Glow starter switch for discharge lamp operation

FIELD OF THE INVENTION

The invention relates to a glow starter switch for discharge lamp operation.

BACKGROUND OF THE INVENTION Known glow-type starters comprise two starter electrodes sealed in a glass envelope which is filled with a gas of a suitable composition and at a suitable pressure. One or both starter electrodes is a bimetallic strip. The operation of the glow starter may be described for the case of a low-pressure mercury- vapor discharge lamp, e.g. a fluorescent lamp. The discharge lamp is operated in series with a ballast inductance on a power supply. The glow starter is connected to the lamp electrodes arranged in the discharge vessel of the discharge lamp, each of the lamp electrodes comprising a filament coated with electron- emissive material. The glow starter contacts are normally open. When the supply is connected, a glow current flows in the glow starter and heats the starter electrodes. If one of the starter electrodes is of bimetallic material, this electrode is arranged to bend towards the other starter electrode, and, when the bimetal is heated sufficiently, the two starter electrodes make contact.

The glow starter is connected such that, when it is closed, the current passes through the lamp electrodes in the discharge vessel, causing them to become emissive. The bimetal electrode is designed such that, when the contacts are closed, the current is insufficient to keep the bimetal in a bent condition at normal ambient temperatures, and the contacts will open. At contact break a voltage surge appears across the inductance and consequently across the discharge lamp, and if the surge is large enough, the discharge will start in the lamp. A small capacitance is normally connected across the starter terminals to assist starting and to suppress radio interference. Once the discharge lamp has started, the voltage across the discharge lamp and consequently across the glow starter is usually about 50% of the supply voltage, and the glow starter is designed such that the voltage across the glow starter during normal lamp operation is too low to maintain a glow discharge, or should there be a small glow, the energy provided

by it will be too small to cause the bimetal to bend sufficiently for closing the starter contacts again.

When the discharge lamp reaches its end of life due to emitter depletion, the glow starter will continuously close and re-open, causing an unpleasant flashing that is often described as " blinking" or "flickering". This flashing may continue for as long as the supply is connected and not only causes visual annoyance, but also has a deleterious effect on the glow starter's life.

Glow starters have been designed in which continuous flashing is prevented if the lamp has not started about 30 seconds after switch-on. This short period of flashing is provided in order to allow a sound discharge lamp to start, since some normal discharge lamps may not start until after a few closures and breaks of the glow starter.

A glow starter of the type described in the opening paragraph is known from British Patent Application GB-A 2 077 999. The known glow starter comprises two starter electrodes of which one or both is/are bimetallic. The known glow starter is provided with an internal stop which limits the movement of a bimetal electrode so that after flashing or

"blinking" with a failed lamp for about 30 seconds any further closure of the starter contacts and further flashing is prevented, the starter being kept in the "locked-out" or open condition while the supply is connected by the heat provided by the glow discharge. This "no-blink" glow starter reverts automatically to its original, unheated open condition a few minutes after the supply has been switched off.

A drawback of the known glow starter is that it is relatively complicated to manufacture.

SUMMARY OF THE INVENTION The invention has for its object to eliminate the above disadvantage wholly or partly. According to the invention, a glow starter switch of the kind mentioned in the opening paragraph for this purpose comprises: a first starter electrode and a second starter electrode in an envelope filled with an inert gas, - the first starter electrode comprising a bimetallic material for making contact between the first and the second starter electrode for assisting in starting an electrical discharge in the discharge lamp,

the second starter electrode comprising a shape memory alloy, the shape memory alloy changing shape at the end of life of the discharge lamp for avoiding contact between the first and the second starter electrode.

A shape memory alloy, also known as memory metal or a smart wire, is a metal (alloy) that remembers its geometry. After the shape memory alloy has been deformed below its activation temperature, it regains its original geometry by itself during heating (oneway effect) or, at higher ambient temperatures, simply during unloading (pseudo-elasticity). Not wishing to be held to any particular theory, the properties of a shape memory alloy are due to a temperature-dependent martensitic phase transformation from a low-symmetry to a highly symmetrical crystallographic structure. Such crystal structures are known as martensite and austenite.

Normal operation of the glow starter is that it assists the discharge lamp in starting up. Upon connection of the power supply, a glow current flows through the glow starter heating the starter electrodes. The first starter electrode being a bimetallic material, this electrode is arranged in the envelope of the glow starter such that it bends towards the second starter electrode. When the bimetal is heated sufficiently, the two starter electrodes make contact. The greater the distance between the two starter electrodes, the higher the temperature required for this.

During life of the discharge lamp the shape memory alloy does not change its shape and the glow starter functions as a normal glow starter. At the end of the normal service life of the discharge lamp, which may be caused, for example, by emitter depletion in (one of) the discharge electrodes, the discharge lamp will no longer start up. In this situation, the glow starter will be in a situation where it continuously tries to cause ignition of the discharge lamp. The glow starter will continuously close and re-open, causing an unpleasant flashing that is often described as "blanking" or "flickering". This flashing may continue for as long as the supply is connected. Due to this "flickering" the temperature in the glow starter will rise to a level which is above the level during normal operation of the discharge lamp (typically above 250°C). This relatively high temperature has an effect on the second electrode of shape memory alloy in that the shape memory alloy (permanently) changes its shape such that contact between the first and the second starter electrode is no longer possible. The second electrode of the shape memory alloy may be so positioned in the end- of-life situation that its change in shape causes it to move or bend away from the first electrode, such that a bimetallic deformation of the first discharge electrode no longer causes any contact between the first and the second starter electrode in the glow discharge.

The use of a shape memory alloy as one of the starter electrodes in a glow starter has the effect that flickering of the discharge lamp stops at end of life. In this manner a possibly unsafe situation at end of life of the discharge lamp is avoided in that heating effects in the glow starter are reduced. Another advantage of the avoidance of lamp flickering is that a waste of of energy due to lamp flickering and continuous ignition attempts is reduced. Yet another advantage of the avoidance of lamp flickering is that most consumers perceive the flickering of the discharge lamp at end of life as irritating. The discharge lamp simply will not ignite anymore at end of life because the glow starter has stopped its attempt to ignite the discharge lamp, contact between the first and the second electrode being no longer possible.

A preferred embodiment of the glow starter according to the invention is characterized in that the shape memory alloy comprises copper-zinc-aluminum, copper- aluminum-nickel, or nickel-titanium alloys. NiTi alloys are generally more expensive but have better mechanical properties compared with copper-based shape memory alloys. Another advantage is a higher activation temperature.

The use of shape memory alloy as material of the second starter electrode enables the glow switch to stay open permanently as a result of the change in shape of the shape memory alloy at the end of life of the discharge lamp. The glow discharge is maintained in the glow starter in this end-of-life situation. It is preferable, however, for this glow discharge to be extinguished in the end-of-life situation. To this end, a preferred embodiment of the glow starter according to the invention is characterized in that the envelope of the glow starter comprises a dispenser means for controllably dispensing hydrogen in the envelope at the end of life of the discharge lamp. As was described above, the temperature in the glow starter rises at the end of life of the discharge lamp to a level which is above the level during normal operation of the discharge lamp (typically in a range of approximately 250°C to approximately 350°C). This relatively high temperature has the effect on the dispenser means that it releases hydrogen into the gas atmosphere of the envelope of the glow starter around and above this temperature. The release of hydrogen causes an extinction of of the glow discharge. It is known that relatively high quantities of hydrogen may cause an arc shutdown of discharge lamps. The presence of released hydrogen in the envelope of the glow starter causes a rise in the voltage required for sustaining the glow discharge in the glow starter. The release of hydrogen into the envelope of the glow starter causes the glow discharge to go out passively.

There are many ways in which the dispenser means can be provided in the envelope of the glow starter. A preferred embodiment of the glow starter according to the invention is characterized in that the dispenser means comprises a hydrogen-containing metal or metal alloy. Such alloys generally comprise an open (internal) structure with a high specific surface area. In addition, such alloys can be relatively easily loaded with relatively large quantities of hydrogen that can be controllably released as a function of temperature, the partial pressure being specific for the material as a function of the metal/hydrogen ratio. In the description of this invention, the term "controllable release" is to be interpreted in the sense that hydrogen is (gradually) set free from the dispenser means, by which release a constant hydrogen equilibrium pressure is obtained in the glow starter during life.

Preferably, the hydrogen-containing metal or metal alloy is selected from the group formed by zirconium, yttrium, titanium, and hafnium. Said metals or metal alloys are very suitable as controllable hydrogen dispenser means for the controllable release of hydrogen in the envelope of the glow starter. The low amount of hydrogen in the glow starter does not affect the lamp properties (voltage, current) during normal service life of the glow starter.

In a preferred embodiment of a low-pressure mercury vapor discharge lamp according to the invention, the dispenser means comprises a metal hydride selected from the group consisting of titanium, zirconium, hafnium, a titanium-zirconium alloy, a titanium- hafnium alloy, and a zirconium-hafnium alloy. A very suitable material is TiH ₂ (titanium hydride).

The location of the dispenser means in the envelope of the glow starter is of influence on the extinguishing of the glow discharge. In this respect, a preferred embodiment of the glow starter according to the invention is characterized in that the dispenser means is disposed on a stem in the envelope of the glow starter, the stem carrying the first and the second starter electrode. In another preferred embodiment of the glow starter, the dispenser means is disposed on the first starter electrode.

BRIEF DESCRIPTION OF THE DRAWINGS These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter. In the drawings: Figure 1 shows a glow starter according to the invention, and

Figure 2 is an electrical circuit diagram including a fluorescent lamp, a ballast, and the glow starter of Figure 1.

The Figures are purely diagrammatic and not drawn to scale. Particularly for clarity, some dimensions are exaggerated strongly. Similar components in the Figures are denoted by the same reference numerals as much as possible.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Figure 1 schematically shows a glow starter according to the invention. The glow starter shown in Figure 1 is provided with a carrier plate 1 of an insulating material. The carrier plate 1 is provided with contacts and means for mechanically coupling the load to a holder. The glow starter has current supply conductors 5 which are connected to respective contacts. The glow starter is a switch S which forms part of a circuit comprising a ballast B and a fluorescent lamp L, as shown in Figure 2. The glow starter shown in Figure 1 comprises a first starter electrode 40 made of a bimetallic material and a second starter electrode 41 in an envelope 42 filled with an inert gas, usually a mix of rare gases. The first and the second starter electrode 40, 41 are mounted on a stem 10 and are bridged by an anti- interference capacitor 43. The first and the second starter electrode 40, 41 are connected to respective current conductors 5. When a voltage is applied across the switch S in the circuit shown in Figure 2, a glow discharge will develop between the first and the second starter electrode 40, 41, thus heating the first and the second starter electrode 40, 41. Said heating causes the first starter electrode 40 made of a bimetallic material to bend, as a result of which the first and the second starter electrode 40, 41 contact each other, which extinguishes the glow discharge. A typical temperature at which contact is established between the first and the second starter electrode 40, 41 is in a range of approximately 120°C to 130°CA tip of the first starter electrode is curved, to facilitate contact between the first and the second starter electrode 40, 41. After electrical contact has been established between the first and the second electrode 40, 41, the ballast B connected in series with the switch S, which has been broken after contact between the first and the second starter electrode 40, 41, generates a pulse across (OK?) the discharge lamp which is connected in parallel with the switch S, as a result of which the discharge lamp can ignite. If the discharge lamp fails to ignite, the process is repeated the moment the contact between the first and the second starter electrode 40, 41

opens up again due to the bimetal material moving back to its original position owing to cooling.

In Figure 1, the carrier plate 1 comprises synthetic resin projections 30 which serve as means for coupling the load to a holder. Said projections comprise a disc 32 at some distance from the carrier plate 1 and a duct 33 accommodating one of the current conductors 5. The disc 32 is partly covered with an electric conductor 20, which is connected as a contact to the relevant current conductor 5.

In Figure 1, the second starter electrode 41 comprises a shape memory alloy, the shape memory alloy changing its shape at end of life of the discharge lamp for avoiding contact between the first and the second starter electrode 40, 41. Preferably, the shape memory alloy comprises copper-zinc-aluminum, copper-aluminum-nickel, or nickel-titanium alloys.

The use of a shape memory alloy as material of the second starter electrode in a glow starter has the effect that flickering of the discharge lamp stops at the end of life of the discharge lamp. A potentially unsafe situation at end of life of the discharge lamp is thus avoided in that heating effects in the glow starter are reduced. At end of life, the discharge lamp simply will not ignite anymore because the glow starter has stopped its attempts to ignite the discharge lamp, contact between the first and the second electrode being no longer possible. In Figure 1, dispenser means 51 are mounted in the envelope 42 of the glow starter. Mounting is achieved by pasting or painting of the relevant material at the predetermined position. The function of the dispenser means 51 is to dispense hydrogen in the envelope 42 in a controllable manner at the end of life of the discharge lamp. Preferably, the dispenser means 51 comprises a hydrogen-containing metal or metal alloy. Preferably, the hydrogen-containing metal or metal alloy is selected from the groups of zirconium, yttrium, titanium and hafnium. Alternatively, the dispenser means 51 comprises a metal hydride selected from the group consisting of titanium, zirconium, hafnium, a titanium-zirconium alloy, a titanium-hafnium alloy, and a zirconium-hafnium alloy.

The use of shape memory alloy as a material of the second starter electrode enables the glow switch to stay open permanently as a result of the change in shape of the shape memory alloy at the end of life of the discharge lamp. In this end-of-life situation, the glow discharge is maintained in the glow starter, but it is preferablyextinguished in the end- of-life situation.

In the example of Figure 1, a dispenser means 51 is mounted on the stem 10 inside the envelope 42 of the glow starter. Alternatively, the dispenser means is disposed on the first starter electrode (not shown in Figure 1).

The dispenser means 51 is inactive during normal operation of the discharge lamp. The dispenser means 51 is activated at the end of life of the discharge lamp. Under end-of-life conditions of the discharge lamp, the temperature in the glow starter rises to a level which is above the level during normal operation of the discharge lamp (typically in a range of approximately 250°C to approximately 350°C). This relatively high temperature has an effect on the dispenser means 51, i.e. around and above this temperature it releases hydrogen into the gas atmosphere of the envelope of the glow starter. The release of hydrogen extinguishes the glow discharge in the glow starter. The relatively high quantities of hydrogen may cause an arc shut-down of discharge lamps. The presence of released hydrogen in the envelope of the glow starter causes a rise in the voltage required for sustaining the glow discharge in the glow starter at the end of life of the discharge lamp. The release of hydrogen into the envelope of the glow starter causes the glow discharge to go out passively.

There are many ways in which the dispenser means can be provided in the envelope of the glow starter. Preferably, the dispenser means comprises a metal hydride selected from the group consisting of titanium, zirconium, hafnium, a titanium-zirconium alloy, a titanium-hafnium alloy, and a zirconium-hafnium alloy. A very suitable material is TiH ₂ (titanium hydride). To obtain a desired performance of the dispenser means at end of life of the discharge lamp, approximately 4 to 10 mg TiH ₂ is coated on the stem 10 of the glow starter. In a testing environment it was found that at end of life of the discharge lamp the flickering of the discharge lamp stopped after 5 minutes from the moment the first flickering was observed. The glow discharge in the glow starter extinguished at around the same time.

It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. Use of the verb "comprise" and its conjugations does not exclude the presence of elements or steps other than those stated in a claim. The article "a" or "an" preceding an element does not exclude the presence of a plurality of such elements. In the device claim enumerating several means, several of these means may be embodied by one and the same item of

hardware. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.