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(11) | EP 0 087 962 A2 |
| (12) | EUROPEAN PATENT APPLICATION |
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| (54) | Beam mode fluorescent lamp having dual cathodes with unipotential ends |
| (57) A beam mode lamp has two thermionic electrodes (33, 34) which when energized by AC
power function alternately as cathode and anode. Electrodes emitted from a cathode
are shaped by the anode into electron beams (e) which excite a gaseous mixture into
radiating ultraviolet light and secondary electrons. A phosphor coating (37) on the
envelope (31) of the lamp connects the ultraviolet radiation into visible light. During
start the electrodes (33, 34) are connected in series. During operation the electrodes
(33, 34) are disconnected from each other. The ends of each electrode are then connected
making the two ends of an electrode at substantially the same electrical potential.
This arrangement results in improved luminous output efficacy and longer operating
life of the lamp. |
BACKGROUND OF THE INVENTION
(1) Field of the Invention:
[0001] The present invention pertains to beam mode discharge fluorescent lamps and more particularly to starting circuits for a beam mode discharge fluorescent lamp.
(2) Description of the Prior Art:
[0002] European Patent Application 82307013.1 filed on December 31, 1982, for a "Dual Cathode Beam Mode Fluorescent Lamp", and assigned to the same assignee as the present invention, discloses a particular embodiment of a fluorescent lamp suitable for replacing the conventional incandescent bulb. Although incandescent lamps are inexpensive and convenient to use, they are considerably less efficient than fluorescent lamps.
[0003] In the above identified application, a beam mode fluorescent lamp, such as illustrated in Figure 1, includes a light transmitting envelope 31 enclosing a fill material which emits ultraviolet radiation upon excitation. A phosphor coating 37 on the inner surface of the envelope emits visible light upon absorption of ultraviolet radiation.
[0004] Two thermionic electrodes 33, 34 for emitting electrons are located within the envelope 31, each electrode has first and second ends. Each electrode is connected between an associated pair of conductors 28, 29 and 35, 36. The electrodes extend lengthwise and parallel to one another in the same plane. One conductor of each electrode is connected to an AC power source. The other conductor of each electrode is connected to a start circuit. These conductors also serve to support the electrodes at.a stationary location within the envelope.
[0005] Each electrode functions as both an anode and cathode under the two alternating polarities of an applied AC voltage. On each half cycle of the AC voltage, the electrode with the positive polarity voltage functions as an anode to accelerate an electron beam which was formed by the electrode with the negative polarity which functions as a cathode to emit electrons forming the electron beam. The accelerated electron beam then enters a drift region.
[0006] Electrons in each electron beam collide with atoms of the fill material in the corresponding drift region, thereby causing excitation of a portion of the fill material atoms and emission of ultraviolet radiation and causing ionization of respective portions of the fill material atoms thereby yielding secondary electrons. These secondary electrons cause further emissions of ultraviolet radiation. The fill material typically includes mercury and a noble gas.
[0007] The lamp includes a base 38 which encloses the start circuit and power source. Both conventional pre-heat and rapid start circuits were suggested as the start circuit.
[0008] The "pre-heat" circuit is a well known fluorescent lamp starting circuit and is described in Section 3.2.2 of Electric Discharge Lamps by John Waymouth. Referring to Figure 2, the electrodes 33, 34 within the lamp are connected in series with a starting switch SW1 and limiting resistor Rl to an ac source of power 9. The electrode is first brought to thermionic emitting temperature by resistive heating from a current flow through the electrodes 33, 34 established by the closing of the starting switch SW1. Subsequent opening of the starting switch SWl places the total applied potential between the electrodes. A discharge is thus formed and sustained. Thermionic emission temperature is maintained by ion bombardment of an electrode during the portion of the ac cycle where the electrode during the portion of the ac cycle where the electrode is acting as a cathode and by electron bombardment during the portion of the ac cycle when the electrode is acting as an anode. In operation, the low potential electrode ends W, Y receive heat energy from bombarding ions and electrons. These electrode ends W, Y are thus heated to a much higher temperature than the rest of the electrode. Most, if not all, thermionic emission emanates from these ends. Evaporation of the filament electrode coating from these end points is also increased due to this high temperature. This factor is responsible for shortening lamp lifetime.
SUMMARY OF THE INVENTION
[0009] Accordingly is an object of the invention to increase usable electrode area for ion and electron bombardment.
[0010] Another object is to increase luminous efficacy, Yet another object to increase lamp lifetime.
[0011] Briefly, a beam mode lamp has two thermionic electrodes. One end of each electrode is connected to an AC power source, the other ends are connected together during starting by a start switch. The electrodes alternate as anode and cathode in response to the AC cycle. Each electrode has a corresponding switch connected between its ends. Both of these switches are open during starting of the lamp allowing current to flow through the electrodes via the start switch. After the electrodes are heated, the start switch opens and a substained discharge occurs between the electrodes. The electrode switches then close and connect the ends of each corresponding electrode. The electrode switches may be thermally responsive so as to open when passing sufficient current, and to close upon cooling. Current limiting resistances may be included. In one embodiment two resistances are used, one in series with each end connecting switch. In another embodiment only one resistor is used, this being in series with the start switch.
DESCRIPTION OF THE DRAWINGS
[0012]
FIGURE 1 represents a lamp suitable for practicing the invention;
FIGURE 2 is a schematic representation of known start circuits and identified as prior art;
FIGURE 3 is a schematic representation of the preferred embodiment of the circuit of the present invention; and
FIGURE 4 illustrates another embodiment of the invention circuit of Figure 3.
DESCRIPTION OF THE PREFERRED EMBODIMENT
[0013] Referring again to Figure 1, a beam mode fluorescent lamp suitable for practicing the present invention is shown. This lamp was described in the aforementioned patent application Serial No. 82307013.1 filed December 31, 1982.., A vacuum type lamp envelope 31 made of a light transmitting substance, such as glass, encloses a discharge volume. The discharge volume contains a fill material which emits ultraviolet radiation upon excitation. A typical fill material includes mercury and a noble gas or mixture of noble gases. A suitable nobel gas is neon. The inner surface of the lamp envelope 31 has a phosphor coating 37 which emits visible light upon absorption of ultraviolet radiation. Also enclosed within the discharge volume of the envelope-31, is a pair of electrodes 33 and 34. These electrodes 33 and 34 function alternately as anode and cathode. At one particular time, one is an anode and the other is a cathode.
[0014] Electrode 33 is connected between conductors 35 and 36 and electrode 34 is connected between conductors 28 and 29 which connect the electrodes 34 and 33,respectively, through enclosure 40 to the AC power supply, and conductors 29 and 35 connect the other ends of electrodes 34 and 33 respective through a vacuum tight seal to a start circuit located in enclosure 40. Electrodes 33 and 34 are a thermionic type and may be described as resistive filaments. The start circuit is the subject of the invention and will be described below.
[0015] After the start circuit is activated by switching on power to the lamp, an AC voltage is applied to electrodes 33 and 34. On the first half cycle of the AC voltage, electrode 33 (for example) will be at a positive polarity with respect to electrode 34. As a result, electrode 34 will function as a thermionic cathode to emit electrons, thereby forming an electron beam as shown by the arrows. Electrode 33 will function as an anode and accelerate the electron beam into a corresponding first drift region 30.
[0016] On the alternate half cycle of the AC voltage, electrode 34 will be positive with respect to electrode 33. Electrode 33 will then function as a thermionic cathode to emit electrons forming a second electron beam, and electrode 34 will operate as an anode and accelerate the formed electron beam into a corresponding second drift region 30 whereupon they collide with atoms of the fill material, thereby causing excitation of a portion of the fill material atoms and emission of ultraviolet radiation and causing ionization of respective portions of th- fill material atoms thereby yielding secondary electrons. These secondary electrons cause further emissions of ultraviolet radiation and are eventually collected by the anode electrode.
[0017] Referring again to Figure 2, there is seen a known preheat circuit wherein the two electrodes are connected in series with AC voltage source 9 and a starting switch SW1. The electrodes 33 and 34 are first brought to thermionic emitting temperature by resistive heating from a current flow through the electrodes established by the closing of starting switch SW1. Subsequent opening of starting switch SWl places the total applied potential between the electrode 33 and 34. A discharge is thus formed and sustained. Thermionic emission temperature is maintained by ion bombardment during the portion of the AC cycle when the electrode filament is acting as a cathode and by electron bombardment during the portion of the AC cycle when the electrode filament is acting_as an anode. In operation, the low potential electrode filament ends W, Y receive heat energy from bombarding ions and electrons. These electrode ends are thus heated to a much higher temperature than the rest of the electrode. Most, if not all, thermionic emission emanates from these ends. Evaporation of the electrode coating from these end points is also increased due to this high temperature. This factor is responsible for shortening lamp lifetime.
[0018] Reference is now made to Figure 3 which is a schematic representation of the preferred embodiment of the invention. Two lamp electrodes 33, 34 are connected in series with an AC voltage source 9 and a starting switch SW1 connected between the high potential ends X, Z, as in the "pre-heat" circuit of Figure 2.
[0019] As a feature of the invention two normally closed thermally responsive switches 40, 41 are connected parallel with electrodes 33 and 34 respectively. Switches 40 and 41 may be commercially available thermal overload switches. A single current limiting resistor 42 is in series with starting switch SW1. The value of resistor 42 is high enough to prevent large transient currents from flowing through the circuit. The starting switch SW1 is initially closed and switch 40, 41 are also closed, shunting electrodes 33, 34. Upon energization from the power source, high current flow is established through thermal switches 40, 41 and starting switch 42, bypassing the two electrodes 33 and 34.
[0020] The thermal switches 40 and 41 quickly open due to heating by this current, allowing current to flow through the electrodes. The electrodes are heated to thermionic emitting temperature. Subsequent opening of the starting switch establishes a sustained discharge between the electrode, particularly at ends W and Y which become very hot.
[0021] The thermal switches 40 and 41 are cooling meanwhile, and after a time delay close, shunting the electrodes so that both ends of each electrode (W and X; Y and Z) is at the same potential during operating modes after ignition. This has the effect of reducing significantly the temperature of the two hot electrode ends W and Y along with increasing the overall emission of the electrodes by directing ion and electron bombardment to filament ends X and Z. Electrode coating evaporation is reduced as the temperature of the electrode ends W and Y are lowered.
[0022] Another embodiment of the invention is shown in Figure 4. Again, use is made of thermally responsive switches 40 and 41 connected in parallel to the electrodes 33 and 34. Two current limiting resistors 43 and 44 are included, one in series with each thermal switch. The value of resistors 43, 44 is chosen to be low enough to open the thermal overload switches 40 and 41 when the starting switch SW1 is closed, but high enough to prevent large transient currents from flowing through the circuit when SW1 is initially closed. The starting switch SW1 is initially closed and switches 40 and 41 are also closed. Upon energization by the power source, high current flow is established which by-passes the two electrodes 33 and 34. The thermal overload switches 41 and 42 quickly open due to heating by this current, allowing current to flow through the electrodes. The electrodes then are brought to thermionic emitting temperature and a discharge is formed. The starting switch SWl is then opened and after an appropriate time delay for cooling the thermal overload switches 41, 42 close, shunting the electrode ends and thereby implementing the unipotential electrode feature of the invention by bringing the ends of each electrode close to same potential.
[0023] Measurement results from a beam mode discharge lamp operated with a standard pre-heat circuit were compared with those obtained with a unipotential electrode arrangement last described. A significant improvement of over a 20% increase in luminous efficacy was obtained using electrodes having unipotential ends. The increase in luminous efficacy is due to the increase in usable cathode emitting area and anode collecting area.
[0024] In both embodiments, luminous efficacy is increased because of the increase of usable electrode area. Operating lifetime is increased through the reduction of electrode temperature and, as a consequence,filament evaporation rate by spreading the area over which heavy ion and electron bombardment of the electrode takes place. Thus the objects of the invention are met.
1. An improved beam mode lamp having a starting mode and an operating mode, and comprised
of an envelope, an excitable mixture of gas contained within said envelope, a first
thermionic electrode and a second thermionic electrode each electrode having a first
and second end and positioned within said envelope, said electrodes when energized
by an AC power source functioning alternately as anode and cathode, wherein the improvement
comprises:
means for connecting the first ends of each electrode to an AC power source;
a first switch means for connecting the second ends of each electrode only during starting modes, whereby electrical current can flow in series through each electrode during starting modes heating said electrodes and allowing a sustained discharge between said electrodes;
a second switch means for connecting the first and second ends of the first electrode during operating modes; and
a third switch means for connecting the first and second ends of the second electrode during operating modes, whereby the first and second ends of the first electrode are at substantially the source elec- trical potential and the first and second ends of the second electrodes are at substantially the opposite electrical potential.
2. The improved beam mode lamp of claim 1 wherein said second switch means and said
third switch means are each comprised of a normally closed thermally responsive switch
which opens upon heating by electrical current during start modes and close upon cooling
during operating modes.
3. The improved beam mode lamp of claim 2 wherein the second and third switch means
are each further comprised of a resistor in series with the thermally-responsive switch,
said resistor allowing sufficient current to flow through the thermally responsive
switch during start modes to cause the thermally responsive switch to rapidly open
allowing current to pass in series through the first electrode, the first switch means
and the second electrode, whereupon after the subsequent opening of the first switch
means, the first and second thermally responsive switches close.
4. The improved beam mode lamp of claim 3 wherein said resistor provides sufficient
resistance to prevent large transient currents flowing while the thermally responsive
switches are closed.
5. The improved beam mode lamp of claim 1 which further includes at least one resistor
in series with the first thermally responsive switch, said resistor allowing sufficient
current to flow through the thermally responsive switch during starting modes to cause
the thermally responsive switch to rapidly open allow current to pass in series through
the first electrode, the first switch means and the second electrode, whereupon after
the subsequent opening of the first switch means, the first and second thermally responsive
switch close.
6. The improved beam mode lamp of claim 5 wherein said resistor provides sufficient
resistance to prevent large transient currents flowing while the thermally responsive
switches are closed.