US2420942A - Electric discharge device and circuit therefor - Google Patents

Description

May 29, 1947. FQULKE I 2,42%,942

' ELECTRIC DISCHARGE DEVICE AND CIRCUIT THEREFOR Filed Feb. 21, 1941 Fig. 3.

I Invenror's: Ted E. FouLk,

Patented May 20, 1947 .ELEGTRIC DISCHARGE oEvIoE AND CIRCUIT THEREFOR Ted E. Foulke, Mayfield Village, ()hio, assignor to General Electric Company, a corporation of New York 1 Y TEN OFFICE 7 Application February 21, 1941', Serial No.ss 0,o42

3 Claims. (01. 315405) i My invention relates to gaseous electric discharge devices and particularly to those of the low pressure type having thermionic electrodes and an operatin atmosphere comprising, a gas and a metallic vapor. Still more particularly it relatesto an improved constructionof gaseous electric discharge lamps and to an improved and simplified method of operating the same, whereby the useful operating life is substantially increased.

Hitherto the use of certain gaseous discharge devices as light sources has been hampered by the large amount of auxiliary equipment necessary for their operation. This necessity arises, first, from the fact that the voltage required to initiate the discharge is, particularly in the case of low pressure lamps, generally much greater than the normal operatin voltage and, secondly, from the fact that unless the thermionic cathodes are preheated for a definite length of time, they are not only damaged themselves by current bombardment, but in addition the envelope walls are blackened by material sputtered from them. This bombardment is particularly prevalent during the starting operation before the cathodes have reached proper operating temperature. It has thus been necessary toprovide a mechanism to delay the application of the voltage between the main electrodes while a preliminary heatin current flows through they thermionic cathodes, and then to provide a high starting voltage. The Foulke Patent No. 2,177,883 is exemplary of arrangements of this type. Such mechanisms are not only objectionable because they add to the overall cst,but also because they tend to burn out or get out of order for other reasons. Further, the operation of the switching mechanisms tends to produce transient voltages which damage the lamp-and the auxiliary equipment. All of these factors affect the reliability of the lighting unit asa Whole.

I have found, however, that byproper design of the lamp and attendant supply circuit in accordance with the invention, it is possible to construct a lamp which is capable of withstanding the heavy bombardment during the starting period without the deleterious efiects of sputtering and blackening and which, at the same time, starts more readily. With such a design, it becomes possible to eliminate much of this cumbersome auxil- 2 iary equipment now necessary with discharge lamps.

It is accordingly an object of the invention to providea gaseous discharge lamp capable of withstanding the effects of starting without undue sputterin or vaporizing'of the cathodes, whereby preheating of the cathodes is made unnecessary. e g It, is a further object of the invention to eliminate the need for the greater part of the auxiliary control mechanisms now necessary with lamps of the type herein described.

Still another object of the invention is to pro vide a more simple and inexpensive lighting unit which ismore reliable in operation and economic'al' in manufacture.

Further and other objects of the invention will appear from the, following detailed description and'from the followin drawings, in which Fig. 1

represents a gaseous discharge lamp of the type here involved and an improved circuit for operating the same, while Fig. 2 illustrates in more detail an improved cathode construction for the same. Fig. 3 represents an enlarged section oi. the windingof our cathode.

Referrin to Fig. 1, the lamp 1 may be of the fluorescent type now prevalent in the art, for example, it may incorporate an envelope 2 having a fluorescent coating 3 on the inside surface thereof and a quantity of vaporizable metal 4, such as mercury, sufficient to produce vapor pressures upto about twenty microns. A quantity of startin gas such as argon or neon or any of the rare gases may be provided at the pressures hereinafter specified. The lamp may also include an activated cathode 5 and a pair of anodes 6, preferably of carbon, the anodes being separated by an insulating baffie 'l. The lamp may be energized by a current supply circuit indicated generally bythe numeral 8, which will be described more in detail below.

Ordinarily, unless such a lamp is operated in a circuit such as that shown in the aforemen-' tioned Foulke Patent No. 2,177,883, which insures preheating of the cathode beforethe main discharge is struck, the useful life is extremely short. This is due to the fact that before the cathode reaches proper operating temperature. it emits an insufiicient number of electrons to materially lower the potential gradient in the tube.

If full voltage is applied beween operating electrodes in this condition, a very high potential gradient exists at the cathode with the result that it may be subjected to very heavy positive ion bombardment. Such positive ion bombardment destroys the coating and sputters part of it and some of the cathode metal itself to the envelope wall, where it collects as a black coating. This coating considerably impairs not only the lighting efficiency, but also the appearance of the lamp. If, however, the cathode has reached proper electron emitting temperature before the voltage is applied, the high potential gradient' at the cathode does not arise and consequently the positive ions do not acquire sufficient energy to,v

materially damage the cathode surface.

Extensive tests have shown that this sputtering and blackening process is largely confined-to the starting period. This is readily understandable in view of what has already been said aboutthe,

cathodes 'having'a large massin order'that a large emittingsurface area might be obtained. I find, howeVerQthat by choosing a form which re- 'duces the mass and at the same time provides a large emitting surface, it is possible to measurablydecrease the heat capacity and, consequent- "ly the heating time. Thus," in one case I sueceded in obtaining measurably improved results by decreasing the'cathode wires to one-quarter the iormefmass per unit length and by providing multiple windingsof' this wire tosupply the necessaryarea; For example; in a standard 85 watt 0.9 ampere R. F. lamp .(rectifying fluorescent) the new'cathode took the form of 13 double turns with inside diameter equal: to 0.060 (60' mil). using coiled coilwire comprising tungsten wire 0100 6""in diameter overwound' with 0.0015 tungstenjwire atabout 4100 turns per inch. These di- 'IlflllSi'OIlS are about the largest allowable if the advantages of the-invention are to be realized. The resulting cathode has considerably less mass than the cathodes hithertousedi and-fstill'has an equivalent emitting surface area. Because of this decreased mass, it' will rise to full operating temperature'much more rapidly than "do the constructions hitherto known. For larger currents or for still faster action, an even smaller filament wire} (for example, a coiled coil of 0.004 wire overwound with 0.0015" wire) 'could' be used and the necessary increasein area obtained by placing'moref than two windings in" multiple. The windings must, however; be positioned close together; otherwisev they will not share the current burdfen'properly. Itwill be thus apparent that by using multiplewindings of fine wire, thejratio ofelectron emitting area to mass. will beg'r eatly increased with an attendant considerable decreas in the heat capacity for the same efiectiveemitting area- The Fig. -2v shows the cathode of Fig. 1 in greater detail and 'is' exemplary of the cathode construction just described. The cathode coil 9 comprises a spiral of inside diameter 0.060"; and having about- 20 double turns per inch. As de scribed, the winding itself comprises the two. wires 10- and IL each' consisting of-six: mil

' causes a greater initial voltage to be induced in the heating coils.

mbit tungsten wire overwound with 1.5 mil wire H) (as shown in Fig. 3). We have found that at least about two of these wires 10 and II are necessary for each ampere of arc current. The dimensions herein given were designed for heating voltage supplies equal to 3.5 volts (3.5 amperes) during starting and 1.7 volts (1.8 amperes) during operation, corresponding to wattages of about 12 and 3 watts respectively. The added wattage during starting aids in the rapid heating of the cathode. It is brought about by placing the cathode heating coils, 24 in the secondary circuit of the power supply circuit to be described below. Here advantage may be taken of the effect of the resonant circuit 38-3! which The end of wires l0 and H are clamped to metal shields l2 and I3 as shown 5 cles of insulatingmaterialused in-manu-facture.

at M. In this manner they are electrically conwhich shows an enlarged section offone of'the wires I For I l the area'is indicated, by the dotted lines, the uncovered portion of the 6 wire being takerr'as "the "projection af b "of the space separating thetu'rns 'of the overwound smaller wire on the"core.,""Actually, the'area willvary withthe'amountof coatingmaterial picked up during the coating process and Wilialso depend =onfhow'muchoi this material has been knocked ofi"during operation. The approximation will,

however; sufiice for rough calculation.

Taking the density of tungsten"to be18.8 a-ms'per' cubic centimeter and the specific heat to'be' 0:084 calorie per gram," the ratio of efiective area in square centimeters tomass in grams is about 1-3,"and the'ratioof effective area in square centimeters to heat capacityin calories about 380'. Useful results can'be obtained when the former of these two ratios is as low as 6, although I have-set 13* as" a practical" lower limit. I would prefer the ratio to' be as higl-r H as 17- to--20-, or higher. Corresponding values bf the area-heat capacity ratio are' 1-76 for useful results and-500 to 590; or-higher', 'for the'preferred "range." It will be understood of coursethat -these ratios are calculated for-tubevoltages' oi the: order of those specified herein, :and that lower ratios' m'ay be used where the 'tube voltages are: such; that the rateof.- positive ioribombardmenti of the cathode WhBnI'COldT is: less. Conversely high ratios *will generally be required: where: thetube voltages are; suchthat they.- 'tend to produce. a greater rate; ofibombardment.

- -A' second factor contributingto the lowlife and early blackening; of lamps-of this type was found to baa: iorm cf "sputtering- 0f loose material from the-anodes andtheir leads. *The 'correetion 'of this; :defect; cannot be ignored lif-r a lamp; of reasonably; long life is. to be obtained.

'Underordinary conditions, a quantity of loose and-decomposable material exists on the anode and its: leads. It. maybe foreign matter of the material: of-w-hich theanode or the: leads are'conr structed. It mayeven=becomposed of-loiosepatti- Under the influence of high velocity ions in the arc, particles of this loose material sputter to the wall of the envelope and thereby add to the blackening. The effect is particularly harmful where the anode voltage is applied before the cathode has been thoroughly heated, as is the case with the lamp of my invention. The reason for this lies in the fact that a much higher voltage gradient will exist in the lamp before the cathode is heated and, therefore, the positive ions which bombard the anodes will have much higher energy. This defect can be largely overcome by a preliminary ion bombardment of the anodes for the purpose of cleaning up the excess loose material on their surface. Ordinary heating of the anodes will not accomplish this result, since the mere process of heating does not impart sufiicient energy to these particles to cause them to leave the anode surface. The high speed positive ions, on the other hand, apparently have sufficient energy to knock the. loose material oif the cathode. ing this preliminary bombardment the action be accomplished by ions having velocities substantially in excess of those which exist under the ordinary operating conditions of the lamp. This can be accomplished by a low pressure discharge between the two anodes. Thus, by reducing the argon pressure to the order of 0.2 to 0.7 mm. and passing a discharge of current strength in the approximate range of 5 to rnilliamperes at 1700 volts between the anodes, a good cleanup can be effected within several minutes. At still lower pressures or higher currents the cleanup can be effected more rapidly. The blackening resulting from this treatment can be burned out of the tube during the subsequent exhaust operations by heating the tube in the presence of air or oxygen. This preliminary treatment greatly decreases the depreciation of the lamp.

The improved method whereby the cathodes are made relatively free of bombardment disintegration gives rise to still a third method of simplifying the auxiliary circuit. Hitherto it has been known that low gaseous pressure was conducive to greater efficiency. However, a practical lower limit was placed upon the pressure by the extremely rapid disintegration of the cathodes at low pressures. In the past it has been found that a compromise between these two factors was best obtained at about 2.2 mm. The improved cathodes of my invention have lowered this practical limit still further and, as a result, I find that I am able to lower the starting voltage sumciently to permit elimination of a high voltage starting winding of the type shown in the aforementioned Foulke patent. Thus, by lowering the argon pressure to the range of about 1.2 to 1.7 mm. I found that reliable starting could be obtained without the high voltage coils. The tube in this case had a length of 52 inches between electrodes and an internal diameter of 1.1 inches. However, as the ratio of tube length to tube diameter is decreased, the pressure at which this same starting voltage is effective to produce reliable starting decreases about in proportion to that ratio. While it is possible to operate below pressures of 0.7 mm., no gain of efficiency is obtained thereby. The omission of the high voltage starting winding or coil will be readily apparent from a consideration of the drawing.

Returning now to a consideration of the operating circuit 8 of Fig. 1, the arrangement shown is generally the same as that shown in the Foulke Patent No. 2,177,883, but is somewhat simplified.

It is necessary, however, that dur In this circuit, the lamp is supplied from a source is of single phase alternating current which, for example, may be a 60-cyc1e, -volt lighting circuit, through the structure represented generally by numeral 8 and comprising a high reactive transformer and smoothing reactor. The core of the. structure comprises two E-shaped members i9 and 20 reversely arranged with respect to each other and with corresponding legs spaced from each other to form magnetic gaps. In the drawing these gaps are shown for simplicity as air gaps, but as the apparatus is manufactured they comprise a suitable non-magnetic material against which the two members are clamped. Between the legs of the member H! are magnetic shunts shown at 2| which increase the flux leakage to provide high leakage reactance.

On each of the outer legs of the core structure isa primary winding consisting of coils 22, a main secondary winding consisting of the coils 22 plus the coils 23 thereby constituting an auto transformer, and a separate auxiliary secondary winding comprising the coils 24 for heating the cathode ,5. The ends of the primary are thus composed of the two coils 22 and are connected by the leads 25 and the switch 26 with the opposite sides of the source It. The opposite ends of the'main secondary composed of the two coils 22 and the two coils 23 connect with the two anodes G-0 while the mid point thereof connects with the cathode. The latter connection includes the conductor 21 and the auxiliary secondary made up of the two coils 24, the purpose of which is to supply heating current to the lamp cathode. The coil 30 and condenser 3i comprise a resonant circuit shunted across the, transformer output and hence across the anodes. They serve to further increase the voltage output with the result that greater voltage output is obtained without resorting to higher winding ratios. At the same time theyserve to increase the initial input to the cathode heating coils 2 3 as already stated.

As shown by the drawing, the magnetic shunts 2| are arranged adjacent the upper ends of the primary coils and extend between the outer legs of the core structure and the middle leg 28 thereof. They are spaced from the legs by magnetic gaps which preferably comprise a suitable nonmagnetic material. It will be seen that by this arrangement the amount of leakage of the primary flux is greatly increased since the shunts provide a relatively low reluctance leakage path. The net result is that the leakage reactance between the primary and the secondary is greatly increased. The middle leg 28 also provides a return path for the unidirectional component of the flux in each of the two outer legs during the operation of the lamp, such a component being present by reason of the fact that the full wave rectifying action of the lamp renders the secondary current unidirectional. By reason of the middle leg, moreover, an exceptionally high reactance is obtained between the two halves or coils of the secondary winding because of the magnetic shunt effect thereof, whereby they are virtually isolated from each other.

The reflector 29' is provided for the purpose of facilitating the starting of thelamp. It is electrically connected to one of the anodes by the high impedance capacitor 32 (0.005 mt), the effect of the connection being to lower the reflector potential with respect to one anode and increase its potential difference with respect to the other. The starting of the lamp is then aided by a capacitative discharge which tends to take place hetweenthereflector and the latter of the twoeanodes. This I starting operation is aided simultaneously by a lowcurrenti cross discharge between the .two anodes.6-L around the bafiie 1.

Returning now to Fig. 2 anda consideration of the shields l2 and; .therein.shown,.it may be notedthat ordinarily at pressures of..l.5 mm. or thereabouts, most of .the blackening .that does occur .in the cathoderegion: spreads out. in the tube in a diffuse mannerl Inaddition, at a short distance from the cathode a Faraday dark space forms and in the usual lampof this type further discoloration deposits in a fairly well defined ring in this space. I have found that by placing :the cylindrical shields l2 and I3 around the cathode, one for each end of the cathode-coil, the Faraday dark space. could be largely eliminated. and the black ring on the tube thereby confined closely to the neighborhood of the cathode. Amethod of eliminating the Faraday .dark space is described in the Foulke Patent No. 2,020,723. The diameter of: the shield should not be small relative to the cathode, otherwise a secondary discharge will occur in the opening and reform-the Faraday dark'space. For example, with the cathode of the dimensions already described, the shields may each have the dimensions of about inch in diameter and about 1% inch in length. In other words, a shield l2, l3 should have an internal opening or diameter not less, substantially, than about four times the outside thickness or diameter of the associated core member or cathode coil 9.

The material of the shields is preferably nickel;

They may be formed by bending a flat nickel strip into the form of acylinder and spot welding the folded endtabs 30 to the leads and cathode coil.

The operation ofthe improved lighting-unit hereinbefore described will be as follows: Upon application of voltage to the cathodes and anodes simultaneously, a heating current will pass through the cathodes to heat them to their rated operating temperature, while at the same time a starting discharge is initiated by the action of the shield or reflector plate 29 as already described. Since the cathodes will heat substantially instantaneously, they will not be damaged by the immediate application of the anode voltage to the tube, as has been the case with lamps of this general type in the past.

It will be understood that the above described circuit and lamp is illustrative of but one modification of my invention and that many variations in the circuit constants and in the form of the circuit and the lamp itself may occur to those skilled in the art to which my invention appertains. For example, it is not necessary that the lamp be of the full wave rectifying type as shown. It may be of type having a single activated electrode at each end, each of these serving alternately'as cathode and anode on each half cycle of alternating current. Further, since it is not essential that the lamp operate on alternating current only, it may be of the direct current type. All such modifications I aim to include within the scope of the appended claims.

What .I claim as new and desire to secure by Letters Patent of the United States is:

l." The'combination with an electric discharge device comprising anenvelope containing an ion izable gaseous atmosphere, associated :anodes,:and

of a metallic shield electrically coupled toione of '7 said electrodes and extendingalong adjacent but outside the envelope, a resonant circuit for applying across thegap between said cathode and anodes a no-load voltage suficiently higher than discharge-maintaining value to initiate discharge with the aid of said shield, and for also applying the lower. discharge maintaining voltage across said gap when the discharge current flows inthe lamp after starting, and. means of connection from said resonant circuit to said electric heating means for applying through the latter, concurrently with the application of discharge initiating voltage across the electrode gap, voltage sufficient to bring the cathode to a temperature ofsubstantial electron emission substantially instantaneously.

2. A positive column electric discharge lamp comprisinga tubular envelope containing an ionizable gaseous atmosphere at low pressure and cooperating spaced apart discharge electrodes whereof at least one is an activated filamentary electrode extending transversely of the envelope tube, and two slightly spaced apart shields substantially coaxial with said filamentary electrode and together defining a cylinder substantially completely surrounding the filamentary electrode and open at both ends, each of said shields being electrically connected to an end of said electrode.

3. A cathode for a low pressure positive column electric discharge comprising a plurality of activated fine wire helical coils arranged in parallel side-by-side relation and connected in parallel across and between a'pair of current lead wires :and each overwound with finer wire for augmenting its activated surface, and characterized by a ratio ofeffective electron emitting surface in square centimeters to heat capacity in calories of the order of substantially 176 to 590.

4. A'cathode for a gaseous electric discharge comprising an activated filamentary electrode and two slightly spacedrapart shields substantially coaxial with said filamentary electrode and together defining a cylinder substantially completely surrounding the filamentary electrode and open at bothends, each of said shields being electrically connected to an end of said electrode.

5. A cathode for an electric gaseous discharge device comprising: a filamentary electrode; a coating of electron-emitting material on said filamentary electrode; and two cylindrical shields coaxial with-said filamentary electrode and spaced longitudinally from each other, each of said shields being placed at, and electrically connected to, amend of said.filamentary'electrode, and each of said'shields being open at both of its ends.

6. The combination ofclaim 5 in which the two cylindrical shields extend beyond the ends of said filamentary electrode.

7. A cathode for a gaseous electric discharge comprising .an activated elongated electrode to beheatedto emissive temperature-and a pair of longitudinally spaced apart cylindrical shields coaxial with said electrode and having an internal opening not less, substantially, than four times the thickness of'said electrode,.each of said shields surroundingand being electrically-connected to an end of said electrodaand being open at both of its own ends.

8. An electrode for coasting with another electrode-in a gaseous electric discharge, comprising a' plurality oi-activated fine wire coils wound and connected in parallel and extending transversely of the direct path between said coasting electrodes,and two slightly spaced apart shields substantially coaxial with'said .coil electrode and.

together defining a. cylinder. substantially com- 9 pletely surrounding the coil electrode and open at both ends, each of said shields being electrical- 1y connected to an end of said coil electrode.

TED E. FOULKE.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,951,112 Wels Mar. 13, 1934 2,177,883 Foulke Oct. 31, 1939 Number Number

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