Electric lamp and discharge devices: systems – With cathode or cathode heater supply circuit – Plural load device systems
Patent
1993-06-10
1996-03-12
Pascal, Robert J.
Electric lamp and discharge devices: systems
With cathode or cathode heater supply circuit
Plural load device systems
315 99, 313574, 313491, H05B 3900
Patent
active
054989302
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The invention relates to a method of dimensioning and operating by A.C. or D.C. at a predetermined external heater power a low pressure discharge lamp having two electrodes between which the discharge is formed, at least one of the electrodes being alkaline earth oxide coated and adapted to form a permanently heated cathode, particularly for use in video matrix displays.
BACKGROUND OF THE INVENTION
In low pressure discharge lamps the discharge is formed between electrodes. These operate alternatively as anode or cathode, respectively, if the lamp is supplied with A.C., or permanently as anode or cathode in case of D.C. operation. Cathodes consist usually of a coiled tungsten wire which is coated with a mixture of alkaline earth oxides to enhance thermionic electron emission.
The life of a fluorescent lamp is mainly determined by the life of the cathode.
The physical behaviour of an oxide coated cathode is complex. Roughly the cathode exists in a sensitive equilibrium of thermionic emission and evaporation of emissive material.
SUMMARY OF THE INVENTION
There is an optimum temperature of the emissive coating at which electron emission is high enough to maintain the discharge and evaporation is low enough to grant sufficient life.
Rapid starting of a low pressure discharge lamp is accomplished by current heating of the cathode--or, in case of A.C. operation, both electrodes--to a temperature which provides for sufficient thermionic emission. The heating mode is permanent, i.e. the heater is externally and permanently heated. The external heater current is normally chosen equal to the discharge current.
In operating low pressure discharge lamps, particularly fluorescent lamps with externally heated cathode(s) the discharge current is superposed to the heater current and therefore forms a locally overheated area (hot spot). Due to temperature difference of about 400-500 K encountered with the known lamps and their operation as described above, the evaporation rate of the emissive oxides increases by orders of magnitude. This, in turn, leads to increased blackening and, ultimately, reduced life of the lamp.
It is an object of the present invention to provide a method as mentioned above in the first paragraph of the description by which the functional life of lamps as indicated above can be increased and the end blackening or end discoloration, respectively, reduced.
This object is met in that an external heater current is used which is approx. 1.5 to 5 times higher than the discharge current and that the heater voltage is approx. 33.33% to 80% lower than that of a conventionally operated cathode or electrode, resp., and that the wattage of the heater circuit is maintained.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a low pressure discharge lamp having two cathodes;
FIG. 2 is a photograph showing a cathode having a hot spot; and
FIG. 3 is a photograph showing a cathode having no hot spot.
BEST MODE FOR CARRYING OUT THE INVENTION
With particular attention to FIG. 1, there is illustrated a low pressure discharge lamp 10 having a U-shaped tube and two electrodes 14 and 16.
The main advantage of the inventive method resides in the fact that to improve the accuracy of control of the desired optimum cathode temperature and thereby enhance lamp life and reduce discoloration the contribution of discharge current heating is minimized and the contribution of the external heater current is maximized so that cathode temperature will not constantly rise and fall to such extents that the adverse effects elucidated above including the formation of hot spots are encountered.
The inventive dimensioning of the external heater current on one side and the heater voltage on the other will normally include to increase cathode wire size and/or to decrease total filament length to permit coil heat to be supplied at higher current and lower voltage.
A preferred embodiment of the method according to the invention is characterized in that for increasing the heater current I to n.multidot.I and decreasin
REFERENCES:
patent: 4734616 (1988-03-01), Northrop
Eich Armin
Kohler Sigurd
Loy Helmut M.
Bessone Carlo S.
GTE Products Corporation
Pascal Robert J.
Ratliff Reginald A.
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