COLD CATHODE DISCHARGE TUBE

Abstract: A cold cathode discharge tube 1 operating at a high frequency includes a capacitor 5 mounted on at least one electrode terminal 3 of the tube 1. The capacitor 5 including a feed­ through capacitor serving as a ballast capacitor is electri­ cally connected in series to the tube 1. The distributed capacitance of power source leads can be neglected in operation because of the presence of the capacitor 5 mounted on the electrode terminal 3 of the tube 1.

Description: BACKGROUND OF THE INVENTION (Field of the Invention) The present invention relates to a cold cathode discharge tube, and more particularly to a cold cathode discharge tube capable of operating at a high frequency. (Description of the Related Art) EP-A-0 162 138 discloses an adapter for a gas discharge tube for connecting the tube to a main voltage socket. To enable a compensation, a wound capacitor can be arranged at the inner or outside circumferential wall of the adapter. A conventional cold cathode discharge tube has a structure as shown in Fig.2. The cold cathode tube 20 has electrodes 20a whose terminals 20b extend to the outside of a glass envelope 20c. The cold cathode tube 20 starts operating when a high frequency power output from an inverter 21 is applied across the terminals 20b via leads 22. Distributed capacitance of the leads 22 can not be neglected because the high frequency power output is used. Therefore, a ballast capacitor 21a has been added heretofore to the inverter 21 for compensating for the distributed capacitance. If the output frequency of the inverter 21 is high, the inverter and its peripheral system can be made small in size. Thus, it is preferable to use a frequency as high as possible. However, with the above conventional cold cathode discharge tube, the higher the frequency becomes, the greater the influence of the distributed capacitance of the leads 22 becomes so that the ballast capacitor 21a becomes unable to compensate for the phase delay, thus resulting in insufficient starting voltage and failure of operating of the tube. As above, the conventional cold cathode discharge tube has a limit of allowable frequency, and of minituarization of the inverter and its peripheral system. Objects of the Invention It is an object of the present invention to solve the above prior art problems and provide a cold cathode discharge tube which can operate at a frequency higher than conventional for which reason the inverter and its peripheral system can be made small in size. Summary of the Invention According to the present invention, a cold cathode discharge tube in accordance with claim 1 which operates at a high frequency comprises a capacitor mounted on at least one electrode terminal of the tube, the capacitor being electrically connected in series to the tube. Therefore, the distributed capacitance of electrode leads can be compensated by the serially connected capacitor. Brief Description of the Drawings The description refers to the accompanying drawings in which: Fig.1 is a cross section showing the main part of an embodiment of a cold cathode discharge tube according to the present invention; and Fig.2 shows a conventional cold cathode discharge tube connected to a high frequency power source. Detailed Description of the Invention A preferred embodiment of the present invention will now be described with reference to Fig.1. A cold cathode discharge tube generally designated by reference numeral 1 is mounted with a capacitor at an electrode terminal 3 connected to an electrode 2 of the tube 1 and extended outside of a glass envelope 4. The capacitor is connected in series to the electrode 2. As particularly shown in Fig.1, the presently preferred embodiment uses as such a capacitor a feedthrough capacitor 5 which is widely used as a noise eliminator. A cap 6 in the form of a tube with a conical bottom portion is mounted on one end portion of the glass tube 4. The cap 6 is made of a conductive material such as a metal and attached to the wall of the glass envelope 4 by suitable means such as adhesive agent. It surrounds the glass envelope 4 with its cylindrical part 6a on a part of its length. The feedthrough capacitor 5 is mounted at a hole formed in the conical bottom portion of the cap 6, and is comprised of a feedthrough electrode 5a connected to the electrode terminal 3, a peripheral electrode 5b connected to the cap 6, and a dielectric member 5c through which the feedthrough electrode 5a passes. With the cold cathode discharge tube 1 constructed as above, a high frequency power from an inverter as described with regard to Fig.2 is coupled to the cap 6 and fed to the electrode 2 via the dielectric member 5c and the electrode lead 3. The feedthrough capacitor 5 connected in series to the electrode 2 serves as a ballast capacitor. More particularly, since the feedthrough capacitor 5 serving as a ballast capacitor is connected to the electrode 2 extremely near to the latter, the distributed capacitance can be compensated to the extent that any practical problem in operation does not occur irrespective of the frequency of the power source, thus solving the problem of phase delay. It becomes possible therefore to apply a sufficiently high starting voltage to the electrode 2. In Fig.1, although one electrode of the cold cathode discharge tube 1 has been provided with the feedthrough capacitor 5, the other electrode may be used as conventional or with the embodiment feedthrough capacitor structure. If the other electrode with the feedthrough capacitor does not require a ballast capacitor, then the lead from the high frequency power source is directly connected to the feedthrough electrode 5a of the feedthrough capacitor 5 instead of the cap 6, to thus disable the ballast capacitor function. As described so far, according to the present invention, a capacitor serving as a ballast capacitor is connected in series to the electrode of the cold cathode discharge tube extremely near to the latter. Therefore, the influence of the distributed capacitance of power source leads and the phase delay can be eliminated so that the starting voltage lowering can be neglected. Accordingly, a frequency higher than conventional can be used for the power source, to thereby allow compact inverter and its peripheral system. While the invention has been disclosed in connection with a preferred embodiment thereof, it will be recognized by those skilled in the art that various modifications of the invention are possible within the scope of the following claims.

Claims: A cold cathode discharge tube (1) having an envelope (4) in which electrodes (2) are provided and extended through said envelope (4) to its outer side in the form of an electrode terminal (3), comprising at least one electrode terminal (3) on which a capacitor (5) is mounted near to said envelope (4) which capacitor (5) is electrically series connected with the discharge tube (1) via that electrode terminal (3) on which it is mounted. The discharge tube (1) of claim 1, characterized in that said capacitor (5) is a feedthrough capacitor. The discharge tube (1) of claim 2, characterized in that a cap (6) of conductive material in the form of a tube with a conical bottom portion is mounted on the envelope (4) and that the capacitor (5) is mounted at a hole formed in said bottom portion. The discharge tube (1) of claim 3, characterized in that a feedthrough electrode (5a) is connected to the electrode terminal (3) and a peripheral electrode (5b) is connected to said cap (6), the feedthrough electrode (5a) and the peripheral electrode (5b) forming said feedthrough capacitor (5). The discharge tube (1) of claim 4, characterized in that a dielectric member (5c) is provided between said feedthrough electrode (5a) and said peripheral electrode (5b). The discharge tube (1) of any one of the claims 3 to 5, characterized in that said cap (6) has a cylindrical portion surrounding said envelope (4) on a part of its length.

Inventors: ICHIKAWA, KEIICHI, NISHIYAMA, KAZUO, SUGAI, TADAO
Applicants: STANLEY ELECTRIC CO., LTD.
Application year: 1988
Main classification: H01J7
IPC codes: H01J61, H05B41, H01J17