EP0184217B1 - Low pressure arc discharge tube having increased voltage - Google Patents

Description

BACKGROUND
• This invention relates to low pressure arc discharge tubes and more particularly to such tubes having an increased voltage drop thereacross.
• It is often desirable to have a higher voltage drop across an arc discharge tube. For example, in ballastless discharge lamps it is desirable to raise the voltage across the lamp to equal the line voltage (110 volts in the U.S.; 220 volts in Europe) in order to minimize other circuit losses.
• In low power. i.e. 40w, metal halide discharge lamps it is desirable to operate at about 50 volts across the lamps to maintain stability (approx. 50-60 volts across the ballast). This higher voltage can be obtained by raising the mercury pressure. However, this increases the probability of explosions. Therefore, it is desirable to raise the voltage without increasing the pressure.
• It is known in the art that the voltage across an arc discharge tube can be increased by varying the operating parameters of the discharge tube, such as the pressure; the current: the cathode fall; etc. However, altering these parameters effects the performance or some other aspect of the discharge tube. In the DE-C-912 243 a low pressure arc discharge Lamp is disclosed with partitions including aligned coaxial openings that fill the cross-section of the arc-tube resulting in a decrease in length of the arc tube and a better illumination of the fluorescent material disposed on the partitions and the envelope. However, the openings are rather big and the partitions seem to be thicker than the electron energy relaxation distance.
BRIEF SUMMARY OF THE DISCLOSURE
• It is therefore, an object of this invention to obviate the disadvantages of the prior art.
• It is another object of the invention to provide an arc discharge tube having an increased voltage drop thereacross.
• These objects are accomplished by the provision of a low pressure arc discharge tube having an envelope of elongate, substantially cylindrical shape and having an electrode located at each end of the envelope. The envelope encloses an inert starting gas and a quantity of mercury. Included within the envelope are structural means for raising the voltage across the arc tube.
• The structural means include at least one partition extending across the envelope and has at least one aperture therein. This partition has a thickness which is less than the electron energy relaxation distance.
• Incorporation of the last-mentioned structural means allows the arc tube to have increased voltage thereacross without changing any of the operational parameters such as the pressure, current, cathode fall, etc.
BRIEF DESCRIPTION OF THE DRAWINGS
• FIG. 1 is an elevational view, partly broken away, of an embodiment of the invention.
• FIG. 2 is a partial cross-sectional view of FIG. 1.
• FIG. 3 is a cross-sectional view as taken along the line 3-3 in FIG. 1.
• FIG. 4 is an elevational view of a prior-art type arc discharge tube used as a control or reference in testing embodiments of the invention.
• FIG. 5 is an elevational view of another embodiment of the invention; and
• FIG. 6 is an elevational view of yet another embodiment of the invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
• For a better understanding of the present invention, together with other and further objects, advantages, and capabilities thereof, reference is made to the following disclosure and appended claims taken in conjunction with the above-described drawings.
• Referring now to the drawings with greater particularity, FIG. 1 shows an arc discharge tube 10 according to a preferred embodiment of the invention. The arc discharge tube 10 includes an envelope 12 of substantially cylindrical shape which is generally made of light-transmitting soda-lime, lead, quartz or other suitable material. An electrode 14 is located within each of the two axially opposed and portions 12a of the envelope. The envelope 12 encloses in ionizable medium including a quantity of mercury and an inert starting gas, e.g. neon at a low pressure in the range of about 0.67 to 5.3 mbar. Structural means 18 of quartz, glass, ceramic or other suitable electrically insulating material for raising the voltage across the arc discharge tube 10 are incorporated within envelope 12. As shown in FIGS. 1-3 structural means 18 comprise a plurality of axially spaced apart partitions 20 each having at least one aperture 22 therein. Each of the partitions extends across the envelope 12 substantially normal to the longitudinal axis 23 and has a thickness T.
• For best results T has a value less than the electron energy relaxation distance d_r of the discharge tube. The electron energy relaxation distance is defined by the equation:
  
  ${\text{d}}_{\text{r}}{\text{⁻¹=[(3P}}_{\text{Hg}}{\text{Q}}_{\text{in}}{}^{\text{Hg}}{\text{+3P}}_{\text{R}}{\text{Q}}_{\text{in}}{}^{\text{R}}{\text{)(P}}_{\text{Hg}}{\text{Q}}^{\text{Hg}}{\text{+P}}_{\text{R}}{\text{Q}}^{\text{R}}{\text{)]}}^{\text{1/2}}$

  

  
  
  where
  P_Hg is the mercury number density in the vapor
  P_R is the rare gas number density
  Q_in ^Hg is the total inelastic scattering cross-section for the electrons by Hg.
  Q_in ^R is the total inelastic scattering cross-section for the electrons by gas
  Q^Hg is the total scattering cross-section for electrons by Hg.
  Q^R is the total scattering cross-section for electrons by gas.
• Each of the partitions 20 contains at least one aperture 22 which constricts the arc within the tube and causes an increase in the voltage across the arc discharge tube. Aperture 22 having a diameter C can be located at the center of the partition 20 or eccentrically located in the partition as in FIGS. 1-3. Locating the aperture 22 remote from the center of the partition, and having the apertures 22 of adjacent partitions 20 positioned in non-alignment provides the added advantage of increasing the effective arc length of the tube. The maximum effective arc length is achieved if the apertures 22 are also located alternatingly about the longitudinal axis of the arc tube and if the apertures intersect a common plane passing through the longitudinal axis.
• The partitions 20 may be sealed hermetically to the interior surface of envelope 12. However, an hermetic seal is unnecessary if the total area between the perimeter of each partition and the interior surface of the envelope is less than the area of aperture 22. The forming of the end portions 12a and the sealing of the electrode 14 leadwires can be performed after the partitions 20 are installed.
• The voltage developed by each partition 20 is inversely proportional to the size of the aperture 22. An increase in voltage can be achieved when the ratio of the internal diameter R of the envelope 12 to the aperture 22 diameter C is as small as approximately 1.1:1. For maximum voltage increase, the aperture 22 diameter C should be made small enough to achieve a ratio B:C of approximately 50:1. Voltage increases of from about 0.5 volts to about 20 volts per partition can be achieved depending on the ratio B:C.
• The invention will now be described more fully with reference to examples and to the drawings in FIGS. 1-6.
• Four arc discharge tubes were constructed from quartz all having an envelope 12 wall thickness of about 1 mm, a length X equal to about 90 mm and an outside diameter of about 25 mm. Each arc tube contained a fill gas of 100 percent neon at a pressure of about 2.7 mbar.
• The arc tube 10 of FIG. 1 was made with six apertured quartz partitions 20, each having a thickness T equal to about 1 mm. The partitions 20 were equally spaced apart from each other by about 10 mm. The distance from either electrode 14 to an adjacent partition 20 was also about 10 mm.
• Each of the six partitions 20 had an aperture 22 diameter C of about 0.5 mm. This results in a ratio B:C of 50:1. The apertures 22 were located alternating about the longitudinal axis of the arc tube 10. The distance D from the longitudinal axis to the midpoint of each aperture was about 2 mm. The apertures also intersect a common plane passing through the longitudinal axis of the arc tube (e.g., the plane of the drawing of FIG. 1).
• FIG. 1 shows a reference or control tube 10' constructed without the apertured partitions.
• FIG. 5 shows an arc tube 10'' according to another embodiment of the invention containing two apertured quartz partitions 24, each having a thickness T equal to about 1 mm. The two partitions were spaced apart from each other by about 25 mm. The distance from either electrode 14 to an adjacent partition 24 was about 20 mm. The two partitions 24 had an aperture 26 having a diameter C of about 2 mm and located coaxially at about the midpoint of the partition.
• FIG. 6 shows an arc tube 10''' made in accordance with yet another invention and having a single quartz partition 24 located at approximately the center of the tube. The partition 24 had a thickness T equal of about 1 mm.
• This partition 24, which was similar to those used in FIG. 5, had an aperture 26 with a diameter C of about 2 mm and was located coaxially at about the midpoint of the partition 24. The partition 24 of FIG. 6 was positioned at about the center of the arc tube envelope 12.
• Table 1 shows the electrical parameters obtained from the four above mentioned arc tubes. Arc tubes 10, 10'' and 10''' were made according to the invention while arc tube 10' denotes the reference control tube.

  
• A comparison of arc tube 10 with the reference arc tube 10' shows that at an arc current of 1.0 amp, the voltage across the arc tube has increased by about 92 percent. This is a voltage increase per partition of 6 volts.
• A comparison of arc tubes 10'' and 10''' shows an increase of 11 volts per partition at an arc current of 1.0 amp and an increase of 8 volts per partition at an arc current of 0.5 amp.
• It is possible to obtain larger than 11 volts per partition as shown in Table I by further reduction in the fill gas pressure. As the pressure is reduced, the discharge at the area around the aperture in the partition becomes reddish which is characteristic of neon excitation. This was observed when about 16 volts was generated around the aperture. For aperture diameters less than about 0.1 mm, the voltage generated can be even greater.
• The present invention is not limited to use in a glow discharge of mercury-inert gas without a phosphor layer. For example, use of partitions according to the invention in a 14'' T12 European fluorescent lamp could increase the voltage from 39 volts to 100 volts. This would cause the voltage across the ballast to drop from 180 volts to 120 volts and thereby reduce the ballast losses by 35%.
• While there have been shown what are at present considered to be preferred embodiments of the invention, it will be apparent to those skilled in the art that various changes and modifications can be made herein without departing from the scope of the invention as defined by the appended claims.

Claims (12)

1. A low pressure arc discharge tube (10) comprising: an envelope (12) having elongated substantially cylindrical shape said envelope (12) having two axially opposed end portions (12a); first and second electrodes (14) each located within a respective one of said axially opposed end portions (12a); an ionisable medium enclosed within said envelope (12) including an inert starting gas for producing a plasma discharge when a predetermined voltage is applied across said electrodes (14); structural means (18) located within said envelope (12) raising the voltage across said arc discharge tube (10); said structural means (18) including at least one partition (20) extending across said envelope (12) and having at least one aperture (22) therein, characterized in that said arc discharge tube includes a quantity of mercury, and in that said partition (20) has a thickness which is less than the electron energy relaxation distance of said discharge tube (10).
2. The low pressure arc discharge tube (10) of Claim 1 wherein said partition (20) is substantially normal to the longitudinal axis of said envelope (12).
3. The low pressure arc discharge tube (10) of Claim 1 wherein said structural means (18) comprises a plurality of said apertured partitions (20) axially spaced apart within said envelope (12).
4. The low pressure arc discharge tube (10) of Claim 3 wherein said envelope (12) has an internal diameter (B) and each of said partitions (18) has an aperture diameter (C), and the ratio of said internal diameter (B) to said aperture diameter (C) is within a range to provide a voltage increase of from about 0.5 volts to about 20 volts per partition.
5. The low pressure arc discharge tube (10) of Claim 3 wherein said ratio of said internal diameter (B) to said aperture diameter (C) is within the range of from about 1.1:1 to 50:1.
6. The low pressure arc discharge tube (10) of Claim 3 wherein each of said apertures (22) is located at the midpoint of each of said partitions (18).
7. The low pressure arc discharge tube (10) of Claims 3 wherein each of said partitions (18) is formed from an electrically insulating material.
8. The low pressure arc discharge tube (10) of Claim 7 wherein said electrically insulating material is quartz.
9. The low pressure arc discharge tube (10) of Claim 3 wherein each of said apertures (22) is eccentrically located in its respective partition (20).
10. The low pressure arc discharge tube (10) of Claim 9 wherein the said apertures (22) of adjacent partitions (20) are positioned in non-alignment.
11. The low pressure arc discharge tube (10) of Claim 10 wherein each of said apertures (22) intersect a common plane passing through the longitudinal axis of said arc tube (10).
12. The low pressure arc discharge tube (10) of Claim 11 wherein said apertures (22) are located alternatingly about the longitudinal axis of said arc tube (10).

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