Fluorescent display tube

A fluorescent display tube having a structure in which only filament-shape cathodes corresponding to required display regions are selectively supplied with electric power and in which elongated cathode attaching plates are disposed apart from one another on an end of an anode substrate, a first support member is secured to the first cathode attaching plate disposed inwards when viewed in the lengthwise direction of the cathodes, while a second support member is secured to the second cathode attaching plate disposed outwardly. Each support member has a base portion secured to the cathode attaching plate, a securing portion for securing the cathode and a connection portion for connecting the base portion and the securing portion to each other. The plural cathodes are stretched to run parallel to one another. A connection portion of the inner support member is longer than a connection portion of an outer support member. The two support members secure the ends of the cathodes at the same positions. Operation voltage is supplied to each of the first or the second cathode attaching plate so that the cathodes divided into two groups are individually operated. Positions at which ends of the cathodes are aligned so that an end cool effect is made to be uniform.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a fluorescent display tube having a 
plurality of filament-type cathodes stretched by stretching members, and 
more particularly to a fluorescent display tube having a structure such 
that stretching members are divided at ends of cathodes to divide the 
cathodes into plural pairs so as to selectively be operated and the shapes 
of the divided pairs are made to be different from each other so as to 
make constant the state of end cool at the end of each cathode. 
2. Description of the Related Art 
A general fluorescent display tube has an envelope, the inside portion of 
which is maintained at high vacuum and which accommodates various 
electrodes and the like. Specifically, an anode is formed on the inner 
surface of a substrate forming a portion of the envelope. In general, the 
anode consists of an anode conductor formed on the substrate and a 
fluorescent layer with which the anode conductor is coated. A control 
electrode is formed above the anode. Filament-shape cathodes are formed 
above the control electrode. Electrons emitted from the cathodes are 
accelerated and controlled by the control electrode so that electrons 
collide with the fluorescent layer of the anode to cause the fluorescent 
layer to emit light. 
One or more filament-shape cathodes are employed to correspond to the 
display area of the fluorescent display tube. In a case where plural 
cathodes are employed, the cathodes are usually disposed to run parallel 
to one another. Two ends of the filament-shape cathodes are secured by 
pairs of stretching members. A usual stretching member has a support 
member for securing an end of the cathode and an anchoring member having 
elasticity to be capable of securing another end of the cathode and 
supplying tension to the cathode. 
In recent years, the fluorescent display tube has been widely used and 
employed so as to be mounted on a vehicle. The display portion of the 
fluorescent display tube which is mounted on a vehicle includes display 
portions, such as the speedometer and the tachometer, which must always be 
displayed during driving and display portions, such as various alarm 
lamps, which are turned on only when required. 
Hitherto, all cathodes corresponding to all of the display portions are 
supplied with electric power and display signals are supplied to the 
cathodes corresponding to required display portions so as to perform 
required display. 
The foregoing conventional method of operating the fluorescent display tube 
has the structure such that operation electric currents are as well as 
supplied to cathodes corresponding to the portions which are not required 
to be turned on. Therefore, electric power has been wasted. 
SUMMARY OF THE INVENTION 
A first object of the present invention is to provide a fluorescent display 
tube having a plurality of filament-shape cathodes and structured such 
that electric power can be supplied to only cathodes corresponding to 
required display regions so as to save energy. 
A second object of the present invention is to provide a fluorescent 
display tube having a plurality of filament-shape cathodes disposed to run 
parallel to one another and divided into plural pairs so as to be 
selectively supplied with voltage and arranged such that positions at 
which ends of the cathodes are held by stretching members are aligned into 
a direction intersecting the lengthwise direction of the cathodes so as to 
make uniform a state where the ends of the cathodes are cooled by the 
stretching members. 
According to a first aspect of the present invention, there is provided a 
fluorescent display tube including: an envelope, the inside portion of 
which is maintained at high vacuum; a plurality of cathodes in the form of 
filaments stretched in parallel to one another in the envelope; and plural 
pairs of stretching members for securing two ends of each of the cathodes 
in the envelope, wherein either side of the stretching members for 
securing the ends of the cathodes are divided into a plurality of groups 
electrically separated from one another and operation voltage can be 
applied to each of the groups. 
According to a second aspect of the present invention, there is provided a 
fluorescent display tube including: an envelope, the inside portion of 
which is maintained at high vacuum; a plurality of cathodes in the form of 
filaments stretched in parallel to one another in the envelope; a 
plurality of cathode attaching plates disposed apart from one another in 
the lengthwise direction of the cathodes at an end of the envelope and 
ejected to the outside portion of the envelope so as to be independently 
applied with voltage for operating the cathodes; and a plurality of 
stretching members each of which is attached to each of the cathode 
attaching plates and which hold ends of the cathodes in such a manner that 
positions at which the cathodes are held are arranged to align in a 
direction intersecting the lengthwise direction of the cathodes. 
According to a third aspect of the present invention, there is provided a 
fluorescent display tube according to the second aspect of the present 
invention, wherein the stretching member has a base portion secured to the 
cathode attaching plate, a securing portion for holding an end of the 
cathode and a connection portion for connecting the base portion and the 
securing portion to each other, wherein at least the base portions of the 
plural stretching members or the connection portions of the same have a 
different shape in order to align the securing portions of the plural 
stretching members in a direction intersecting the lengthwise direction of 
the cathodes. 
According to a fourth aspect of the present invention, there is provided a 
fluorescent display tube according to the third aspect of the present 
invention, wherein the connection portion is not in contact with the 
cathode. 
Other objects, features and advantages of the invention will be evident 
from the following detailed description of the preferred embodiments 
described in conjunction with the attached drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
A first embodiment of the present invention will now be described with 
reference to FIGS. 1 to 3. A fluorescent display tube according to this 
embodiment is structured so as to be mounted on a vehicle. The fluorescent 
display tube according to this embodiment has a box-shape envelope formed 
by airtightly combining substrates made of insulating material. Air in the 
inside portion of the envelope is evacuated and thus the same is made to 
be high vacuum state in the manufacturing process. 
Referring to FIG. 1, reference numeral 1 represents a rectangular anode 
substrate 1 forming a portion of the envelope. Anode conductors and wire 
conductors (not shown), connected to the anode conductors, are formed on 
the anode substrate 1. Moreover, a fluorescent layer is applied to the 
upper surface of the anode conductor so that an anode serving as a 
light-emission display portion is formed. A control electrode (not shown) 
is formed on the anode. 
As shown in FIG. 1, a plurality of filament-shape cathodes 2 are stretched 
to run parallel to one another at positions above the control electrodes 
of the anode substrate 1. Each of the cathodes 2 is stretched in the 
lengthwise direction of the rectangular anode substrate 1. The two ends of 
each of the cathodes 2 are secured to the two lengthwise directional ends 
of the anode substrate 1 by stretching members. The stretching member 
includes a support member 3 for securing an end of the cathode 2 and an 
anchor member for elastically holding the other end of the cathode 2 so as 
to apply a tension to the cathode 2. 
As shown in FIG. 1, two cathode attaching plates 4a and 4b are disposed 
above the end of the rectangular anode substrate 1. Each of the cathode 
attaching plates 4a and 4b is a plate elongated in a direction (that is, 
the lengthwise direction of the anode substrate 1) perpendicular to the 
lengthwise direction of the cathodes 2, the cathode attaching plates 4a 
and 4b being disposed apart from each other at a predetermined interval in 
the lengthwise direction of the cathodes 2. The cathode attaching plates 
4a and 4b are made of 426-alloy having a thermal expansion coefficient 
approximating that of glass. 
Two ends of each of the cathode attaching plates 4a and 4b are connected to 
spacer frames 5 during the manufacturing process. The spacer frame 5 is a 
member formed into a rectangular shape and having electrode members and 
outer leads 6 of the fluorescent display tube attached thereto. The two 
cathode attaching plates 4a and 4b are opposite to each other while 
interposing a bent groove 7 except the two ends of the cathode attaching 
plates 4a and 4b in a state where the cathode attaching plates 4a and 4b 
are connected to the spacer frames 5. A rectangular through hole 8 is 
formed at an end of the first cathode-attaching-plate 4a connected to the 
spacer frames 5. Also a rectangular through hole 9 is formed at another 
end of the second cathode-attaching-plate 4b connected to the spacer 
frames 5 at a position opposite to the through hole 8 with respect to the 
anode substrate 1. By cutting the two ends of the two cathode attaching 
plates 4a and 4b along the cutting lines A--A and B--B shown in FIG. 1, 
the two cathode attaching plates 4a and 4b are left as connection 
terminals 10a and 10b each having an end which is branched into two 
sections, and then extended to the outside of the anode substrate 1. 
A first support member 3a, which is a first stretching member, is secured 
to the first cathode-attaching-plate 4a located at an inner position with 
respect to the lengthwise direction of the cathodes 2. The first support 
member 3a has a base portion 11a which is secured to the first 
cathode-attaching-plate 4a. A substantially L-shape connection portion 13a 
is stood erect from the base portion 11a. A securing portion 12a is formed 
at the leading end of the connection portion 13a. An end of the cathode 2a 
is, by welding or the like, secured to the securing portion 12a. In this 
embodiment, the first support member 3a has the common base portion 11a, 
two connection portions 13a and two securing portions 12a so as to stretch 
two cathodes 2a. 
A second support member 3b, which is a second stretching member, is secured 
to the second cathode-attaching-plate 4b located at an outer position with 
respect to the lengthwise direction of the cathodes 2. The second support 
member 3b has a base portion 11b which is secured to the second 
cathode-attaching-plate 4b. A substantially L-shape connection portion 13b 
is stood erect from the base portion 11b. A securing portion 12b is formed 
at the leading end of the connection portion 13b. An end of the cathode 2b 
is, by welding or the like, secured to the securing portion 12b. In this 
embodiment, the second support member 3b has the common base portion 11b, 
four connection portions 13b and four securing portions 12b so as to 
stretch four cathodes 2a. 
The first support member 3a and the second support member 3b have the same 
structure because each of the support members 3a and 3b has the base 
portions 11a and 11b secured to the corresponding first and second cathode 
attaching plates 4a and 4b, the securing portions 12a and 12b for securing 
the ends of the cathodes 2a and 2b and the connection portions 13a and 13b 
for connecting the base portions 11a and 11b and the securing portions 12a 
and 12b. However, as shown in FIGS. 1 to 3, the first support member 3a 
secured inwards with respect to the lengthwise direction of the cathodes 2 
has the connection portion 13a having a longer size, while the second 
support member 3b secured outwards with respect to the lengthwise 
direction of the cathodes 2 has the connection portions 13b having a 
shorter size. The difference in the length between the connection portions 
13a and 13b of the two support portions 3a and 3b is determined in such a 
manner that the securing portions of the two support members 3a and 3b are 
aligned in a direction perpendicular to the lengthwise direction of the 
cathodes 2. That is, the support members 3a and 3b, mechanically divided 
so as to be individually operated by electric power, secure the ends of 
all of the cathodes 2 at the same positions in the lengthwise direction of 
the cathodes 2. 
If the support members 3a and 3b have the same shape, the positions at 
which the two support members 3a and 3b secure the ends of the cathodes 2 
are made to be different in the lengthwise direction of the cathodes 2. If 
the ends of the cathodes 2 are shifted, effective display areas 
corresponding to the respective cathodes 2 are unintentionally made to be 
different from one another. The reason for this is that a portion of 
generated heat is absorbed by the support member and the emission is 
therefore deteriorated at the ends of the cathodes 2. The foregoing 
phenomenon is called an "end cool effect". In this embodiment, all of the 
ends of the cathodes 2 are aligned so that the ends of the display areas 
are free from irregularities. As a result, the layout of each display 
portion can be determined significantly freely. 
Another end of each of the cathodes 2 is anchored by the anchoring member 
so that appropriate tension is applied to each of the cathodes 2. Each of 
the anchoring members is secured to the common cathode attaching plate. 
The cathode attaching plate is ejected to the outside portion of the 
envelope so as to be connected to a cathode operation circuit. 
The fluorescent display tube according to this embodiment is structured to 
be mounted on a vehicle such that the four cathodes 2b supported by the 
second support member 3b are used to always display images, such as 
display areas for the speedometer and the tachometer. On the other hand, 
the two cathodes 2a supported by the first support member 3a are cathodes 
for display portions which are not always displayed. Operation voltage 
supplied from the cathode operating circuit is always supplied to the 
cathodes 2b supported by the second support member 3b of the second 
cathode-attaching-plate 4b. Usually, electric power is not supplied to the 
cathode 2a supported by the first support member 3a of the first 
cathode-attaching-plate 4a. Only when a necessity of displaying an alarm 
lamp arises, operation voltage is supplied to the first 
cathode-attaching-plate 4a to which the first support member 3a is 
attached. Therefore, the fluorescent display tube according to this 
embodiment enables electric power to selectively be supplied to the 
cathode corresponding to a required display region. Thus, electric power 
can be saved. 
However, since the support members 3a and 3b, as shown in FIG. 3, secure 
the ends of all of the cathodes 2 disposed to run parallel to one another 
at the same position in the lengthwise direction of the cathodes 2, the 
support members 3a and 3b have connection portions 13a and 13b having 
different lengths. The cathodes 2a and 2b are in contact with the 
horizontal portions of the connection portions 13a and 13b having the 
different lengths. Thus, it can be considered that heat of the cathodes 2 
is also transmitted to the support members 3a and 3b in the 
above-mentioned portions. Since the length of contact with the cathode 2 
is different, the quantity of heat deprived in the foregoing portion is 
different between the first support member 3a and the second support 
member 3b. Therefore, a consideration can be performed that degree of 
deterioration in the emission is different for each of the support members 
3a and 3b. 
Accordingly, as modifications of the embodiment shown in FIGS. 1 to 3, 
second and third embodiments will now be described with reference to FIGS. 
4 and 5. These embodiments are modifications of the first support member 
3a according to the first embodiment. A first support member 20 according 
to the second embodiment shown in FIG. 4 has a base portion 21, a securing 
portion 22 and a connection portion 23 arranged to connect the base 
portion 21 and the securing portion 22 to each other and bent in the 
stepped-shape. In a second support member (not shown), by making the 
length for which the connection portion is in contact with the cathodes 2 
and the length of contact for which the horizontal portion of the securing 
portion of the connection portion 23 of the first support member 20 is in 
contact with the cathode 2 to be the same, the difference in the end cool 
effect occurring attributable to the difference in the shape of the 
support member does not arise. 
A first support member 30 according to the third embodiment shown in FIG. 5 
has a base portion 31, a securing portion 32 and a connection portion 33 
for connecting the base portion 31 and the securing portion 32 to each 
other. In a second support member (not shown), by making the length for 
which the connection portion is in contact with the cathodes 2 and the 
length of contact for which the horizontal portion of the securing portion 
of the connection portion 33 of the first support member 30 is in contact 
with the cathode 2 to be the same, the difference in the end cool effect 
occurring attributable to the difference in the shape of the support 
member does not arise. 
The fluorescent display tube, according to the present invention and having 
a plurality of filament-shape cathodes, the two ends of which are secured 
by the stretching members, is structured such that either side of the 
stretching members for securing the ends of the cathodes are divided into 
a plurality of groups electrically separated from one another and 
operation voltage can be applied to each of the groups. Therefore, 
operation voltage can be applied to only required cathodes so that 
electric power is saved. 
Moreover, the stretching member is secured to each of the plural cathode 
attaching plates disposed apart from one another in the lengthwise 
direction of the cathodes, and shape of the stretching members are 
determined in such a manner that the positions at which the ends of the 
cathodes are secured by the stretching members are aligned when viewed in 
the lengthwise direction of the cathodes. Therefore, the cool effect is 
obtained in the same portions of the plural cathodes divided electrically 
and mechanically. As a result, display irregularities in the display areas 
can be prevented. 
Although the invention has been described in its preferred form with a 
certain degree of particularity, it is understood that the present 
disclosure of the preferred form can be changed in the details of 
construction and in the combination and arrangement of parts without 
departing from the spirit and the scope of the invention as hereinafter 
claimed.