Electric lamp and discharge devices: systems – Current and/or voltage regulation
Reexamination Certificate
1999-01-04
2001-01-09
Wong, Don (Department: 2821)
Electric lamp and discharge devices: systems
Current and/or voltage regulation
C315S307000
Reexamination Certificate
active
06172468
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to gas discharge lamps generally and to methods and apparatus for starting such lamps in particular.
BACKGROUND OF THE INVENTION
Gas discharge lamps are well known in the art and their operation is described in FIGS.
1
-
4
, to which reference is now made.
FIG. 1
generally illustrates a gas discharge lamp and indicates that such a lamp includes a bulb
10
, two electrodes
12
and
14
, and gas
16
within the bulb
10
. The lamp is controlled by a ballast
18
which includes an igniter
19
therein. Prior art gas discharge lamps are discussed in the
OSRAM Metal Halide Lamps Technology and Application Handbook,
July 1996, pp. 35-39 and 52.
To start the lamp, igniter
19
provides a spark, of typically 2-4 kV for a cold start and 20-40 kV for a hot start, between the two electrodes
12
and
14
. The spark causes the electrode acting as the cathode, such as electrode
12
, to emit electrons which ionizes the gas
16
. The ionized gas then provides a low current path between the electrode
12
and the electrode
14
, acting as the anode, thereby reducing the amount of voltage needed to close the circuit.
To ensures that the spark becomes established as a stable steady-state arc discharge, the spark must be of a high voltage (2-40 kV), the electrical energy of the spark must be high, the ballast must provide a quick current flow and the ballast must have an adequate open circuit voltage, typically of 250V.
The spark
20
is shown in the voltage-time graph of FIG.
2
. Once ignition has occurred, the gas
16
is ionized and the voltage needed to maintain a current through the lamp drops to a low, operating voltage of about 20V, remaining there until the AC voltage direction changes. If the electrodes
12
and
14
are not sufficiently warm (i.e. they do not emit enough electrons), the ionization of the gas
16
cannot be maintained and the current path is broken. Accordingly, when the voltage changes direction, the gas must be reignited.
The reignition continues until the electrodes
12
and
14
are warm enough to maintain the ionization during the voltage direction change. This typically takes 10-100 cycles, where the length T
1
of half of each cycle is typically on the order of 2.5 msec. Once this occurs, the operating voltage rises to the nominal operating voltage of the lamp which is typically between 50 and 130VAC and depends on the type of the lamp.
FIG. 3
shows the cycles and the changing operating voltage over time.
The high power ignition pulses cause localized “hot spots” on the electrode, melting of the metal and sputtering of the electrodes
12
and
14
which erodes them. The sputtering blackens the inside walls of the bulb
10
, thereby reducing the amount of light (as measured in lumens) that the lamp provides, a phenomenon known as “lumen degradation”. Furthermore, the sputtering removes material from the electrodes, as shown in FIG.
4
.
FIG. 4
shows electrode
12
with a very uneven end
22
. As more and more material is removed, the distance between the electrodes
12
and
14
is increased and, if the distance is too far, the spark does not successfully reach from one electrode to the other. Due to the two effects of sputtering and blackening, the lamp light output degrades dramatically and, eventually, the lamp fails.
Mechanisms are known for igniting the gas with a DC voltage and, once the gas is ignited, switching to AC operation. Since the voltage never changes direction, the gas
16
remains ionized. However, in such lamps, the current only attacks the electrode
14
acting as the anode, causing sputtering and warming up electrode
14
significantly more than electrode
12
. The result is that portions of electrode
14
melt down, causing more severe damage than that seen with AC ignition.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a novel method of igniting a gas discharge lamp which provides minimal or no reignition operations.
There is therefore provided, in accordance with a preferred embodiment of the present invention, a gas discharge lamp which includes a lamp, a ballast for providing an AC voltage to the lamp and a frequency changer which selects an initial AC frequency at which the ballast provides the AC voltage and which changes the AC frequency to a second AC frequency, wherein the second frequency is higher than the initial frequency.
Moreover, in accordance with a preferred embodiment of the present invention, the frequency changer selects the second AC frequency once the gas discharge lamp has substantially achieved a standard operating voltage.
Still further, in accordance with a preferred embodiment of the present invention, the initial AC frequency has a period which is long enough to maintain gas ionization during an AC voltage direction change.
Additionally, in accordance with a preferred embodiment of the present invention, the change from the initial AC frequency to the second AC frequency can be any increasing function, such as a step or a ramp function.
Finally, in accordance with a preferred embodiment of the present invention, there is provided a method of operating a gas discharge lamp. The method includes the steps of initially operating the lamp at an initial alternating current (AC) frequency; and later operating the lamp at a second AC frequency.
The second AC frequency is higher than said initial AC frequency.
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patent: 4331905 (1982-05-01), Owen
patent: 4630005 (1986-12-01), Clegg et al.
patent: 5565740 (1996-10-01), Hiramatsu et al.
patent: 5569984 (1996-10-01), Holtslag
patent: 5572094 (1996-11-01), Yamashita et al.
patent: 5623187 (1997-04-01), Caldeira et al.
patent: 5677602 (1997-10-01), Paul et al.
patent: 5705894 (1998-01-01), Krummel
patent: 5770924 (1998-06-01), Osterried
patent: 0239420B1 (1992-02-01), None
patent: 0311424B1 (1994-03-01), None
patent: WO 97/43879 (1997-11-01), None
OSRAM Metal Halide Lamps Technology and Application Handbook, Jul. 1996,pp. 35-39 and 52.
D Chuc Tran
Darby & Darby
Metrolight Ltd.
Wong Don
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