The present invention relates to a discharge tube having a positive glow discharge characteristic in which the tube voltage increases as the discharge current increases and a negative arc discharge characteristics in which the tube voltage decreases as the discharge current increases due to an increase in emission of thermoelectrons.
It is known that cold-cathode tubes, hot-cathode tubes and semi-hot cathode tubes are used in discharge-tube applications.
Cold-cathode tubes have positive discharge characteristics and have the advantages of long life, low power consumption, low heat dissipation, and the ability to be easily lit and quenched. The disadvantage of cold-cathode tubes is their low intensity. Hot-cathode tubes, which are known as fluorescent lamps, have negative thermionic discharge characteristics and the advantage of high intensity. Hot-cathode tubes, however, have the disadvantages of short life, high power consumption, high heat dissipation, and cannot be lit and quenched in themselves. Semi-hot-cathode tubes are arranged such that no filament electrode is energized by external circuits and have the disadvantage that a relatively long time is required to reach the desired intensity after energization. Also, semi-hot-cathode tubes are impractical in terms of life and intensity.
In Japanese Patent Application No. 63-172761, the present inventor proposes a discharge tube of the type having a discharge characteristic in which the negative discharge characteristic of a cold-cathode tube is combined with the positive discharge characteristic of a hot-cathode tube. More specifically, the proposed discharge tube comprises a pair of electrode assemblies which are disposed opposite each other in a discharge space. Each of the electrode assemblies includes a cup-shaped electrode for glow discharge and, a filament electrode for arc discharge which is disposed in the cup-shaped electrode. This arrangement is intended to achieve long life with glow discharges and very high brightness with arc discharges. However, it is necessary that such discharge tube be provided with an automatic control circuit for controlling discharge current in order to stably maintain the state of discharge. In this prosed type of discharge tube, several hours after energization the temperature of the cup-shaped electrode for glow discharge increases due to emission of thermoelectrons and a transition from glow discharge to arc discharge occurs. As a result, a phenomenon such as snaking or flickering takes place and the discharge becomes unstable. For this reason, the above electronic control circuit is used to control discharge current. However, the cost of the discharge tube becomes expensive owing to the necessity of such a control circuit and, even with this control circuit, it is still difficult to eliminate perfectly flickers of high frequency.
US-A-2 314 134 discloses an electric discharge device of the luminescent tube type used in luminous signs. It comprises a sealed tube of glass or other suitable transparent material, the inside surface of which is coated with fluorescent powder for light generation. The tube contains an inert gas at a low pressure and also mercury. A respective electrode is provided at each end of the tube.
An illustrated electrode is supported by a current-carrying lead sealed in the glass tube and extending inwardly to a larger diameter portion of nickel or iron. About the end of the larger diameter portion there is wrapped, and welded thereon, one end of a filamentary electrode member composed of a tungsten wire. The tungsten filament is in the form of a coiled coil and is coated with an emittive material comprising a mixture of barium and strontium oxides.
On the outside of the larger diameter portion, a tubular quartz insulating member is secured and has an exterior annular recess in which is secured the lower end of a second electrode member in the form of a frusto-conical shell of nickel which may, if desired, be coated interiorly with emittive material. At two or three points along the inner edge or the shell, a metal, for instance thorium, having comparatively high emission characteristics when cold is deposited. The filament is connected at its inward end to the shell by means of a connector wire.
When a proper voltage is applied to the tube, an arc builds up instantly on the interior of the shell but, owing to the presence of the insulating member, cannot reach the lead except through the filament with the result that the tungsten filament heats up sufficiently to serve as the main electrode during continued operation.
In a second embodiment for operation at relatively low currents, an electrode arrangement includes a tungsten filament, a thin frusto-conical metallic shell interiorly coated with emittive material, and an outer, generally cylindrical, shell of non-conductive material, a plate connecting the lead wire with the interior of the shell to form part of a supplementary electrode.
It is an object of the present invention to provide a discharge tube in which the above-described problems can be overcome.
It is also an object of the present invention to provide a high-intensity long-life discharge tube which has the functions of both arc discharge and glow discharge and in which a transition from glow discharge to arc discharge is not caused by a rise in the temperature of an electrode for glow discharge.
The present invention provides a discharge tube comprising: a tubular member, including a tubular wall having opposite closed ends defining a gas charged space; first and second electrode assemblies respectively disposed at said closed ends in said gas charged space; and means for applying an a.c. voltage to said first and second electrode assemblies; wherein: each electrode assembly comprises a first, cup-shaped, electrode having a closed end, an open end and a central axis, and a second electrode extending along said axis, the second electrode of each electrode assembly of each electrode assembly including a coating of a thermoelectron emitting material, said electrode assemblies producing, in use, an arc discharge therebetween; each cup-shaped electrode has an outer surface directly facing and open to said tubular wall; and each cup-shaped electrode has an inner surface directly facing the associated second electrode; characterized in that: each cup-shaped electrode is a glow discharge electrode; voltage is applied to the cup-shaped electrodes at their closed ends; each cup-shaped electrode is electrically connected in parallel with the associated second electrode; and said electrode assemblies produce, in use, glow discharge between said cup-shaped electrodes and arc discharge between said arc discharge electrodes.
When a voltage from the same high-frequency electric source is applied to these electrodes, a high intensity arc discharge and a glow discharge are stably formed in the discharge space at the same time, whereby a very high intensity of 35,000 nit or more can be obtained by the synergistic effect of the arc discharge and the glow discharge. For instance, if metal, such as nickel, is used for electrodes for glow discharge, stable glow discharge will be obtained up to a current level of about 3 mmA. However, in general, at current levels of 4 mmA or more the discharge characteristic of the electrodes for glow discharge enters an arc-discharge region where glow discharge does not stabilize. In contrast, the above-described arrangement according to the present invention makes it possible to provide stable glow discharge even when the current increases, and an intensity of 10,000 nit can be achieved with glow discharge alone.
Ways of carrying out the invention will now be described with reference to the accompanying drawings, in which:
Fig. 1 shows a discharge tube to be described as a background to the present invention. As illustrated, a rod-like sintered electrode 2 and a filament coil electrode 3 are disposed at each end of a transparent glass tube 1 which has a diameter of approximately 6 mm and a length of approximately 260 mm. At each end, the rod-like sintered electrode 2 and the filament coil electrode 3 are arranged in parallel and close to each other, i.e., in a non-contact state. Each of the rod-like electrodes 2 has a diameter of 2 mm and a length of 6 mm and is prepared by mixing tungsten powder, zirconium, nickel and barium carbonate, forming the mixture with a press, and sintering the formed mixture. Each of the filament coil electrodes 3 has a good electron emission characteristic and is prepared by coating a solution of barium hydroxide over the peripheral surface of a tungsten wire, sintering the tungsten wire to form a coat of barium carbonate, and forming it into a coil. In the glass tube 1, each of these electrodes 2 and 3 is supported by a tungsten wire or rod-like member 7 and connected to one end of a lead wire 8 via the tungsten rod-like member 7. One end of each of the lead wires 8 extends into the glass tube 1 through a glass end wall thereof. The inner surface of the glass tube 1 is coated with a fluorescent film 4 and the interior of the glass tube 1 is charged with argon gas pressurized at a pressure of 4000 Pa (30 torr) and 5 mmg of mercury 6.
A strip or trigger coating 9 is formed on the outer surface of the glass tube 1 to extend along the length thereof. A lead wire 10 is led from the trigger coating 9 and connected to the lead wire 8 which is led from one end of the glass tube 1. The other ends of the respective lead wires 8 are connected to an A.C. power source 11.
Fig. 2 shows a discharge tube (not, however, embodying the invetnion) according to a modification of the discharge tube shown in Fig. 1. In Fig. 2, the same reference numerals are used to denote the same elements as those shown in Fig. 1. This modification differs from the first embodiment in that each filament coil electrode 3a is arranged to surround a corresponding rod-like sintered electrode 2a in a non-contact state.
For example, if a sine wave of oscillation frequency 40 kHz and effective voltage 1,500 V is applied to the lead wires 8 at both ends of the discharge tube, a highly stable discharge can be achieved with a discharge current of 20 mA and an intensity of 35,000 nit. The temperature of the tube wall of the portion of the discharge tube which is adjacent to each electrode assembly is approximately 15 degrees higher than room temperature and the amount of heat generated can be reduced compared to conventional arrangements. Accordingly, it is possible to reduce power consumption. In addition, since no control circuit for stabilizing discharge is needed, the discharge-tube driving circuit can be made compact.
Fig. 3 shows another modification (not, however, embodying the invention) of the discharge tube of Fig. 1 and only the electrode assembly which differs from that shown in Fig. 2 is illustrated. This modification is similar to the modification of Fig. 2 in that a rod-like sintered electrode 2b is surrounded by a filament coil electrode 3b, but the filament coil electrode 3b is densely coiled in cup-like form with each of its ring segments held in close contact with the adjacent ring segment.
Fig. 4 shows an embodiment of the invention constituted by yet another modification of the discharge tube of Fig. 1 and an electrode assembly which differs from that shown in Fig. 3 is illustrated. In the illustrated assembly, a sintered metallic electrode 2c for glow discharge is formed into a cup-like shape, and a filament coil electrode 3c for arc discharge extends straight along the axis of the assembly. The cup-like electrode 2c may be formed into the shape of a hollow cylinder with a bottom.
It is desirable that the discharge tubes have a glass-tube diameter of about 4 mm to about 10 mm.
Instead of the sintered electrode shown in Fig. 4, there can be used an electrode formed in such a way that a nickel or tungsten wire is densely coiled in a shape similar to that of the electrode 3b shown in Fig. 3 while coating a nickel or tungsten powder over the peripheral surface of the coiled wire. Further, then the filament coil electrode 3c is set at the center of the thus-formed electrode.
A sine-wave oscillating voltage was applied across the discharge tube shown in Fig. 2 under the following conditions:
The relationship between voltage (V) and discharge current (mA), shown in Fig. 5, was obtained by gradually raising the voltage (V) from 0 V.
As is apparent from Fig. 5, glow discharge was started between the opposite sintered electrodes at 400 rmsV, and the filament coil electrodes started discharges at approximately 500 rmsV. Even if the voltage was raised to 500 rmsV or more, a positive discharge characteristic was maintained between the sintered electrodes. In other words, it was proved that the glow discharge could be maintained even at a voltage level of 500 rmsV or more. In addition, it was proved that, at a voltage level of 500 rmsV or more, a negative discharge characteristic could be obtained between the filament coil electrodes, whereby arc discharge could be maintained.
As is apparent from the foregoing, since the two kinds of discharge, glow discharge and arc discharge, are realized within a single discharge tube, very high intensity of illumination can be achieved. Also, since the filament coil electrodes are heated by glow discharge, arc discharge can be generated by using a relatively low voltage. In addition, since the two kinds of electrodes are arranged in a non-contact state, the sintered metallic electrodes are not heated by the heat generated in the adjacent filament coil electrodes. Accordingly, since no thermorunway takes place in the sintered metallic electrodes, glow discharge does not proceed with arc discharge and the glow discharge can be kept highly stable between the sintered metallic electrodes.
Each of the filament coil electrodes is coated with an active oxide such as barium, strontium or the like in order to accelerate emission of thermoelectrons. Accordingly, particles may be scattered due to evaporation or peeling caused by ion bombardment or heating and fall on the inner tube wall of the discharge tube, thereby causing the shading phenomenon in which dark shades are formed on the inner tube wall of the discharge tube. However, when the cup-shaped sintered metallic electrode shown in Fig. 4 is employed, scattered particles stick to the inner wall of the cup-shaped sintered metallic electrode and the stuck particles or active oxide can be reused. In addition, since it is possible to prevent the shading phenomenon by suppressing the phenomenon in which scattered particles stick to the inner tube wall of the discharge tube, the lifetime of the discharge tube can be improved. The present inventor conducted a lifetime test with a discharge tube having such electrode assemblies, and the shading phenomenon was not substantially observed even after running of 10,000 hours or thereabouts.
Fig. 6 shows another embodiment of the discharge tube of the present invention. This embodiment is like the discharge tube of Fig. 2 but a top glass plate 1c and a bottom glass plate 1d have ribs 12a and 12b formed on their facing surface, respectively. It is possible to obtain a postcard-size surface light source made from a glass plate with a plate thickness of approximately 4 mm. However, if the size is further increased, the thickness of the glass plate also increases to an impractical extent. By means, however, of the ribs 12a and 12b, the strength of each of the top and bottom glass plates 1c and 1d can be increased to a considerable extent. Accordingly, a surface light source having a light weight and a considerably large size can be obtained.
If the gaps A and B between the tips of the ribs 12a and 12b and the inner surfaces of the opposing glass plates are respectively selected to be approximately 0.5 mm to 0.1 mm, the discharge impedance in each of the gaps A and B becomes high and the discharge space is substantially divided into a plurality of small discharge spaces X, Y and Z. Electrode assemblies E, each of which is similar to that shown in Fig. 4, are respectively arranged at the opposite ends of each of the small discharge spaces X, Y and Z, whereby a discharge plate is obtained which is constructed as if a plurality of discharge tubes were arranged side by side. In this arrangement, arc discharge is reliably produced in each of the small discharge spaces, whereby it is possible to prevent the phenomenon in which arc discharges are concentrated upon a specific one of the opposite electrodes (this phenomenon easily occurs at a temperature of chiefly 5°C or less). Further, since the inner surfaces of the glass plates including the ribs 12a and 12b are coated with the fluorescent film, the surface portions of the glass plates corresponding to the respective ribs are not darkened. In addition, since the area occupied by the fluorescent surface increases owing to the formation of the ribs, the total intensity of the surface light source rises. In Fig. 9, reference numeral 13 denotes an end plate made of glass, and the lead wires 8 extend through the end plates 13 to hold the corresponding electrode assemblies E.
Preferably, sintered metal is used for the electrodes for glow discharge. Although intensity is somewhat low, nickel may also be used.
As is apparent from the foregoing, since the filament coil electrodes are heated by glow discharge, emission of thermoelectrons is accelerated and rapid lighting (several tens of seconds) is enabled.
Since active oxide can be reused and the shading phenomenon can be suppressed, a lifetime as long as 20,000 hours can be achieved.
Since glow discharge and arc discharge coexist, a very high intensity of 35,000 nit or thereabouts can be realized.
Since the electrodes for arc discharge can be heated by the respective electrode assemblies themselves, no external preheating device is needed and power consumption can be reduced to a considerable extent. In addition, the amount of heat generated can be reduced.
The electrodes for glow discharge are not forced to the state of arc discharge and stable discharge can therefore be achieved.