Electric lamp and discharge devices: systems – Combined load device or load device temperature modifying... – Filament – electric heater – or resistance in shunt with the...
Reexamination Certificate
2000-01-19
2001-04-24
Vu, David (Department: 2821)
Electric lamp and discharge devices: systems
Combined load device or load device temperature modifying...
Filament, electric heater, or resistance in shunt with the...
C315S056000, C315S059000
Reexamination Certificate
active
06222320
ABSTRACT:
TECHNICAL FIELD
The invention proceeds from a metal halide lamp in accordance with the preamble of Claim
1
. At issue here are metal halide lamps with a starting aid fitted outside the ceramic discharge vessel.
PRIOR ART
The use of starting aids which reduce the starting voltage has been known for a long time. An earlier alternative was to add a radioactive gas (Kr 85) to the starting gas in the lamp.
U.S. Pat. No. 5,355,053 has already disclosed a metal halide lamp with an external starting aid. When metal halide lamps are started, the electric flashover is produced by a high-voltage pulse which is applied between the two electrodes located in the ceramic discharge vessel. The absolute value of this high voltage is determined by the geometrical dimensions of the discharge vessel and, in particular, by the cold filling pressure of the inert gas (mostly xenon) located therein. A high cold filling pressure leads, on the one hand, to high light yields and good maintenance, but on the other hand requires correspondingly high starting voltages which are not directly available.
A remedy is found in an electrically conducting, metal starting aid fitted outside on the discharge vessel. It is either a metal wire or a strip, which is sintered onto the ceramic discharge vessel. The separate part can likewise have the shape of a starting strip which bears against the discharge vessel and is pressed on, for example, by means of a bimetal. During operation, the bimetal lifts this starting aid up off the discharge vessel. This is required, since the starting aid is electrically connected to one of the two electrodes, and so there is a steep gradient of the electric field strength present between the starting aid and the second electrode, which leads to diffusion of the sodium through the wall of the discharge vessel.
A bimetal is dispensed with in the case of starting aids lacking direct electric contact with the system voltage. Instead of this, use is made of axial or helical starting strips surrounding the discharge vessel. The starting aid is coupled in this case to the starting pulse only capacitively. Since it is at a freely floating potential, sodium diffusion is prevented. Such a design is also used for metal halide lamps, where it is fitted, in particular, on the outer bulb (EP-A 732 870). This lamp has a substantially smaller fraction of sodium in the filling.
The flashover forms in like manner in both cases, that is to say both for direct and for capacitive coupling. Firstly, a discharge is produced between the first electrode, at which the high-voltage pulse is present, and the nearest point on the ceramic wall, on which the starting aid is seated outside. Discharge propagates on the ceramic wall until there is finally a flashover to the second electrode.
In the case of starting aids lacking direct electric contact with the system voltage, because of the capacitive coupling there is set up on the starting aid a potential which is between that of the high-voltage pulse at the first electrode and the zero potential of the second electrode. The potential difference between the high-voltage pulse and the starting aid is consequently less than when the starting aid is at the potential of one of the electrodes. The level of the starting voltage is decided by the field strength forming in the space between electrode tip and starting aid. In this case, the geometry and the spacings influence the level of the starting voltage.
A further previously known solution for, metal halide lamps having a discharge vessel made from quartz glass is to provide UV emitting, gas-filled chambers outside the discharge volume (EP-A 722 184). Starting is facilitated here by the ionizing effect of UV radiation.
SUMMARY OF THE INVENTION
It is the object of the present invention to provide a metal halide lamp in accordance with the preamble of Claim
1
which starts relatively unproblematically and whose starting aid is simple and cost-effective to produce. A further object is to reduce the electric field strength required for starting, in particular for lamps with a high metal halide dose, as required principally in metal halide high-pressure lamps which are virtually or completely free from mercury, and thus to facilitate starting and arc acceptance in these lamps.
This object is achieved by means of the characterizing features of Claim
1
. Particularly advantageous refinements are to be found in the dependent claims.
Metal halide lamps with little mercury (corresponding to less than 1 mg/cm
3
Hg), chiefly mercury-free metal halide lamps, exhibit substantial difficulties in starting reliably. In these lamps, which contain a high dose of metal halides (for example, 10 to 250 &mgr;mol/cm
3
), there is the difficulty of metal halide layers condensing on the electrode surfaces as the lamp cools. As a result, the process of releasing electrons from the surface of the electrodes, which is decisive for multiplying the charge carriers, proceeds substantially less efficiently than in the case of an electrode surface which is uncoated or coated with metal (Hg). For example, in the case of Hg-containing lamps small Hg droplets condense on the electrode surface.
The use of a single-ended starting aid results in the formation of a capacitive auxiliary discharge which is directed away laterally from the electrode and preionizes the discharge path between the electrodes. To be precise, the auxiliary discharge emits high-energy UV radiation into the volume between the electrodes. This process is so efficient that the multiplication of the charge carriers in this volume, and thus the flashover, is strongly promoted.
According to the invention, the starting aid is constructed such that it produces a strong inhomogeneity of the electric field strength between the starting aid and the assigned electrode. As a result, the efficiency of the starting aid is substantially increased for the same value of the applied starting voltage.
The starting aid can be produced from heat-resistant metal (typically tungsten) in the case when the electrode bears permanently against the discharge vessel. If it is desired to have only a temporary contact with the discharge vessel in the cold state of the lamp, the use of a thin bimetal strip is to be recommended.
In the case of lamps having a ceramic discharge vessel, no indication has been established of sodium diffusion induced by the bearing of a metallic starter electrode, at least over a burning life of approximately 5000 h.
The design is also helpful, in particular, in ceramic metal halide lamps where addition of radioactive fractions in the starting gas, such as Kr 85, for example, is dispensed with. Furthermore, the hot starting of ceramic metal halide lamps is facilitated when the starting electrode bears permanently.
A first embodiment of the starting aid consists in creating at one end (or also at two ends) a punctiform contact of a starting aid on the outside of the wall on the discharge vessel, approximately at the level of a first electrode, advantageously in the vicinity of the electrode tip or in the bordering region of electrode shaft. In this case, the direct spacing between the starter electrode and electrode should be substantially smaller than the spacing between the two electrodes in the discharge volume.
The punctiform design creates an extremely strong inhomogeneity in the electric field strength. This punctiform starting aid (starting electrode) is connected to the lead of the other, second electrode.
The outer starting electrode can bear on the outside of the discharge vessel temporarily or else permanently. In the case of lamps capped at one end, the starting electrode is fitted (welded) on the frame which holds the discharge vessel and, proceeding from there, touches the discharge vessel in the vicinity of the first electrode, while the frame leads to the second electrode.
The fact that a starting aid bears against the ceramic discharge vessel produces a capacitive partial discharge between the starting aid and internal electrode. Because of the smaller geometrical sp
Mc Neill William H.
Patent Truehand-Gesellschaft fuer elektrische Gluelampen mbH
Vu David
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