Classifications

• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
  • H01J61/00—Gas-discharge or vapour-discharge lamps
  • H01J61/02—Details
  • H01J61/04—Electrodes; Screens; Shields

Definitions

• the present invention refers to a cathode unit for fluorescent tubes with a cathode which is permanently mounted in relation to the tube wall and which is surrounded by a cathode shield which consists of electrically conductive material and which is not connected electrically to the cathode.
• the service life for a fluorescent tube counted in hours of burning time is mainly determined by the service life of the tube cathodes.
• the cathodes When the cathodes have lost a certain portion of their emission material consisting of alkaline earth oxides, their electron emitting capacity has dropped to such an extent that the tube will either not start or will enter a flickering stage which rapidly pulverizes the remaining emission material.
• the barium tungstate thus formed remains as an intermediate layer between the tungsten and the actual emission substance throughout the service life of the cathodes while the barium is continuously diffused as vapour through the substance.
• the barium tungstate layer entails a damping of the reaction according to the above formula, i.e. a reduced formation of barium. As a result. of this, all of the barium will not have been vaporized until after about 30 000 hours of continuous burning of a normal fluorescent tube. The stresses on the tube cathodes during the start process are, however so large that the service life is reduced by a factor of 2-3 during the normal use of a fluorescent tube, i.e. with a mean connection period of 2-3 hours a time.
• Loss of the cathode material which serves as emission substances and the concomitant reduction in the service life of the fluorescent tube are caused, in principle, by three different processes, namely 1) removal of emission material due to ion bombardment, particularly in conjunction with insufficient cathode temperatures; 2) vaporization of emission material; and 3) chemical reactions between the emission material and gaseous impurities in the tube.
• the cathode is surrounded in a normal fluorescent tube construction by a rare gas atmosphere with a pressure of about 2.5 10 2 Pa.
• the vaporisation of the emission substance is comparatively constant during continuous operation but takes place more rapidly after each start and in the minutes following a start due to the increased cathode temperature.
• a cathode for a long-life tube must be designed so that vaporized ions and molecules are reflected bapk to the cathode surface to a considerable extent and so that the cathode temperature remains moderate during the actual start period.
• the purpose of the presen invention is to solve the problems outlined above and thus to achieve a cathode design which is inteded for use in a fluorescent tube and which markedly increases the service life of the tube.
• the cathode shield consisting of a box-shaped casing, the bottom of which has an opening permitting the cathode to be inserted into the interior of the box, and through sealing the end of the box with a disc provided with a centrally located hole and made of electrically insulating material.
• a cathode unit of this type means that ions and molecules which have been released from a cathode surface through ion bombardment and ions and molecules which have been vaporized from the cathode surface are reflected back to the cathode surface to a far greater extent.
• the cathode shield should preferably consist of iron or nickel.
• the disc which must consist of a material which is not pulverized during the ion bombardment, can consist to advantage of mica.
• the hole in the disc must have as small diameter as possible so as to reduce the blackening of the inside of the tube wall to a minimum. Too small a hole diameter will, however, mean that the starting voltage of the fluorescent tube will rise in undesirable manner. Consequently, the hole in the disc should have a diameter which is as small as possible while bearing in mind the fact that the starting voltage of the tube may not exceed a predetermined value.
• the most suitable hole diameter for a normal fluorescent tube with a tube diameter of 38 mm has proved to be 10-12 mm.
• the bottom of the cathode shield should preferably have an area which is at least equal in size to the area of the hole in the disc.
• Fig. 1 shows one end of a fluorescent tube provided with a cathode unit designed and constructed in accordance with the invention.
• Figs. 2a and 2b show - in a vertical cross section and in a view from beneath respectively - a cathode shield used for the cathode unit.
• Fig. 3 shows, in plan, a mica disc intended to cover the open end of the cathode shield illustrated in Fig. 2a and Fig. 2b.
• Fig. 4 is a chart which illustrates the dependency of the starting voltage and the degree of blackening on the hole diameter of the mica disc.
• Fig. 1 shows, in section, one end of a fluorescent tube designed and constructed in accordance with the invention.
• the glass wall (l) of the tube is sealed by a foot (2) at one end in the conventional manner.
• This foot serves simultaneously as a base for the cathode supports (4) which support the tube cathode (3).
• These cathode supports which are electrically conductive, are connected by means of supply wires (5) fused into the foot (2) through which current can be made to pass through the cathode (3) and heat up the cathode.
• the cathode (3) is surrounded by a cathode shield (6) which should preferably be of iron or nickel.
• the shield (6) is supported by a brace (7) fused into the foot (2) and is electrically insulated from the cathode (3).
• the cathode shield (6) is shaped like a box, in the bottom of which an oblong opening (8) has been made for inserting the cathode (3) and parts of the cathode supports (4).
• the open end of the cathode shield (0) is sealed with the aid of a mica disc (9), the thickness of which should preferably amount to 0.10 - 1.15 mm.
• tbe mica disc (9) is provided with a centrally located hole (10), preferably circular in shape.
• the hole (10) has a diameter of 10-12 mm for a normal fluorescent tube with a tube diameter of 38 mm.
• a smaller diameter than this will, admittedly, reduce the blackening of the inside of the tube wall but will, simultaneously, increase the starting voltage to unacceptable values, as is illustrated in fig. 4, which shows the starting voltage U in volts as well as the relative degree of blackening S as a function of the dia meter D 10 in mm of the hole (1).
• a larger hole diameter will reduce the starting voltage no more than insignificantly but will increase the blackening of the tube wall considerably.
• the disc (9) be of mica or some other electrically non-conductive material which does not emit a gas since the ion bombardment would, if the disc were made, for example, of iron, give rise to further pulverized material and thus increase the blackening of the tube wall.
• the design described above offers a further advantage, namely during the half periods when the spiral (3) acts as an anode. Since the discharge must pass through the mica disc (9) provided with a hole, a marked increase will be obtained in the electron density adjacent to the spiral (3) which functions as an anode. The anode drop will thus be reduced. This will entail a reduction in the cathode temperature and thus reduce the rate of vaporization.
• the tube be evacuated by means of a pump process in which vacuum pumping is combined with "internal pumping" achieved by permitting mercury drops to strike the hot tube.
• a drop of this type is illustrated schematically at (11) in fig. 1. When the drop strikes the heated fluorescent tube (wall (1) and or foot (2)) it is vaporized explosively and the mercury vapour thus formed will rapidly flow out.
• the arrows (12 and 13) indicate in a schematic manner the most important flow paths taken by the vapour.
• the mercury vapour which follows the path marked by the arrow (13) must not be obstructed by the construction formed from the cathode shield ( 6 ) and the mica disc (9) if the carbon dioxide which exists at the emission layer and which was formed through conversion from carbonates to oxides is to be removed efficiently and if the internal pumping is to be effective.
• the hole diameter in the mica disc (9) shoxild exceed 10 mm (for a fluorescent tube with a tube diameter of 38 mm) and the bottom opening (8) in the cathode shield (6) must have an area which is at least as large as the hole area of the mica disc but preferably larger.
• the cathode design described above makes it possible, while retaining a normal burning time of 3 hours per connection, to achieve al service life which is 3-4 times longer than that of conventional fluorescent tubes.

Landscapes

• Discharge Lamp (AREA)
• Electrodes For Cathode-Ray Tubes (AREA)
• Vessels And Coating Films For Discharge Lamps (AREA)
• Cathode-Ray Tubes And Fluorescent Screens For Display (AREA)
• Physical Vapour Deposition (AREA)
• Nitrogen Condensed Heterocyclic Rings (AREA)
• Discharge Lamps And Accessories Thereof (AREA)

Priority Applications (4)

Applications Claiming Priority (2)

Publications (1)

Family

ID=20339270

Family Applications (1)

Country Status (25)

Cited By (8)

Families Citing this family (4)

Citations (10)

Family Cites Families (3)

• 1979
  • 1979-11-07 SE SE7909213A patent/SE435332B/sv not_active IP Right Cessation
• 1980
  • 1980-11-04 PT PT72017A patent/PT72017B/pt unknown
  • 1980-11-04 DE DE19808029380U patent/DE8029380U1/de not_active Expired
  • 1980-11-04 IT IT25764/80A patent/IT1134172B/it active
  • 1980-11-04 DE DE3041548A patent/DE3041548C2/de not_active Expired
  • 1980-11-05 BE BE0/202699A patent/BE886030A/fr not_active IP Right Cessation
  • 1980-11-05 FR FR8023573A patent/FR2473785A1/fr active Granted
  • 1980-11-06 JP JP55502594A patent/JPH0250582B2/ja not_active Expired - Lifetime
  • 1980-11-06 GB GB8120702A patent/GB2077033B/en not_active Expired
  • 1980-11-06 YU YU2839/80A patent/YU39696B/xx unknown
  • 1980-11-06 CA CA000364156A patent/CA1150340A/en not_active Expired
  • 1980-11-06 MX MX184636A patent/MX147322A/es unknown
  • 1980-11-06 ES ES1980263202U patent/ES263202Y/es not_active Expired
  • 1980-11-06 AU AU65704/80A patent/AU543221B2/en not_active Expired
  • 1980-11-06 CH CH8265/80A patent/CH649653A5/de not_active IP Right Cessation
  • 1980-11-06 WO PCT/SE1980/000279 patent/WO1981001344A1/en active IP Right Grant
  • 1980-11-06 HU HU801205A patent/HU181318B/hu unknown
  • 1980-11-06 BR BR8008906A patent/BR8008906A/pt not_active IP Right Cessation
  • 1980-11-07 AT AT0547480A patent/AT377385B/de active
  • 1980-11-07 CS CS807548A patent/CS250206B2/cs unknown
  • 1980-11-07 PL PL1980227738A patent/PL132221B1/pl unknown
• 1981
  • 1981-07-03 DK DK296081A patent/DK158177C/da active
  • 1981-07-06 FI FI812128A patent/FI68928C/fi not_active IP Right Cessation
  • 1981-07-06 SU SU813315649A patent/SU1218936A3/ru active
  • 1981-07-06 NO NO812294A patent/NO153946C/no unknown
  • 1981-07-07 RO RO104806A patent/RO81624B/ro unknown

Patent Citations (11)

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