EP0473679B1

EP0473679B1 - Discharge lamp unit with variable light intensity

Info

Publication number: EP0473679B1
Application number: EP90908615A
Authority: EP
Inventors: Jozsef Ladanyi
Original assignee: Individual
Current assignee: Individual
Priority date: 1989-05-25
Filing date: 1990-05-25
Publication date: 1995-02-22
Anticipated expiration: 2010-05-25
Also published as: EP0473679A1; ATE118955T1; HU202701B; ES2071098T3; DE69017209D1; RU2095946C1; DE69017209T2; WO1990014746A1

Abstract

Discharge lamp unit with variable light intensity, especially for light display boards which comprises a discharge lamp with a heated cathode, an ignition circuit coupled to the lamp, a direct current source coupled to anode and cathode electrodes of the lamp and a light intensity adjusting circuit connected in the anode to cathode path of the lamp, wherein the light discharge lamp is formed by a compact discharge tube (1) with two filaments (2, 4), two ends of the first filament (2) are connected to terminals of a heating source (3), the negative terminal (V-) of the direct current source is connected to the heating source (3), and the light intensity adjusting circuit comprises a controlled switch (5). In an embodiment with increased lifetime the heating source (3) comprises a heating transformer, the end terminals of the first filament (2) are connected to the secondary winding of the heating transformer, and the negative terminal (V-) of the direct current source is connected to central tap point (9) of the secondary winding.

Description

The invention relates to a discharge lamp unit with variable light intensity, especially for light display boards, comprising a discharge lamp having a compact discharge tube with two filaments arranged at two end regions thereof and serving as a cathode and an anode, respectively, an ignition circuit coupled to the lamp, a direct current source coupled to anode and cathode electrodes of the lamp and a light intensity adjusting circuit connected in the anode to cathode path of the lamp.
Discharge lamp units with variable light intensity are generally used for large light display boards, in which the basic picture element of the board is made of a gas discharge element of special design, wherein several anodes are arranged in a common envelope opposite to a common heated cathode electrode, and in the interior of the envelope fluorescent layers emitting different colored light are associated with the respective anode regions.
The control and operation of such boards are connected with a number of problems, of which a few will be discussed. For obtaining a good picture resolution, the size of the gas discharge tubes designed to such purposes must be small. In principle, a voltage of 33 ≈ 35 V would be sufficient for maintaining the gas discharge process. In contrast thereto actual embodiments use much higher voltages such as 50 ≈ 60 V in order to provide a safe ignition and a continuous discharge. The use of a voltage much higher than the value required for maintaining the discharge process decreases the efficiency and leads to unnecessary losses of heat.
A further drawback comes from the limited lifetime of the filament of the discharge tubes used in pulse mode. The emission is uneven along the cathode surface, and the extent of emission starts to decrease in the end regions of the filament even after a shorter use. This desrease slowly moves toward the central portion of the filament. By the time the emission decrease reaches the central portion of the filament, the cathode looses its emission capability and the tube cannot be used any more.
The special discharge tubes used in display boards are not manufactured by mass-scale production, therefore they are comparatively expensive.
US-A-4777409 describes a discharge lamp unit according to the prior art comprising a discharge lamp having a compact discharge tube with two filaments arranged at two end regions thereof, wherein the filaments are connected in series with each other through a bimetall switch. In this arrangement both filaments are heated in the closed state of the bimetall swich, which is opened when the discharge process has started. During the discharge process neither of the filaments is heated. A DC supply voltage is applied to the discharge tube through a controlled switch, which is regulated by means of a switching regulator such that the current drawn by the lamp is maintained at a substantially constant value.
The object of the invention is to provide a discharge lamp unit which can provide an increased cathode lifetime, which has an increased light utilization effectivity and which enables the use of mass-produced discharge lamps.
To achieve these objects a discharge lamp unit with variable light intensity has been provided especially for light display boards, which comprises:a discharge lamp having a compact discharge tube with two filaments arranged at two end regions thereof, both ends of the first filament being connected to terminals of a heating source, said first filament being permanently heated during operation and serving as a cathode, said second filament being left unheated and serving as an anode; a direct current source with a positive and a negative terminal, the negative terminal being connected to the heating source; a light intensity adjusting circuit coupled between the positive terminal of the direct current source and the second filament, said light intensity adjusting circuit comprising a controlled switch periodically enabling or disabling at least a major portion of the current corresponding to maximum light intensity to pass therethrough with an on-off ratio that corresponds to the required light intensity; and an ignition circuit coupled to the lamp.
The compact discharge tube with two filaments can be exposed to pulse control mode, and if one of the cathodes looses its emission or gets broken, then the tube can be inserted in opposite position into its socket, whereby the filament which was used previously as an anode will thereafter be used as a cathode. This possibility doubles the lifetime of the compact discharge tube.
A further increase in lifetime comes from the use of an alternative current heating source i. e. a heating transformer, wherein the end terminals of the first filament are connected to the secondary winding of the heating transformer, and the negative terminal of the direct current source is connected to central tap point of the secondary winding.
In a preferable embodiment both end terminals of the second filament which forms the anode electrode are interconnected and they are coupled directly or through a resistor to the controlled switch and through the switch to positive terminal of the direct current source.
The pulse control can be solved in an easier way if a second resistor is connected between the positive terminal and the anode with a resistance value being by at least one order of magnitude higher than that of the resistor in the switched path.
According to the invention the display board has preferably in each picture point three discharge lamp units, and every one of these lamp triplets comprises compact discharge tubes emitting red, green and blue light, respectively.
The invention will now be described in connection with preferable embodiments thereof, in which reference will be made to the accompanying drawings. In the drawing:

Fig. 1: shows the circuit diagram of a light discharge unit,
Fig. 2: shows the schematic arrangement of a display board,
Fig. 3: shows time curves of selected voltages measurable within the light discharge tube.

Fig. 1 shows the control circuit of a compact discharge tube 1. Filament 2 of the compact discharge tube 1 is coupled to source 3 which provides a constant filament voltage, while the tube has a second filament 4 which, in operation, serves as an anode and it is not heated. The source 3 is formed by heater transformer coupled to the mains network, the transformer has a secondary winding that provides a 7 V heating voltage for the tube 1 and a central tap point 9 is lead out from the secondary winding. The central tap point 9 is coupled to negative terminal V- of a DC power supply which forms an earth point.
The positive terminal V+ of the power supply is connected to a first resistor R1 and to a series member consisting of a controlled switch 5 and a second resistor R2. The other ends of the resistors R1 and R2 are interconnected and coupled through wire 10 to both terminals of filament 4 which is arranged in an end region of the compact discharge tube 1. This end region lies opposite to the one in which the filament 2 that forms the cathode is arranged.
The controlled switch 5 has a control input which receives rectangular pulses with adjustable width. During the on periods of these pulses the controlled switch 5 is in conducting state, whereby the resistors R1 and R2 are connected in parallel. During the off periods of the pulses the controlled switch 5 is in non-conducting state, therefore between the positive terminal V+ and the filament 4 the current can flow only through the resistor R1.
In case of the embodiment shown in Fig. 1 the ignition of the compact discharge tube 1 is enabled by a capacitive ignition electrode 6 covering a portion of the envelope of the tube 1. The ignition electrode 6 is connected through pulse transformer 7 to pulse generator 8 which provides ignition pulses in 1.5 ≈ 2 second intervals.
If several ones of the compact discharge tubes 1 together with the associated circuits (as shown in Fig. 1) are arranged in rows and columns, a matrice-like light display board 11 shown in Fig. 2 is obtained with pixel points formed by respective single compact discharge tubes. A color display board is obtained if each pixel element is formed by three compact discharge tubes la, lb and lc having red, blue and green colors. The U-shaped design of conventional compact discharge tubes and their standard width of 25 mm enable the formation of a display board 11, in which the pixel size is about 60-70 mm. In Fig. 2 the compact tubes were shown at the first pixel point only, and all other points have similar design.
Before describing the operation of the circuit arrangement according to the invention reference will be made to Fig. 3, in which the operational voltage between the filaments 2 and 4 is shown as a function of time.
If a nominal heating voltage, i. e. an AC voltage of 50 Hz frequency and 7 V effective value is continuously applied to filament 2 of the compact discharge tube 1, then in case of the circuit arrangement of Fig. 1 the zero line of the direct current supply voltage will vary as shown in Fig. 3. The heating voltage of 7V is symmetrical to the zero line of the DC voltage only therefore because the heating source 3 supplies a voltage which is symmetrical to the ground potential. The AC voltage of 7 V applied to the two ends of the filament 2 is evenly distributed along the filament. With respect to the filament 4 (which serves as an anode) in Fig. 3 curve 2a shows the potential of the first end of the filament 2, curve 2b shows the potential of the second end of the filament 2, while the zero axis designated as 2c shows the potential of the central point of the filament 2. The potentials of the two ends of the filament 2 with respect to the anode correspond to waveforms, in which respective AC voltages of ± 3.5 V are superimposed on the DC anode voltage.
In the compact discharge tube 1 the voltage required for maintaining gas discharge is between about 33 ≈ 35 V. If a ground Potential is provided at the central point of the filament 2 as suggested by the present invention, then the filament 2 will always have an at least 50 % long section which has a potential with respect to the opposite filament 4, i.e. to the anode, that is at least as high as the voltage of the V+ terminal or higher, and this section is sufficient for maintaining the discharge process. The DC voltage V+ can thus be adjusted to correspond to 33 ≈ 35 V. If the point of the filament 2 is considered as the momentary maximum of the current emission which is the most negative compared to the anode, then in view of the curves of Fig. 3 it will be clear that in each period of the heating voltage this point slides continuously and mildly along the filament 2, i. e. the maximum emission cannot be associated with any discrete filament section. From this property it follows that in use the abrasion of the active cathode material will be evenly distributed along the length of the filament which increases the active lifetime of the cathode.
If the negative terminal of the DC supply voltage were applied to an end point of the filament 2, then the Potential of this end point would form the zero line 2c (shown on Fig. 3) and the voltage of the other end point would correspond to the curve 2a but in that case the effective value of the voltage would be 7V instead of the 3.5 V in the previous case. If the potential of this other end point lies in the positive half period relative to the zero line, then the momentary value of the anode-cathode voltage is smaller than the DC voltage along the whole filament, and the discharged state in the tube can be maintained only if a DC voltage is applied to the filament 4 that is by 7 V higher than in the previous case. This means that a DC voltage of about 40 V must be used instead of the 33V minimum permitted value, however, the average light intensity will not be higher. Such a circuit arrangement would lead to a substantial decrease in efficiency.
It can also be understood, that in cases wherein the negative terminal of the DC supply voltage is applied directly to an end point of the filament 2, in the first half period of the filament voltage the first end of the filament will be the most negative point compared to the anode, while in the other half period this point will be the other end of the filament. The maximum emission will therefore alternatively lie at the two end points of the filament. This fact means that the maximum load will fall to the two end points of the filament and the current of the gas discharge will follow an uneven distribution along the filament, whereby the wear of the filament will also be uneven. If the zero potential of the DC voltage is shifted to the central point of the filament as suggested by the present invention, then an increased expected lifetime is obtained and a decreased DC voltage can be used.
If the two end points of the filament 4 which forms the anode are interconnected, then a more even potential distribution along this filament 4 can be obtained. The advantages offered thereby are not so significant but they are not negligible.
Following the short analysis of the emission properties a preferable way of the pulse control of the compact discharge tube 1 will be described.
Firing pulses are coupled to the ignition electrode 6 through the pulse transformer 7. Although the discharge process can be started by a single ignition pulse, these pulses are repeated in 1 ≈ 2 second long periods in order to ensure the ignited state of the compact discharge tube 1 also in cases if the anode current has broken due to any accidental reason.
The value of the discharge current is determined by the state of the controlled switch 5. In the on state of the switch 5 the resistors R1 and R2 are connected in parallel, and their values are chosen so that they ensure a tube current of about 150 mA. In that case the tube lights with maximum intensity. In the off state of the controlled switch 5 a minimum current of 1 ≈ 1.5 mA is allowed by the resistor R1 to flow through the tube which is just sufficient for maintaining the discharge process, whereby noticeable light is not generated. The maintained discharge process ensures that the current of the tube can be increased instantaneously i.e. without any delay. The light intensity is determined by the ratio of the on and off times of the pulses coupled to the control input of the controlled switch 5 and this ratio can be changed easily.
In every point of the display board 11 any required color and up to a maximum any intensity can be adjusted by the appropriate control of the tubes from which respective different colored triplets are arranged in these points. The speed of this control can be higher than the time resolution of the human eye, whereby moving inscriptions and moving pictures can also be displayed.
The advantages of the invention lie first in the increased lifetime of the compact discharge tube 1 owing to the suggested way of heating, second in the interchangeability of the filaments 2 and 4 because if the heated filament gets broken or looses its emission, the tube can be inserted in an opposite position in the socket and the positions of the filaments 2 and 4 get interchanged, and the filament 4 which was used previously as an anode can be used as cathode and the filament 2 can serve as the anode even in broken form or if it has no emission any more. By this possibility the increased lifetime can be doubled. The decrease of the operational DC voltage spares energy. A further substantial advantage comes from the possibility of using mass produced, commercially available compact discharge tubes for forming elements of the display board 11, since this reduces manufacturing costs by a significant extent.

Claims

A discharge lamp unit with variable light intensity, especially for light display boards, comprising: a discharge lamp having a compact discharge tube (1) with two filaments (2, 4) arranged at two end regions thereof, both ends of the first filament (2) being connected to terminals of a heating source (3), said first filament (2) being permanently heated during operation and serving as a cathode, said second filament (4) being left unheated and serving as an anode; a direct current source with a positive and a negative terminal (V+ and V-), the negative terminal (V-) being connected to the heating source (3); a light intensity adjusting circuit coupled between the positive terminal (V+) of the direct current source and the second filament (4), said light intensity adjusting circuit comprising a controlled switch (5) periodically enabling or disabling at least a major portion of the current corresponding to maximum light intensity to pass therethrough with an on-off ratio that corresponds to the required light intensity; and an ignition circuit coupled to the lamp (1).
The discharge lamp unit as claimed in claim 1, characterized in that the heating source (3) comprises a heating transformer, the end terminals of the first filament (2) are connected to the secondary winding of the heating transformer, and the negative terminal (V-) of the direct current source is connected to central tap point (9) of the secondary winding.
The discharge lamp unit as claimed in claims 1 or 2, characterized in that both end terminals of the second filament (4) which forms the anode electrode are interconnected and they are coupled directly through a resistor (R2) to the controlled switch (5) and through the switch to positive terminal (V+) of the direct current source.
The discharge lamp unit as claimed in claim 3, characterized in that a second resistor (R1) is connected between the positive terminal (V+) and the anode with a resistance value being by at least one order of magnitude higher than that of the resistor (R2) in the switched path.
A display board, with three discharge lamp units, each as claimed in claims 1 or 2, arranged in each picture point of the display board, each of these lamp triplets comprising compact discharge tubes (1) emitting red, green and blue light, respectively.