Electric lamp and discharge devices: systems – Periodic switch in the supply circuit – Silicon controlled rectifier ignition
Reexamination Certificate
1999-12-15
2001-05-15
Vu, David (Department: 2821)
Electric lamp and discharge devices: systems
Periodic switch in the supply circuit
Silicon controlled rectifier ignition
C315S224000, C315SDIG005
Reexamination Certificate
active
06232725
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a circuit arrangement for operating a high pressure discharge lamp with a current having opposite polarities in successive periods, which lamp is provided with at least two main electrodes being spaced on an electrode distance from each other, the circuit arrangement comprising:
input terminals for connecting a supply source,
output terminals for connecting the high pressure discharge lamp, and
means, coupled to the input terminals, for supplying the lamp current to the high pressure discharge lamp, which lamp current, in the successive periods has a predetermined shape.
Such a circuit arrangement is known from U.S. Pat. No. 5,608,294. The known circuit arrangement provides a measure to suppress flickering of a high pressure discharge lamp and is in particular suitable for operating a high pressure discharge lamp in a projection system like a projection television apparatus. In the known circuit arrangement, the lamp is supplied with successive block shaped current pulses of opposite polarity. The suppression of flickering is achieved by supplying, during periods of the lamp current, additional current pulses with the same polarity at the end of a predetermined fraction of such a period of the lamp current. By means of the thus reshaped current pulses, the temperature of the electrode is raised to a relatively high value, which high temperature increases the stability of the discharge arc, because the discharge arc originates from the same place on the electrode in each cathodic phase and so flickering is substantially suppressed. The additional current is supplied in a regular sequence, preferably during each successive pulse. Although it is known that AC operation of high pressure discharge lamps with a low frequency alternating lamp current prevents a rapid erosion of the electrodes of the high pressure discharge lamp (further also referred to as the lamp) and allows operation of the lamp with a relatively high efficacy, it has occurred that lamps operated with the known circuit arrangement showed to have a continuous increase of the arc voltage over an operating time of several hundred hours, which voltage increase appeared to continue when the lamp was experimentally operated for several thousand hours. As a luminous output of the lamp being fairly constant over the life of the lamp is of vital importance for use in a projection system, a continues arc voltage increase forms a serious drawback in reaching a long lamp life.
In case a high pressure discharge lamp is operated with an AC current, each electrode of the lamp alternately functions as a cathode and as an anode during successive periods of the lamp current. During these periods the electrode is said to be in the cathodic phase and the anodic phase, respectively. Electrode material, that is removed from the electrode in the anodic phase, returns to the electrode as a stream of ions in the cathodic phase. These transport processes further complicate the behavior of the electrode temperature during each period of the lamp current, since the time dependency of the electrode temperature in the anodic phase differs from that in the cathodic phase.
SUMMARY OF THE INVENTION
It is an object of the invention to provide a circuit arrangement for operating a high pressure discharge lamp in a way which substantially overcomes the mentioned drawback and also maintains the substantial suppression of flickering of the lamp during its operation.
According to the invention, a circuit arrangement of the kind mentioned in the opening paragraph is characterized in that that the circuit arrangement is provided with
means for detecting a first parameter indicative for the electrode distance and forming a first signal dependent on the first parameter, and with
means for reshaping the lamp current in dependence on the thus formed first signal.
It has surprisingly occurred that with a controlled reshaping of the lamp current it is possible to substantially overcome the problem of continuous increase of the lamp voltage without significantly affecting lamp flicker suppression.
Further improvement with regard to discharge arc stability is achieved when the circuit arrangement further comprises:
means for detecting a second parameter indicative of the occurrence of lamp flicker and forming a second signal dependent on the detected second parameter, and
means for further adjusting the shape of the lamp current in successive periods in dependence of the second signal.
Because the shape of the current flowing through the lamp is changed in accordance with the detection of occurrence of flickering, it is possible to suppress both the flickering to a level fully acceptable for optical projection and to simultaneously substantially control alterations in the electrode distance and thus counteract a continuous tendency of lamp voltage increase.
In an embodiment the first parameter is provided by the lamp voltage, preferably averaged over several periods.
In an embodiment of the circuit arrangement according to the invention the lamp voltage during each successive period provides the second parameter. Use of the lamp voltage for the second parameter has the advantage that the first and second parameter are both based on lamp voltage. This simplifies the circuit arrangement. In a first preferred embodiment the shape of the lamp voltage during each period is detected and used for forming the second parameter. Preferably this is realized by means in the circuit arrangement which measures the lamp voltage at selected intervals during such a period and compares the thus found values with each other. In a second preferred embodiment for forming the second parameter it is the value of the lamp voltage in successive periods at a fixed moment during each period, preferably at a moment of a constant lamp current, which are detected. In a practical embodiment this is preferably realized by means for measuring the lamp voltage at a moment close to the end of each period and comparing the outcome of consecutive periods having the same polarity. In a further embodiment the second parameter is formed by the luminous output of the lamp, for instance by means of optical detectors placed around a display area of a projection system, for instance at the edge of the display area.
Good results were obtained in cases where the frequency of the periods of opposite polarity of the lamp current was selected from the range 45 Hz-500 Hz.
REFERENCES:
patent: 5608294 (1997-03-01), Derra et al.
patent: 5880561 (1999-03-01), Miyazaki et al.
patent: 0944294 (1999-09-01), None
patent: WO9714275 (1997-04-01), None
Derra Gunther H.
Fischer Hanns Ernst
Ganser Hans G.
Krucken Thomas
Moench Holger
U.S. Philips Corporation
Vu David
LandOfFree
Circuit arrangement for operating a high-pressure discharge... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Circuit arrangement for operating a high-pressure discharge..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Circuit arrangement for operating a high-pressure discharge... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2513243