This invention relates to high intensity discharge (HID) lamps, more particularly to HID lamps comprising an improved electrode for enhanced dimming characteristics of the lamp. The invention relates also to a manufacturing method of such improved electrodes.
Most of the commercially available known HID lamps are used for several purposes, for example low power metal halide lamps are used for indoor lighting applications. HID lamps have electrodes without preheating features. The lamps have to start reliably with cold electrodes, and perform the specified electrical and light characteristics both initially and through their life after reaching steady-state operation temperatures. These requirements set different and often contradicting conditions to the electrode design.
In addition to these requirements, dimming is a great advantage in applications in which light output of the lamp has to be adjusted to some reduced value, or actual conditions allow reduction of lamp power and energy consumption.
The most important types of HID lamps are the high-pressure mercury, high-pressure sodium, metal halide, discharge automotive, and special type (projection, studio, etc.) lamps. Continuous mode dimming is particularly important in the low wattage range of HID lamps intended for interior lighting or possibly for vehicle lighting.
Conventional electrode designs allow dimming down only about to 60% of the nominal wattage, which makes them unusable for certain applications.
According to the solutions disclosed in U.S. Pat. Nos. 2,887,603 and 2,951,171, a special pair of electrodes is used in a lamp. One of them comprises a thorium-oxide emission material in a conical cavity. The other one discloses an electrode with coil that provides a nest for the emission material in order to reduce the loss rate thereof and consequently to lengthen the life of lamp.
Use of thorium-oxide as emission material in the form of tablet or pellet is indicated in U.S. Pat. No. 3,619,699 that relates to electrodes of discharge lamps. Penetration of the arc terminus into an electrode cavity is assisted by vapor breathing of the emission material, which injects plasma ingredients into the cavity during AC re-ignition after current zero. Such breathing is very desirable in high-pressure low-current lamps. Breathing is favored by a cavity, which has a depth not substantially greater than the terminus penetration depth. High temperatures deep within the cavity are advantageous, and are achieved by providing enhanced thermal coupling between the forward end of the cavity member and the cooler radiation shield surrounding it, and also by thermal insulation between the sides of the cavity member and the cooler shield. Disposing the emission material within the lower portion of the cavity favors deeper terminus penetration. Projection of the radiation shield beyond the cavity member is avoided because such projection would favor formation of a spot mode arc terminus on the shield.
This teaching provides hints to use a spiral member on the tip of the electrode, but the spiral member does not really form a cavity. The spiral member is used to make a reservoir for the emission material. The reservoir is not completely filled with the emission material, but an element that holds this material in place fills up the cavity completely. It is also complicated to manufacture such electrodes due to the emission material insert and large number of electrode components.
Another solution is known from the published patent application US 2006/0238127, in which the discharge vessel has a first and a second mutually opposed neck-shaped portion provided with a pair of electrodes, each of which is tubular over its entire length. The rod and spiral combination is declared in this document as having several drawbacks such as hardly controllable thermal contact between them. Therefore the electrode is manufactured with a tungsten tube mounted onto a tungsten rod with an intermediate member. A tungsten tube of small diameter is very expensive and requires non-conventional electrode manufacturing technology that may further increase the costs of production and may involve dimension-control issues primarily at electrodes of submillimeter size.
There is a need for HID lamps with cavity electrodes that require simple and cost effective manufacturing technology.
Recently emerging requirement is that HID lamps are dimmable with a continuous and wide range of wattage. There is a particular further need for meeting this requirement by suitable electrode structure.
In an exemplary embodiment of the present invention, there is provided a high intensity discharge lamp comprising a discharge vessel, two electrode rods having substantially flat ends facing to each other in opposite positions within the discharge vessel. A spiral coil of wire is wound at least on a part of the surface of at least one of the electrode rods. The spiral coil protrudes over said end of the corresponding electrode rod and thus forms a hollow cavity for extending dimmable wattage range of the lamp.
In an exemplary embodiment of another aspect of the present invention, an electrode for high intensity discharge lamps is provided. The electrode comprises an electrode rod having a substantially flat end. A spiral coil of wire is wound at least on a part of the surface of the electrode rod, and the spiral coil protrudes over said end of the electrode and thus forms a hollow cavity for extending dimmable wattage range of the lamp.
Finally a method for manufacturing such an electrode is provided, the method comprising the step of winding turns of at least one spiral coil layer of wire onto the surface of an electrode rod, while the spiral coil layers protrude over said end of the electrode rod, and thus a hollow cavity is formed.
In exemplary embodiments of the method, the spiral coil layers and turns thereof are pressed together in a press mould comprising a cylinder, an inner core and one or two concentric ring shaped movable pistons.
This invention has several advantages over the prior art. It broadens the dimmable wattage range of the HID lamps with respect to the dimmable wattage range of the known HID lamps significantly. The flexible cavity shape configuration of the lamp electrodes provides a better luminous efficiency; more well defined arc attachment and consequently more stable operation of the lamp. The electrode structure of this invention can be produced by only minor modifications applied to the existing technologies of manufacture, which in turn results in cheap and easy production of the lamp.
The invention will now be described in detail with reference to the enclosed drawing, in which
Referring first to
The discharge vessel 2 is illustrated in
Two modified embodiments of this last two-layer type electrode follow in the illustrations of
In
The radius r3 of the wire in the second layer 18 can be substantially identical with or different from the radius r2 of the wire in the first layer 17. As a common practical rule for the sizes, it can be suggested that radiuses r2 and r3 of the wires of the spiral coils be less than ⅗ of the radius r1 of the corresponding electrode rod 9 of cylindrical shape. This rule originates from constraints of state-of-art manufacturing technology.
In the course of manufacturing, a step of winding turns of at least one spiral coil layer of tungsten or tungsten alloy wire onto the surface of an electrode rod 9 is implemented while the resultant spiral coil layers will protrude over the end 16 of the electrode rod 9 and thus the hollow cavity 11 will be formed.
The end part of the electrode is thus made of spiral coils 10. This means that the electrical and mechanical contact between the neighboring turns of the coil may occasionally be insufficient. In order to enhance the quality of the contact, the spiral coil layers and the turns thereof may be pressed together during manufacture.
The layer structure of
In the diagram of
The two electrodes of
The invention is not limited to the shown and disclosed embodiments, but other elements, improvements and variations are also within the scope of the invention. For example, it is clear for those skilled in the art that different cross-section shapes of the electrode rod, such as polygonal cross-section, may also be applicable for the purposes of the present invention.
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Entry |
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PCT/US2008/064239 International Search Report, mailed Feb. 16, 2009. |
Number | Date | Country | |
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20080315769 A1 | Dec 2008 | US |