistor 25 of 10 to 100 kΩ is generally used for this purpose. A main series capacitor 26 is connected between a leakage transformer 27 and the discharge lamp 20. This embodiment can be also employed in a case wherein the metal vapour discharge lamp according to the embodiments shown in Figs. 6 and 7 is lit by using a capacitive ballast. All of the above-described embodiments exhibit the following advantages in addition to an advantage that the discharge lamp can be assuredly and stably lit by means of the non-linear capacitor 15 and the non-fusible metal wire 12: Since the high voltage pulses generated for the purpose of starting the discharge lamp are applied to the non-linear capacitor 15 via the non-fusible metal wire 12, discharge between the two electrodes of the non-linear capacitor 15 is prevented even if the rare gas leaks into the outer vacuum envelope 16 from the luminous tube 11 at the final stage of the useful light emission life of the discharge lamp 20. Even if discharge does occur, since the non-fusible metal wire 12 serves as a current limiting resistor, passage of large electric current through the non-linear capacitor 15 is prevented. Furthermore, when a large volume of air is introduced into the evacuated outer envelope 16 due to its breakage, the non-fusible metal wire 12, which is at a high temperature due to heat from the luminous tube, is oxidised and thereby becomes incandescent and breaks. As a result of this, the circuit of the starter is opened. Furthermore, since the luminous tube is also caused to leak due to creation of cracks in the sealed portion caused from hardening by virtue of oxidation of the niobium tubes by which the tube electrodes are supported, the discharge lamp does not emit light, and generation of high voltage pulses is also stopped. However, in order to obtain the above-described effects, the size, the shape, the value of resistance and the like of the non-fusible metal wire 12 need to be properly determined. In a case wherein the present invention is applied to a high pressure sodium lamp, and in particular in a case wherein a lamp with a small output of 150 W or less is used, the preferable structure of the above-described coiled non-fusible metal wire 12 is as follows: a single wire with a diameter of 0.04 to 0.1 mm and a length of 260 to 1200 mm is formed to a single coil with an inner diameter of 0.1 to 0.6 mm and a length of 15 to 100 mm. The reason for employing the single coiled form in the small output lamp lies in that: since such small capacity lamp comprises a small size discharge tube, the light-shielding effect due to the presence of the non-fusible metal wire needs to be reduced as much as possible. If the diameter of the wire is less than 0.04 mm or the inner diameter of the coil is less than 0.1 mm, the spring effect is reduced, causing the contact of the coil with the outer wall of the discharge tube to deteriorate, and the starting effect to be reduced. If the diameter of the wire exceeds 0.1 mm, the non-fusible metal wire cannot be burnt through even if air is introduced due to breakage of the outer vacuum envelope. As a result of this, high voltage pulses continue to be generated. In an intermediate or large output lamp exceeding 180 W, the coiled non-fusible metal wire 12 is preferably formed in such a manner that: a single wire with a diameter of 0.03 to 0.1 mm and a length of 260 to 3500 mm is formed to a double coil constituted by a primary coil with an inner diameter of 0.06 to 0.2 mm and a length of 50 to 800 mm and a second coil with an inner diameter of 0.15 to 0.5 mm and a length of 7 to 132 mm. The reason for employing a double coil in an intermediate or large output lamp lies in that; since the intermediate or the large output lamp comprises a relatively long discharge tube and the starting voltage thereof is relatively high, the use of the double coil which has excellent contact performance will help starting. If wire of a diameter less than 0.03 mm is used as the non-fusible metal wire, the non-fusible metal wire is self-heated at the time of passage of electric current through the starter, causing the resistance of the metal to be raised. This leads to the lowering of the pulse voltage and the starting characteristics become poorer. On the other hand, if a coil formed by a wire with a diameter exceeding 0.1 mm, the non-fusible metal wire cannot be burnt through, even if air is introduced upon breakage or the like of the outer vacuum envelope. As a result of this, generation of the high voltage pulses is continued. If the inner diameter of the double coil is 0.15 mm or less, the spring performance of the double coil is poor so that excellent contact with the discharge tube cannot be obtained. On the other hand, if the inner diameter is 0.5 mm or more, the luminous flux of the lamp can be reduced excessively due to increase in effect of shielding of the discharge tube. The coiled non-fusible metal wire is arranged in such a manner that the cold resistance is 3 to 40Ω. Since the effective current which passes through the non-fusible metal wire is small enough to be neglected, the voltage drop due to the non-fusible metal wire is relatively small if the above-described resistance is within the above-described range. Therefore, the level of the voltage of the generated pulses is not affected. A specific design example in accordance with the present invention will be described. A design example is described in which the starter according to the second embodiment shown in Fig.6 was applied to a 360 W-high pressure sodium lamp. The size of the discharge tube 11 was such that the outer diameter was 9 mm and the distance between electrodes was 107 mm. As the coiled non-fusible metal wire 12, a double coil formed by a single wire of diameter 0.069 mm and length of 1210 mm was employed, the double coil being constituted by a primary coil with an inner diameter of 0.125 mm, a pitch of 200%, and a length of 275 mm and a second coil with an inner diameter of 0.5 mm, a pitch of 160%, and a length of 48 mm. The resistance of this non-fusible metal wire 12 was 20Ω in a cold state (non-heating state), but it was 240Ω in a hot state (heating state). As the thermal response switches 13 and 14, bimetal switches were employed which were on at the time of the cold state and which were turned off due to heat from the luminous tube 11 and the above-described non-fusible metal wire 12. As the non-linear capacitor 15, a type was employed which has a saturation characteristic such that the voltage-charge hysteresis characterics thereof becomes saturated charge Q = 33 [µc] when the saturated voltage E = 300 [V]. As the semiconductor switch 23, a TRIAC with a breakthrough voltage V BO : 220 [V], a peak off current I DRM : 10 [µA] (max), a breakthrough current I BO : 0.5 [mA] (max), a holding current I H : 50 [mA], and an on-state voltage V T : 3.0 [V] (max) was used. When the thus-constituted high pressure sodium lamp having the starter was lit after connection to a commercial AC power source of 100 [V], 50 [Hz] via a choke coil ballast for lighting a 400 [W] high pressure mercury lamp,high voltage pulses with a peak voltage of substantially 2600 [V] were generated and the high pressure sodium lamp was immediately lit. When the starter was operated, the above-described non-fusible metal wire 12 was in a non-heated state, and the resistance was relatively low value of 20Ω. As a result of this, the level of the generated high voltage pulses was not effected. Next, the lamp was lit after introducing slight amount of xenon gas into the outer vacuum envelope of the above-described lamp similarly to a state wherein rare gas in the luminous tube has leaked. In this case, although discharge was generated between the electrodes of the non-linear capacitor, the size of current passing through the starter is several amperes at the largest, therefore, no failure or breakage of the ballast or the like occurred. Then, the lamp was lit after introducing air into the outer vacuum envelope of the above-described lamp similarly to a state wherein the outer vacuum envelope has leaked or broken. The temperature of the non-fusible metal wire reached 1100°C or higher due to heat from the discharge tube after the lamp had been lit. The introduced air carried the metal wire to incandesce and break within a period of about ten seconds and several minutes, and the luminous tube to leak. As a result of this the discharge lamp was brought to a state wherein it did not emit light and the generation of high voltage pulses was stopped. As can clearly be understood from the above description, the metal vapour discharge lamp according to the present invention can be started assuredly and stably even if a relatively small size ballast is employed. Furthermore, if the non-linear capacitor is brought into a short circuit state due to a leakage failure of the discharge tube, passage of a large current through the starter can be prevented. Furthermore, there is an advantage that if leakage or breakage failure occurs in the evacuated outer envelope, the circuit of the starter can be opened.

Claims: A metal vapour discharge lamp comprising a starter (12-15) which is connected in parallel with a luminous tube (11) and is accommodated together with the tube (11) in an evacuated outer light-transmissible envelope (16) of the lamp, characterized in that said starter comprises a coiled non-fusible metal wire (12) which is wound on or laid along the outer wall of said luminous tube (11) in a contacting manner and a non-linear capacitor (15) which is connected in series to said non-fusible metal wire (12). A metal vapour discharge lamp according to claim 1, wherein said starter further comprises a semiconductor switch (23) connected in series to the non-fusible wire (12) and the non-linear capacitor (15). A metal vapour discharge lamp according to claim 2, wherein said semiconductor switch (23) has a resistor (24) which is connected in parallel therewith. A metal vapour discharge lamp according to claim 1, 2 or 3, wherein a resistor (25), which is connected in parallel with said starter, is accommodated in said outer envelope (16). A metal vapour discharge lamp according to any of claims 1 to 4, wherein thermal response switches (13,14) which are turned on when cold, are connected to the ends of said non-fusible metal wire (12). A metal vapour discharge lamp according to any of claims 1 to 5, wherein said coiled non-fusible metal wire (12) is, in the case of a small capacity lamp of less than 150 W, constructed of a single wire of diameter of 0.04 to 0.1 mm formed as a single coil with an inner diameter of 0.1 to 0.6 mm. A metal vapour discharge lamp according to any of claims 1 to 5, wherein said coiled non-fusible metal wire (12) is, in the case of a lamp exceeding 180 W, constructed of a single wire with a diameter of 0.03 to 0.1 mm formed to a double coil constituted by a primary coil with an inner diameter of 0.06 to 0.2 mm and a second coil with an inner diameter of 0.15 to 0.5 mm.

Inventors: IIDA TAKENOBU, SHIINA JOJIRO, YASUKAWA MINORU, IIDA, TAKENOBU, SHIINA, JOJIRO, YASUKAWA, MINORU
Applicants: IWASAKI ELECTRIC CO LTD, IWASAKI ELECTRIC CO., LTD.
Application year: 1988
Main classification: H01J61
IPC codes: H01J61