US2714681A - Electric discharge device - Google Patents

Description

A118. 2, 1955 R. L. KEIFFER ET AL 2,714,681

ELECTRIC DISCHARGE DEVICE Filed Aug. 27, 1948 2 Sheets-Sheet l Fig 6. F3957 i Inven tors Gaavge E. Inman, Raymond L. Kei++e1-,

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United States Patent 2,714,681 ELECTRIC DISCHARGE DEVICE Raymond L. Keiifer, Chagrin Falls, and George E. Inman, East Cleveland, Ohio, assignors to General Electric Company, a corporation of New York Application August 27, 1948, Serial No. 46,506 1 Claim. (Cl. 313-109) Our invention relates to electric and more particularly to low pressure electric discharge devices such as low pressure fluorescent lamps.

In some types of low pressure electric discharge devices, such as low pressure fluorescent lamps, it has been observed that a lamp itself produces a considerable amount of radio interference. That is, the change in the current flowing between the electrodes of the lamp, and the associated phenomenon, apparently have caused the production of electro-magnetic radiation, which has a low frequency 240 cycle hum when the lamp is operated on a commercial source of 60 cycle alternating current. The reason is that apparently during each cathode half cycle on a 60 cycle power source two bursts of radio frequency energy occur, each for a duration of about 0.1 millisecond. The two times of occurrence are apparently coincident with the transition time between the low-current discharge condition and the high-current discharge condition.

The present invention concerns the construction of electric discharge devices and lamps, and methods of operating the same, to reduce or minimize the production of such undesirable high frequency oscillations which produce radio interference.

It is an object of our invention to provide a new and improved electric discharge device.

It is another object of our invention to provide a new and improved low pressure fluorescent lamp which does not produce appreciable or noticeable radio interference.

It is a further object of our invention to provide a new method of operating low pressure electric discharge devices employing thermionic electrodes.

Generally speaking, in accordance with our invention we provide a new and improved fluorescent lamp of the low pressure type wherein the principal medium for supporting an arc discharge is a metallic vapor, such as mercury vapor, and in which the filling or starting gas composition and pressure are determined to control and minimize the generation of electromagnetic radiations by assuring the continuous and gradual change of cathode discharge current during starting conditions, or during transition between low and high current conditions. For example, we may employ various compositions of inert gases, such as, krypton, Xenon, or mixtures thereof, in combination with argon to provide a gaseous medium having excitation and ionization potentials intermediate that of mercury and argon. In this manner, the electric field or cathode fall phenomenon within the vicinity of the thermionic electrode and which controls the electron emission from the thermionic electrode is controlled not only in magnitude but also as to rate of change in order to minimize the generation of high frequency radiation.

More particularly, we find that the amount of radio interference can be reduced to substantially zero by employing in such an electric device a mixture of inert gases from the group consisting of argon, krypton and discharge devices,

xenon. Where the starting gas composition consists of equal proportions of krypton, xenon and argon, the range of total pressure which was found to be most effective in meeting the requirements of radio interference elimination and reasonable starting and operating voltages is from 1 mm. to about 1 mm. inclusive, and preferably within the range of 2 to 5 mm. inclusive, of mercury. Of course, it will be appreciated that in carrying out our invention we are not to be limited to equal portions of these gases and that substantial variations therein may be made without departing from the spirit and scope of our invention. Furthermore, where the starting gas composition comprises argon and Xenon, we have found that the optimum range of total pressure is from 1 to mm. inclusive of mercury, without incurring an inordinate increase in starting voltage.

For a better understanding of our invention reference may be had to the following description taken in connection with the accompanying drawings and its scope will be pointed out in the appended claims. Fig. l illus trates an embodiment of our invention as applied to a low pressure fluorescent lam Fig. 2 is: an enlarged view showing one type of thermionic electrode which may be employed in the fluorescent lamp shown in Fig. 1; Figs. 38 show the range of radio frequency noises produced by a prior art device; Fig. 9 represents diagrammatically a circuit which we and determination of the amount terference produced by the lamps investigated.

Referring now more particularly to the embodiment of our invention shown in Fig. 1, we have there illustrated our invention as applied to an electric discharge device of the low pressure type, such as a low pressure tric current through the ionizable medium described hereinafter. Within the envelope 1 we provide a plurality of spaced electrodes 3 and 4 which are of the thermionic type. It will be appreciated that in carrying out our invention only one of these electrodes may be of there mionic type, and our invention is applicable with equal facility to such an arrangement. These electrodes are illustrated in detail in Fig. 2 which is an enlarged view of such an electrode. Electrodes 3 and 4 are supported by lead-in wires 5, 6 and 7, 8, respectively, which also serve as electrical connections to the electrodes from external accessible contact pins 9, 10 and 11, 12, respectively, which are supported by bases 13 and 14. The use of two pins at each end of the lamp for connection is, of course, optional, the form and arrangement of such electrode structures depending upon the nature of the electrodes employed and the operating circuits connected thereto.

Within the envelope 1 we employ an ionizable medium, such as a quantity of mercury, indicated by the globule 15; the quantity of which mercury used may be somewhat in excess of that required during normal operation of the lamp; and the pressure of the mercury vapor during operation may range from about 3 to 20 microns, having a cold pressure of about 1 to 3 microns. The stated operating range of mercury vapor pressure is generally related to that value of envelope temperature which is optimum for producing the most efficient conversion of input energy to ultraviolet energy which is generated upon the passage or conduction of the electric current through the mercury vapor.

Fig. 2 is an enlarged View of one type of thermionic electrode which may be employed in carrying out our invention. These electrodes may be of the filamentary type constructed of a refractory metal such as tungsten,

and may or may not be provided with activating coatings, such as that of the alkaline earth metals, that is, oxides or carbonates thereof in suitable mixtures. For example, the electrode may be formed as a coiled-coil, comprising primary turns 16, which are additionally wound into secondary coils 17, and which primary and secondary turns are covered with the activating coating 18. As to whether the activating coating 18 covers the secondary turns and extends therebetween, or covers merely the primary turns of the assembly, is purely optional. The lead-in wires 5 and 6 may be formed in the manner shown to provide anode structures 19 and 20, or these lead-in wires may' be terminated immediately beyond the bends at positions 21 and 22 thereby dispensing with the anode structures if such is desired.

In many of the prior art low pressure lamps employing filamentary electrodes and in which a single filling or starting gas such as argon has been employed, the increase and decrease of current during each half cycle of operation on an alternating current circuit, and the increase and decrease of current upon starting and stopping opera tion of such lamps on direct current circuits has produced noticeable noise or radio interference, due to the fact that such change or arc current has produced electromagnetic radiations of various frequencies depending upon the design and construction of the lamp. For example, referring to Figs. 3-8, inclusive, which are reproductions of oscillographs showing the several radio frequencies generated by lamps superimposed on the current wave shape of the lamps. These oscillographs were, of course, taken from lamps not employing the invention disclosed herein and are used to indicate the nature of phenomena which are eliminated by employment of the present invention. They are characteristic of lamps containing either a single starting gas or a mixture of starting gases not having graded ionization potentials in accordance with the invention. The most readily available example of discharge lamps showing these characteristics are the common commercially available 20, 30, and 40 watt fluorescent lamps containing argon as a starting gas. It is evident from Figs. 3-8 that noise occurs close to zero current value twice during each half cycle over the range of 200 kilocycles to 1500 kilocycles. The sawtooth eifect at the top of the current Wave in Figs. 7 and 8 is noise coincident with anode oscillation and can be made to disappear when the anode oscillations disappear. This latter noise appears in the range of approximately 100 kilocycles to 450 kilocycles and so is not evidenced in the broadcast band, when observing a 40 watt lamp.

By using the testing circuit shown in Fig. 9, by investigating not only the prior art lamps but also lamps constructed in accordance with the present invention, we have found that the amount of noise or the magnitude of the radio interference may vary considerably, such as ranging from about 150 microvolt noise in the broadcast band by use of the prior art lamps, to less than 1 microvolt noise in the same band by using lamps constructed in accordance with our invention. The microvolt noise referred to herein is defined as being the average microvolt noise in the broadcast band. The small amount of radio interference noise produced by lamps constructed in accordance with our invention could not be seen if plotted on curves to the same scale as in Figs. 3-8 and the curve would appear merely as a smooth sinusoidal wave.

In order to produce a low pressure electric discharge device such as a fluorescent lamp employing thermionic electrodes or a single thermionic electrode, and which does not produce appreciable or noticeable radio interference during operation, we have found that we may substantially eliminate such radiation by assuring a gradual and continuous change in the cathode discharge current by using a gas, or mixtures of gases such as krypton, or xenon, or argon to provide a successive range or gradation.

of excitation and ionization potentials, preferably intermediate those of argon and the metallic vapor employed. For example, where mercury vapor is employed we have found that a mixture of krypton, xenon and argon, having a total pressure not less than 1 mm. will assure such a gradual and continuous change in cathode discharge current within the vicinity of the thermionic electrodes during the increase and decrease of arc-path current thereby minimizing or substantially eliminating the generation of radio interference. The upper limit of the total pressure of the starting gas composition is determined by starting voltage and efficiency considerations. As one particular example, we have found that equal quantities of argon, krypton and xenon, each at a partial pressure of 1.17 mm., providing a total pressure of approximately 3.5 mm., are highly satisfactory for the substantial elimination of radio interference. The provision of the plurality of starting gases having graded excitation and ionization potentials eliminates the necessity for a rapid change of cathode voltage drop in the vicinity of the electrode, during the rising current phenomenon and the falling current phenomenon, which is occasioned by the relatively great difference in ionization potentials of mercury and argon in the prior art type lamps. For instance, in the case of a mercury argon lamp, the ionizing potentials of mercury and of argon are 10.4 and 15.4 volts respectively. With this combination, xenon and krypton, whereof the ionizing potentials are 12.1 and 13.3 volts respectively, provide a suitable gradation of ionizing potentials. On the other hand, neon and helium, whereof the ionizing potentials are 21.5 and 24.6 respectively, would be unsuitable be cause the required gradation would not be achieved. By following the principles explained in accordance with our invention, other suitable combinations may be achieved for different metallic vapors. Reference may be made to a table of atomic properties of the elements containing the critical potentials for more complete information as to the gradations possible. A suitable table is contained in the text Gaseous Conductors by I. D. Cobine, McGraw-Hill Book Company, Inc., 1941, pages 579-583.

It has been our observation that the generation of the radio interference is probably caused at the transition points in the arc-path current where the arc characteristic changes from the low-current region to the highcurrent region, and also at the reverse point where the arc current changes from the high-current region to the low-current region as referred to a half-cycle of alternating current operation, which it will be understood, involve the matter of differences between the excitation and ionization potentials of mercury and the starting gas composition employed.

We also believe that this phenomenon which we have discovered is related to cathode heat, speed of ionization of the medium employed, and thickness of the cathode sheath, and that all of these phenomena are affected by the filling gas composition employed.

In lamps constructed in accordance with our invention we have found that the amount of radio interference caused, as measured in terms of average microvolt noise in the broadcast band, can be reduced to values less than 1 microvolt, which characteristic compared to prior art developments or characteristics, is practically a negligible amount of interference.

Our invention is particularly adaptable to electric discharge devices such as low pressure fluorescent lamps employing electrodes which during normal operation, that is, during starting, or upon operation from an alternating current source, undergo a transition from a low-current characteristic to a higher current characteristic at which time there is substantial change or decrease from a high cathode voltage drop to a low cathode voltage drop, which change or sudden rate of change is believed to cause the production of the objectionable radio interference or noise.

While the term thermionic as employed herein is intended to cover not only activated but also unactivated type electrodes, it is within the purvue of our invention to include any electrode structure with respect to which there is the above-described change from a low-current region of operation to a high-current region of operation.

Referring now to Fig. 9, we have there illustrated the testing circuit which has been employed in testing not only the prior art lamps to determine the amount of radio frequency generated thereby but also to test lamps embodying our invention. The lamp is illustrated by the element indicated by the numeral 1, to which power is supplied from a source of alternating current comprising conductors 23 and 24, which of course are shielded. An insulating transformer 25 is connected in the manner shown and power is supplied to the lamp 1 through the inductive ballast 26. A by-passing condenser 27 is con- 7 nected between one terminal of the lamp 1 and ground intermediate the lamp terminal and the ballast 26 to prevent radio frequency current from appearing in the ballast or power lines from where it might be radiated and interfere with the measurements being made. In series relation with the lamp 1 we provide a resistance 28 to obtain a voltage which varies in accordance with the lamp current. A shielded cable is employed to bring this voltage to the input circuit of the noise meter 29 and to the input circuit of the radio receiver 30. Headphones connected to either of these instruments produce the familiar sound of noise. This audio signal is brought to the oscilloscope 31 through an isolating condenser 32 where it is seen as short duration pulses superimposed on the lamp current wave shape. The noise meter 29 shows the microvolt magnitude of the radio frequency being picked up. The maximum time delay in travel of these pulses from the radio to the oscilloscope is several microseconds. The shortest time interval which can be seen on the oscilloscope is approximately A of a half cycle of 60 cycle alternating current, or about 100 microseconds, so the occurrence of the pulses is accurately portrayed with respect to the current wave shape.

An electronic switch 33 is employed to show alternately lamp voltage and the audio pulsed current wave shape, so that the three signals seem to appear at the same time. Thus, lamp current, lamp voltage, and the radio noise generated by the lamp 1 are seen at the same instant on the screen of the oscilloscope. The IR drop method of pickup of the radio frequency energy from the lamp was selected as the method which is least subject to errors, and is a method now generally accepted as being highly accurate in the determination of such noise levels.

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

A fluorescent lamp of the low pressure type comprising an enclosing envelope, a fluorescent material on the inside surface of said envelope, a pair of electrodes within said envelope and at least one of which is a coiled filamentary cathode activated with alkaline earth oxides, and an ionizable medium comprising mercury vapor and a gaseous atmosphere for providing graded excitation and ionization potentials and consisting of argon, krypton and xenon at substantially equal partial pressures and at a total pressure of approximately 3.5 mm. in order to minimize the production of high frequency radiation incident to the increase and decrease of cathode discharge current.

References Cited in the file of this patent UNITED STATES PATENTS 1,726,107 Hertz Aug. 27, 1929 2,346,522 Gessel Apr. 11, 1944 2,407,379 Morehouse Sept. 10, 1946 FOREIGN PATENTS 233,305 Great Britain 1926

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