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(11) | EP 0 634 884 A1 |
| (12) | EUROPEAN PATENT APPLICATION |
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| (54) | Glow switch starter |
(57) The invention relates to a glow switch starter comprising
According to the invention, the getter comprises an alloy of zirconium and palladium to which oxygen is bonded. It is achieved thereby that the glow switch starter ignites a discharge lamp comparatively quickly also when no or only a small quantity of radioactive material has been provided in the discharge vessel. |
[0001] The invention relates to a glow switch starter comprising
- a glass discharge vessel which is sealed in a gastight manner and is provided with an ionizable medium,
- a pair of electrical conductors which are passed through a wall of the discharge vessel,
- a bimetallic element conductively connected to one of the electrical conductors, and
- a getter.
[0002] Such a glow switch starter is known from US Patent 5,001,391. The operation of such a glow switch starter is as follows. When used for igniting a discharge lamp, the glow switch starter shunts the discharge lamp and is connected in series with electrodes of the discharge lamp. A voltage present across the discharge lamp generates a glow discharge between the electrical conductors of the glow switch starter. This glow discharge heats the bimetallic element connected to one of the electrical conductors, which element is so deformed under the influence of this heating that it makes contact with the other electrical conductor. The creation of this contact extinguishes the glow discharge, and a current flows through the electrodes of the discharge lamp via the electrical conductors and the bimetallic element of the glow switch starter. This current brings the electrodes of the discharge lamp to a temperature at which emission of electrons takes place to a sufficient degree for enabling the discharge lamp to ignite. During heating of the electrodes of the discharge lamp, the bimetallic element of the glow switch starter cools down and is so deformed thereby that the contact between the two electrical conductors of the glow switch starter is broken. Breaking of the contact also interrupts the current through the electrodes of the lamp and generates an ignition voltage pulse in an inductive element which is connected in series with the lamp. If this ignition voltage pulse generates a discharge between the electrodes of the discharge lamp, the voltage across the discharge lamp, and thus the voltage between the electrical conductors of the glow switch starter, drops so strongly that no further glow discharge occurs. If on the other hand the ignition voltage pulse generates no discharge between the electrodes of the discharge lamp, the process described above will repeat itself.
[0003] A problem which often arises with glow switch starters is that impurities in the ionizable medium can considerably slow down the creation of the glow discharge. This is especially the case when the glow switch starter has been present in a room with a low luminous intensity during a comparatively long time before use. Possible impurities are, for example, water, nitrogen, oxygen and also hydrogen. Often, depending on the manufacturing process of the glow switch starter, hydrogen is the impurity which is present in the greatest quantity. A source of hydrogen in the glow switch starter may be, for example, a metallic emitter layer which is electrolytically provided on the bimetallic element and in which hydrogen is stored during this deposition. This hydrogen is subsequently evolved by diffusion and ageing processes and then mixes itself with the ionizable medium. A second source of hydrogen is formed by water which is released from the wall of the discharge vessel and which is reduced by metals present in the discharge vessel.
[0004] The undesirable effect of impurities is compensated in many known starters by the application of a quantity of radioactive material. This, however, increases the cost price of the glow switch starter while also the use of radioactive materials in products for a mass market may be regarded as undesirable.
[0005] A second way of counteracting the effect of impurities is to provide a getter in the discharge vessel of the glow switch starter as described, for example, in the US Patent cited above. The getter described therein consists of bismuth or lead. Most of the impurities are removed from the ionizable medium to a certain extent by such a getter so that the creation of the glow discharge in the glow switch starter is accelerated. Such a getter, however, exhibits a comparatively low activity in removing hydrogen from the ionizable medium especially at comparatively low temperatures. This means that the use of the known getter in glow switch starters in which the impurity in the ionizable medium consists for a comparatively large portion of hydrogen is incapable of preventing an often considerable delay in the creation of the glow discharge.
[0006] The invention has for its object inter alia to provide a glow switch starter in which a glow discharge arises comparatively quickly during use, also when a comparatively large quantity of hydrogen has entered the discharge vessel during manufacture. According to the invention, this object is achieved in that the getter comprises an alloy containing palladium and zirconium, in which the molar ratio palladium:zirconium lies in the 0.004-0.18 range, while in addition oxygen is chemically bonded to the alloy, the molar ratio of oxygen (O) to zirconium lying in the 0.02 to 1 range.
[0007] The getter in a glow switch starter according to the invention is comparatively cheap and simple to provide and has a high activity in binding hydrogen even at comparatively low temperatures (approximately 100 °C). As a result, hydrogen is removed from the ionizable medium to a comparatively high degree in a glow switch starter according to the invention. It was found that the delay before the glow discharge is generated is considerably shortened by this, also when the glow switch starter has remained in a room of a low luminous intensity for a comparatively long period before use.
[0008] It should be noted that European Patent EP 291123A1 mentions the use of (inter alia) the palladium/zirconium alloy mentioned above in an electric lamp. The alloy in such a lamp mainly acts as a water getter. Although the activity of the alloy in gettering hydrogen is also mentioned, no experimental results obtained at a temperature below 200 °C are given. Seen against the use of the palladium/zirconium alloy as described in EP 291123A1, it is highly surprising that the alloy when used in a glow switch starter according to the invention at a temperature (depending on the surroundings of the glow switch starter, sometimes no higher than approximately 100 °C) at which the activity in gettering water is negligibly small, should show a comparatively high activity in gettering hydrogen.
[0009] It was found to be possible in many cases to realise glow switch starters according to the invention with a comparatively small ignition delay without the use of radioactive material. If it should be desirable, however, to reduce the delay in the creation of the glow discharge still further, it is possible to provide a radioactive material such as, for example, tritium in the form of titanium-tritium in the discharge vessel. It was found that the glow discharge is generated relatively quickly in the presence of only a comparatively small quantity of radioactive material. It was found to be possible in all cases to keep the delay in the generation of the glow discharge within acceptable limits with a quantity of radioactivity of the radioactive material of less than 50 nanoCuries.
[0010] An advantageous embodiment of the glow switch starter according to the invention is characterized in that the molar ratio palladium:zirconium lies in the 0.09 to 0.11 range. It was found for a composition of the alloy within this range that a very good activity in gettering hydrogen was obtained.
[0011] A further advantageous embodiment of a glow switch starter according to the invention is characterized in that a portion of the electrical conductors which is in contact with the wall of the discharge vessel consists of copper. This is the case, for example, when the conductors are made from a core formed from an Ni-Fe alloy surrounded by a copper sheath. The copper at the surface of the portion of the conductor which is in contact with the discharge vessel wall is in the oxidized state. A good gastight connection to the discharge vessel wall is realised by means of this oxidized surface. Since the ionizable medium of the glow switch starter according to the invention contains only a very small quantity of hydrogen, the oxidized surface of the conductor is not reduced during ageing of the glow switch starter. Such a reduction gives rise to leaks between the conductors and the discharge vessel wall in many known starters. In other known starters, the conductors are made from a Cr-Ni-Fe alloy to prevent the occurrence of leaks. These latter conductors, however, are considerably more expensive than the former.
[0012] An embodiment of a glow switch starter according to the invention will be explained in detail with reference to a drawing.
[0013] In the drawing, Fig. 1 is a diagrammatic picture of a glow switch starter according to the invention.
[0014] In Fig. 1, A denotes a discharge vessel which is sealed in a gastight manner and has a wall of lead-free glass and a volume of approximately 0.7 cm³. The discharge vessel is filled with an ionizable medium of approximately the following composition: Ar = 60%, Ne = 29%, He = 11%. The pressure of the ionizable medium is 18 Torr at room temperature. C and D are electrical conductors which are passed through a wall of the discharge vessel. The electrical conductors consist of a core formed from a Ni-Fe alloy and a sheath of Cu.
[0015] E denotes a bimetallic element which is conductively connected to one of the electrical conductors. The bimetallic element comprises an active side formed from Ni(20%), Fe(74%) and Mn(6%) and a passive side formed from Ni(36%) and Fe(64%). Both sides of the bimetallic element are covered with an electrolytically applied emitter layer consisting of Zn. F denotes a getter provided inside the discharge vessel against the wall thereof. This getter consists of approximately half a milligram of an alloy containing palladium and zirconium in which the molar ratio palladium:zirconium lies in the 0.004-0.18 range, while oxygen is chemically bonded to the alloy, the molar ratio of oxygen (O) to zirconium lying in the 0.02 to 1 range.
[0016] In Table 1, the glow switch starter shown in Fig. 1 is compared with two glow switch starters serving as references and denoted reference 1 and reference 2. The glow switch starter denoted reference 1 contains no getter but is in all other respects of the same composition and construction as the glow switch starter shown in Fig. 1. The glow switch starter denoted reference 2 contains, instead of a getter, a quantity of radioactive material in the form of titanium-tritium with an activity of approximately 50 nanoCuries. The glow switch starter denoted reference 2 was constructed in all other respects in the same way as the glow switch starter shown in Fig. 1. The Table indicates for these three glow switch starters the average time which elapses between the application of a potential difference between the electrical conductors and the moment the glow discharge is created: t-delay. It is apparent that this duration can be very considerable (reference 1) for a glow switch starter which contains neither radioactive material nor a getter. It is also visible that this duration is negligibly short both for a glow switch starter according to the invention and for a glow switch starter which contains a certain quantity of radioactive material. It is demonstrated thereby that it is possible to avoid the use of radioactive materials in glow switch starters and to replace this material with an effective getter such as in a glow switch starter according to the invention.
TABLE I
| Glow switch starter type | t-delay (sec.) |
| Glow switch starter with Zr/Pd-getter | < 0,1 |
| Reference 1: glow switch starter without radioactive material and without getter | > 5 |
| Reference 2: glow switch starter with radioactive material | < 0,1 |
1. A glow switch starter comprising
- a glass discharge vessel which is sealed in a gastight manner and is provided with an ionizable medium,
- a pair of electrical conductors which are passed through a wall of the discharge vessel,
- a bimetallic element conductively connected to one of the electrical conductors, and
- a getter,
characterized in that the getter comprises an alloy containing palladium and zirconium, in which the molar ratio palladium:zirconium lies in the 0.004-0.18 range, while in addition oxygen is chemically bonded to the alloy, the molar ratio of oxygen (O) to zirconium lying in the 0.02 to 1 range.2. A glow switch starter as claimed in Claim 1, characterized in that the molar ratio
palladium:zirconium lies in the 0.09 to 0.11 range.
3. A glow switch starter as claimed in Claim 1 or 2, characterized in that a portion
of the electrical conductors which is in contact with the wall of the discharge vessel
consists of copper.