US2976449A - Lamp and method - Google Patents

Description

March 21, 1961 D. A. LARSON ETAL 2,976,449

LAMP AND METHOD Filed Oct. 10, 1957 3 Sl'leets-Shee'rl 1 l March 21 1961 D. A. LARsoN ET AL 2,976,449

LAMP AND METHOD 3 Sheets-Sheet 2 Filed Oct. l0, 1957 March 21, 1961 D. A. LARsoN ET AL 2,976,449

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INV ENTOR5 AMP ca/e/P-A/r /A/ Muna/7F15 WZVEX United States Patint LAMP AND NmTHoD Daniel A. Larson, Cedar Grove, and Luke Thorington, Berkeley Heights, NJ., assignors to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania Filed Get. 10, 19:57, Ser. No. 689,395

2 Claims. (Cl. S13- 109) This invention relates to discharge lamps and to methods for operating same and, more particularly, to a discharge lamp which `operates. with a positive voltampere characteristic and to a method for operating a lamp with a positive volt-ampere characteristic.

The usual type of discharge lamps, such as fluorescent or high-pressure mercury-vapor lamps, all operate with what is known as a negative volt-ampere characteristic. In the operation of such fluorescent and HPMV lamps, as the current through the lamp increases, the resistance of the lamp discharge decreases, thereby creating a runaway discharge which would destroy the lamp if not controlled. In order to overcome this runaway discharge characteristic, such lamps are normally provided with a series reactance which limits the current and prevents the discharge from running away. Such reactances are expensive and in addition constitute a power loss and are therefore objectionable.

It' is the general object of this invention to avoid and overcome the foregoing and other diiiiculties of and objections to prior-art practices by the provision of a gas-discharge lamp which operates with a positive voltampere characteristic; that is, the discharge is inherently stabilized and will not run away.

It is the further object to provide a method for operating a gas-discharge lamp so that it will have a positive volt-ampere characteristic. I

The aforesaid objects of the invention, and other objects which will become apparent as the description proceeds, are achieved by providing -a hydrogen-discharge lamp which has a substantial current-limiting electron space charge formed about the lampv cathode. This causes the potential drop between the cathode and the anode to occur in the region of the anode and this produces a self-limiting type of discharge which will not run away. In addition, there has been provided a method for operating such a lamp so that the discharge will be self-limiting.

For a better understanding of the invention, reference should be had to the accompanying drawings., wherein:

Fig. 1 is an elevational View, partly in section, of a hydrogen-discharge lamp and the operating circuit therefor; A

Fig. 2 is an elevational view, partly in section, of an alternative embodiment of the lamp Vshown in Fig. l;

Fig. 3 is a representation of the Voltage drop which occurs across the electrodes of the usual fluorescent lamp; l

Fig. 4 is a representation of the voltage drop which occurs between the cathode and the anode for a lamp as illustrated in Fig. l;

Fig. 5 is a graph of lamp-operating voltage versus lamp current for a lamp as shown in Fig. 1 when lled with hydrogen at a pressure of 200 microns mercury;

Fig. 6 corresponds to Fig. 5 and illustrates operating characteristics for a lamp as shown in Fig. l when lled with hydrogen at a pressurekof 0.5 millimeter;

Fig. 7 corresponds to Fig. 5 except that the lamp hydrogen till pressure is 2.5 millimeters;

Fig. 8 corresponds to Fig. 5 exceptthat the lamp hydrogen fill pressure is 4 millimeters. l

With specific reference'to the form ofthe invention illustrated in the drawings, the numeral 10 ,illustratesV generally a hydrogen-discharge lamp which operates with a positive volt-ampere characteristic and which generally comprises a light-transmitting envelope 12 having a vitreous mount 14 hermeticallysealed at the neck thereof. This mount has sealed therethrough three lead conducto-rs, 16, 18 and 20. Two of the lead conductors 16 and 13 support an oxide-coated cathode 22 between their inwardly-extending extremities. Provided on the inner surface of the envelope is a light-transmitting, electricallyconductive layer Z4 which may be fabricated of tin oxide, for example, and such electrically-conducting layers are well known. Coated on the inner surface of the electrically-conducting layer 24 is a phosphor material 26 for converting ultraviolet radiations and electrons into visible light and such phosphor material will be considered hereinafter. The lead 4conductor 20 connects to the electrically-conductive layer 24 and la source 28 of relatively high potential is connected between one side of the cathode 22 and the electrically-conductive layer 24.

inches and it may be fabricated of the well-known sodalime-silica glass. The cathode 22 comprises a tungsten coil which has the general contiguration of a coiled-coil having a barrel length of approximately 3A; inch and a major coil diameter of approximately /lf; inch. Contained Within the turns of the inner coil are forty milligrams, for example, of a mixture of alkaline-earth oxideswhich may comprise 50% by weight barium oxide, 30% by Weight calcium oxide and 20% by weight strontium oxide.

materials may be used, if desired.

The phosphor material which is coated onto the elecf trically-conductive layer 24 may comprise any phosphor material which is excitable by relatively low-voltage electrons and ultra-violet radiations to produce visi-ble radiations. As examples, such phosphors are manganeseactivated zinc fluoride, manganese-activated zinc orthogermanate and zinc-activated zinc oxide and these phosphors are generally known in the art. The source of relatively high potential is shown as variable, but for large-scale use of the lamp such ra source would desir-r For the purposes ably be iixed with respect to voltage. of operating the lamp under different conditions, "however, the source 28 of relatively high potential is variable from 0 to about 120 volts. The source 30 of Vrelatively low potential which is used to provide cathode-heater current is also variable from 0 to about 5 volts, for'example, for the purpose of operating the lamp under varying conditions, as explained hereinafter.

While specific dimensions for the lamp constructions have been listed in detail hereinbefore, it should be 'understood that the envelope configuration may be varied as well as the cathode construction, the electrical connections `for the lamp, the conducting coatings Yand the type of power sources which may be used. YAs an example, the cathode heater current source 30 may be an alternetting-current source, if desired, and the operating power These percentages may be varied considerably and single oxides or other well-knownV electron-emitting source 28 may also be an `alternating-current source if the lamp is to operate on half cycles only. Many other materials are also suitable for providing the thin, trans parent electrically-conductive layer 24 and cadmium or zinc oxide, for example, may be substituted in place of the tin oxide.

In Fig. 2 is illustrated an alternative embodiment of the lamp shown in Fig, l wherein the thin, transparent, electrically-conductive layer 24 is dispensed with and is replaced with a vacuum-metalized reflecting coating 32 of aluminum or silver, for example, The phosphor ina-- terial 26 is coated over this aluminum or silver coating and the end section 34 of the envelope is left light transmitting in order to pass the light which is generated by the phosphor.

The lamps which are shown -in Figs. l and 2 are essentially photoluminescent-cathodoluminescent lamps wherein the electrons which excite the phosphor are supplemented by additional electrons and ultra-violet radiations which are generated in the hydrogen gas discharge, as will be explained in detail hereinafter. Cathodoluminescent lamps are known `and such a lamp is generally illustrated in U.S. Patent No. 2,177,705 to Friederich, which lamp as disclosed by Friederich has `an evacuated envelope. This curtails the current which can be carried, which correspondingly limits the light output of the lamp. An incandescent-cathodoluminescent lamp is described by Thorington, one of the coinventors herein, in U.S. Patent No, 2,759,119. When such a lamp as described by Thorington in this aforementioned patent is operated as a vacuum lamp, the current which is available for cathode-ray excitation of the phosphor is limited. When a gas is added to the lamp to increase the electron current, the discharge is not self-limiting and must be ballasted. Thorington also suggests utilizing the phosphor coating as the limiting resistor, but in such a ease a large proportion of the power drawn by the lamp would be dissipated `by the potential drop across the phosphor. Accordingly, such a lamp as described by Thorington in his aforementioned patent is not eilicient as a cathodoluminescent lamp, per se, but `rather utilizes a limited amount of cathodoluminescent response to supplement the visible output from the incandescent filament,

Hydrogen-discharge sources are also known and are primarily used for experimental purposes as the ultraviolet emission of a hydrogen discharge is substantially continuous. Such lamps do not operate with a self-limiting ltype of discharge and require a current-limiting reactance as in the case of the usual gas-discharge lamp.

In Fig. 3 is shown a schematic representation of the voltage drops which occur from cathode to anode in the usual type fluorescent lamp. In such a lamp there is a fairly-sharp voltage drop at the cathode and thereafter the voltage gradient is less in the positive column.

In Fig. 4 is shown a representation for the voltage drops which occur across the instant lamp. In the region of the cathode, the substantial electron space charge actually causes a negative voltage to occur and all of the voltage drop across the lamp electrodes is realized in the region of the anode. This is verified by the fact that when operating the instant lamp, there is no cathode glow, hot spot, varc or any other light emission occurring in the region of the cathode and all of the light emitted from the lamp occurs in the region of the anode. This can only be attributed to the fact that there is a very low electric field in the region of the cathode and suppression of a field in this manner can only be accomplished by a substantial electron space charge. In addition, the operating characteristics for the instant lamp substantiate this visual observation,

Fluorescent and other types of discharge lamps which operate with a negative volt-ampere characteristic normally operate with what is known as a hot spot on the cathode or with a cathode-glow type of operation, as in a so-called cold-cathode fluorescent lamp. The hot spot or cathode glow, as the case may be, is caused by positive-ion bombardment of the cathode. Such cathode positive-ion bombardment, however, is indicative of the fact that there is a relatively-strong electric eld in the region of the cathode, which of course precludes the existence of a substantial electron space charge in the region of the cathode. For this reason, hot-spot or cathode-glow operation of the instant lamps is to be avoided or a non-self-limiting discharge will result.

In Fig. 5 are illustrated the performance characteristics of the lamp as shown in Fig. l when lled with hydrogen at a pressure of about 200 microns. In the operation of the lamp three parameters were varied, namely the lamp-operating potent-ial which was applied between the cathode and the anode, the electron emissivity of the cathode which was varied by varying the potential for the so-callcd heater current source 36, and the hydrogen till pressure, This resulted in varying the currents which were drawn by the lamp with varying results, as explained hereinafter, At a hydrogen till pressure of 200 microns and a cathode heater current of 6.5 amperes, the current which the lamp drew increased substantially with increasing lamp operating potential until a lamp operating potential of 30 volts was applied. Thereafter, the cathode glowed brightly and considerable light was emitted from the region of the cathode, This is attributable to an electric field in the region of the cathode resulting from dissipation of the electron space charge. Immediately thereafter a destructive arc occured through positive-ion bombardment of the cathode and the discharge would have run away if not stopped. As the heater current was increased to 7.3 amperes to increase the electron space charge, the lamp was capaable of drawing 330 milliamperes before a destructive arc occurred and as the heater current was increased still further, no destructive are occurred, even when the lamp was drawing 500 milliamps. As shown in Fig. 5, as the heater current was increased to increase the emissivity of the cathode, less potential was required to cause the lamp to draw an equivalent current, This is attributable to the fact that increased electron emissivity resulting from increased heater current will cause a larger electron space charge in the region of the cathode and this will cause an increased voltage gradient in the region of the anode, which will accelerate the electrons in the lamp more rapidly to increase the current drawn by the lamp. For the specific cathode construction, a potential of 3,8 volts across the cathode resulted in a cathode heater current of 8.2 amps., see Fig. 5. Of course, the cathode operating characteristics will normally be different for alternative cathode constructions as may readily be used.

In Fig. 6 is shown the same lamp operated with a hydrogen fill pressure of 0.5 millimeter. At a cathode heater current of 6.8 amps., the electron space charge which was formed was relatively weak and as the lamp operating potential was increased to 40 volts and the lamp drew a current of milliamps., the discharge converted to a destructive discharge. As the heater current was increased, as indicated, to increase the electron space charge in the region of the cathode, the resulting electron space charge proved sucient under the applied operating voltages to limit the current and the lamp was able to draw currents up to 500 milliamperes at either of the indicated increased cathode heater currents.

In Fig. 7 is shown the same lamp, but operated at a hydrogen-till pressure of 2.5 millimeters. With the indicated heater currents, no destructive discharge occurred even when the lamp was drawing 500 milliamperes.

In Fig, 8 is shown the same lamp operated at a hydro gen pressure of 4 millimeters with the cathode heater current as indicated.

It will be observed that as the hydrogen-till pressure in increased, a greater lamp-operating voltage was required in order to cause the lamp to daw the same 5 amount of current. This is attributable to the fact that the higher hydrogen-till pressures oler lwhat might be termed more impedance to the discharge and thus require a greater lamp-operating voltage to cause the lamp to draw an equivalent current. It should be pointed out, however, that the greater the hydrogen pressure, the greater the number of electrons -which are formed in the hydrogen discharge and the greater the intensity of the resulting ultra-violet radiations which the hydrogen discharge generates. Thus for the same current drawn, the lamp which has the greater hydrogen-lill pressure will be brighter. Still-greater hydrogen till pressures such as about 6 mm. may =be utilized, if desired, so that the lamp will operate across a 110 volt line. As can be seen from the foregoing, the hydrogen ll pressure acts to establish the lamp operating voltage and loading and hydrogen till pressures greater than 6 mm. may be used to increase the lamp operating voltage and loading still further.

Summarizing the operation of the lamp, the lamp cathode is heated to an electron-emitting state by the application of a relatively-low potential thereacross. It should be understood that if the cathode had a potential applied thereacross which was equivalent tothe lamp operating potential, a destructive arc would occur across the cathode and to eliminate such cathode-arcing, the cathode is provided with a relatively low resistance so that it can be operated with a low potential. When the operating voltage is applied between the cathode and the anode, this causes a iiow of electrons from the cathode toward the anode. Because of the voltage gradient which is present, as shown in Fig. 4, the electrons are not accelerated until they reach the region of the anode, whereupon the hydrogen gas iill is ionized to form a hydrogen discharge. This generates additional electrons and ultraviolet radiations. A substantial portion of the additional electrons will be directed toward the lamp anode because of the proximity of the discharge to the lamp anode. These electrons and the ultra-violet radiations excite the phosphor material to produce visible light. Thel space charge which is formed in the Iregion of the cathode acts to limit any positive-ion bombarment of the cathode since the eld gradients which are present are confined to the region of the anode. Thus the lamp cannot go over to the so-called hot-spot or cathode-glow operation since positive-ion bombardment of the lamp cathode is prevented.

As an alternative embodiment, the thin, transparent, electrically-conducting coating may be dispensed with and a conducting material such as zinc oxide or niobpentoxide incorporated with the phosphor, as decribed in U.S. Patent No. 2,177,705 to Friederich; The phosphor and admixed material then act as the lamp anode.V

As an additional alternative embodiment, the lamp operating potential may Ibe alternating current, asnoted hereinbefore, and insuch a case, the lamp will operate to produce light only an alternate half cycles. For this reason, direct-current operation of the lamp is preferable.

It will be recognized that the objects of the invention have been achieved by providing a discharge lamp which operates with a positive volt-ampere characteristicand by providing a method for operating same.

While in accordance with the patent statutes, one bestknown embodiment of the invention has been illustrated l envelope at least a portion of which is light transmitting v Y and described in detail, -it is to be particularly under@ j l stood that the invention is not limited thereto or thereby.. e

We claim:

l. The method of operating with a positive volt-ampere y characteristic a hydrogen-discharge lamp having, a thermionic cathode comprising a coil carrying an electi'onsaid cathode to cause same to become strongly electronV emitting Iand to fonn an electron space charge thereabout, simultaneously applying a relatively high predetermined electric potential between said cathode land said anode comprising phosphor material, and controlling the potential applied across said cathode with respect to the potential applied between said cathode and said anode to create Y therebetween a hydrogen gas-discharge only in the region of `said anode. v

2. An electric gas-discharge lamp which is operable with a positive volt-ampere characteristic, said lamp comprising, a cathode comprising a coil coated with electronem-issive material and adapted to be heated to an ellectron-emitting state by the application of a predetermined electric potential which is substantially less than the predetermined operating electric potential for said lamp, an

surrounding said cathode, an anode comprising phosphor material excitable to visible luminescence by electrons and ultraviolet radiations carried on the inner 'surface of said envelope, Ia hydrogen gas lill at a pressure of from about 200 microns to about 6 mm. within said envelope,

lead conductors sealed through said envelope, two of said l lead conductors electrically connecting to opposite sides of said cathode, another of said lead conductors electri cally connecting to said lamp anode, the two fof said lead conductors connecting to opposite sides of said cathode adaptedto have a relatively low predetermined electric potential applied thereacross to cause said cathode to become strongly electron emissive, and enact said lead conductors which connects to said catho-de and the other of said lead conductors .which connectsto said anode adapted to have a relatively gh predetermined lamp operating potentialappllied thereacross.

References Cited iny the tile of this patent UNITED STATES PATENTS 2,748,308 Kolkman' May 29, 1956 2,759,119 Thorington Aug. 14, 1956A 2,803,775 Jaumann Aug. 20, 1957 2,832,912 Lake Apr. 29, 1958- Laporte May 20,

' OTHER REFERENCES,

Fundamental Processes of-Eleotrica-l Discharges inl Gases, Loeb, published Yby Wiley and Sons, 1939.y

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