Electric lamp and discharge devices: systems – Pulsating or a.c. supply
Reexamination Certificate
2001-06-05
2003-07-01
Wong, Don (Department: 2821)
Electric lamp and discharge devices: systems
Pulsating or a.c. supply
C315S057000, C315S150000, C315S360000
Reexamination Certificate
active
06586891
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to high-intensity discharge lamps and high-intensity discharge lamp operating apparatuses. In particular, the present invention relates to high-intensity discharge lamps such as metal halide lamps, high pressure mercury lamps, and high pressure sodium lamps that are provided with a trigger wire.
Conventionally, high-intensity discharge lamps such as metal halide lamps have been used for general illumination or spot illumination. In recent years, they are also widely used as a light source of OHPs and liquid crystal projectors.
A metal halide lamp includes, for example, an arc tube (luminous bulb) made of quartz glass and a pair of electrodes spaced apart with a predetermined distance in the arc tube, and mercury and a metal halide are enclosed as luminous materials in the arc tube. The arc tube is sealed with electrode sealing portions at both ends, and the pair of electrodes are connected to respective external lead wires via metal foils hermetically sealed in the electrode sealing portions. The external lead wires are electrically connected to an operating circuit (driving device) of the lamp. The operating circuit includes a ballast for restricting current flow to less than a predetermined amount during operation and means for applying a high-voltage pulse voltage.
The lighting operation of metal halide lamps is as follows. In order to start lighting operation, first, it is necessary to apply a voltage to the pair of electrodes to cause insulation breakdown to start discharge. A voltage necessary to cause the insulation breakdown is called “breakdown voltage”, and the breakdown voltage is generally a high voltage of several hundreds times higher than the lamp voltage during steady-state operation.
In order to achieve the compactness and the low cost of the operating circuit, it is preferable that the breakdown voltage is as low as possible. As a technique for lowering the breakdown voltage, a configuration where a trigger wire (close conductor) is wound around the outer circumference of the arc tube is known, which is disclosed in Japanese Laid-Open Patent Publication No. 8-69777, for example.
FIG. 11
shows a metal halide lamp having this configuration.
The metal halide lamp shown in
FIG. 11
has an arc tube
101
and electrode sealing portions
102
a
and
102
b
, and a trigger wire
108
having a first end
108
a
and a second end
108
b
is spirally wound around the circumference of the arc tube
101
. A pair of electrodes
103
a
and
103
b
are opposed to each other inside the arc tube
101
. The pair of electrodes
103
a
and
103
b
are connected to external lead wires
105
a
and
105
b
via the metal foils
104
a
and
104
b
in electrode sealing portions
102
a
and
102
b
. The second end
108
b
of the trigger wire
108
is connected to the external lead
105
b
, and the first end
108
a
of the trigger wire
108
is wound around the end (near the base) of the electrode sealing portion
102
a
on the side of the arc tube
101
. The external lead wires
105
a
and
105
b
are electrically connected to an operating circuit (driving device)
111
, and the operating circuit
111
is electrically connected to a power supply
107
.
This metal halide lamp is provided with the trigger wire
108
, so that the breakdown voltage can be reduced. This is caused by the following mechanism. When a high-voltage pulse voltage is applied from the operation circuit
111
connected to the power supply
107
to the electrodes
103
a
and
103
b
, the trigger wire
108
has the same electric potential as that of the electrode
103
b
. As a result, the trigger wire
108
causes an electrical field having a large electric potential gradient to be formed inside the arc tube
101
. This electrical field is likely to cause insulation breakdown of Xe gas between the electrodes
103
a
and
103
b
and thus the breakdown voltage can be reduced.
However, in the case of the conventional metal halide lamp provided with the trigger wire
108
, although the breakdown voltage can be reduced, color change, an increase of the lamp voltage, or devitrification, which is a phenomenon that the transparency is lost by opaqueness of the arc tube, may occur during operation of the lamp. Furthermore, the luminous flux maintenance factor is reduced, or the lamp cannot be turned on, and thus the lifetime of the lamp tends to be reduced.
As a result of the factors causing the above-described phenomena in depth, the inventors of the present invention found that the electrical field generated by the trigger wire
108
is a large factor. For example, in the case where a driving voltage (lamp voltage) of 65V is applied across the electrodes
103
a
and
103
b
during operation, the voltage between the trigger wire
108
connected to the external lead wire
105
b
and the electrode
103
a
is also 65V. When the distance between the electrodes
103
a
and
103
b
is 3.7 mm, the shortest distance between the first end
108
a
of the trigger wire
108
and the electrode
103
a
is 1 mm, then the electric potential gradient of the electrical field formed between the electrodes
103
a
and
103
b
is 17.6V/mm. On the other hand, the electric potential gradient of the electrical field formed between the electrode
103
a
and the trigger wire
108
is 65V/mm, which is more than three times higher than 17.6V/mm, in the largest portion.
In the lamp where a high-voltage pulse voltage is applied, the insulation breakdown of Xe gas is caused, and thus discharge is started, the temperature of the inner wall of the arc tube
101
is increased by subsequent discharge. When the temperature of the inner wall of the arc tube
101
is increased, enclosed material such as metal halide enclosed in the arc tube
101
is evaporated and further ionized. The ionized luminous material is affected more by the electrical field between the trigger wire
108
and the electrode
103
a
than by the electrical field between the electrodes
103
a
and
103
b
, and thus attracted more to the trigger wire
108
. More in details, in the case where the applied driving voltage is alternating voltage, when the trigger wire
108
is in the negative electric potential, the luminous material such as sodium in the form of positive ions are attracted to the trigger wire
108
, and sodium having a small ion radius moves in the quartz glass and leaks out of the arc tube
101
. Thus, the amount of the luminous material in the arc tube
101
is reduced, so that the optical characteristics (color temperature, lamp voltage, luminous flux maintenance factor etc.) are significantly changed, which was found by the inventor of the present invention. Furthermore, sodium or the like breaks the amorphous structure of the quartz glass while moving in the quartz glass, and crystallization of the quartz glass (phase transition to cristobalite) occurs, which causes opaqueness or devitrification of the quartz glass.
SUMMARY OF THE INVENTION
Therefore, with the foregoing in mind, it is a main object of the present invention to prolong the lamp lifetime of a high-intensity discharge lamp provided with the trigger wire.
A high-intensity discharge lamp of the present invention includes an arc tube including a pair of electrodes opposed to each other therein; and a trigger wire made of a conductive material provided in an outer circumference of the arc tube. The trigger wire is turned to be in a conductive state with one electrode of the pair of electrodes when a start-up voltage is applied across the pair of electrodes, and discharge is started between the pair of electrodes in a state where an electrical field is formed between the trigger wire that is in the conductive state and the other electrode of the pair of electrodes.
In one embodiment of the present invention, a part of the trigger wire is arranged close to an external lead wire in an insulating state, the external lead wire being electrically connected to the one electrode, and when the start-up voltage is applied, insulation breakdown is caused between the p
Kaneko Yuriko
Kiryu Hideaki
Minamihata Ryo
Murase Takayuki
Takahashi Kiyoshi
Harness & Dickey & Pierce P.L.C.
Matsushita Electric - Industrial Co., Ltd.
Tran Thuy Vinh
Wong Don
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