US2184709A - Rectifier tube - Google Patents

Description

Dec. 26, 1939. .1. BRUIJNl-:s ET Al.

RECTIFIER TUBE Original Filed Aug. 6, 1954 Patented Dec. 26, 1939 RECTIFI'ER TUBE Johannes Bruijnes, Mount Vernon, N. Y., and Daniel Marie Duinker and Johannes Gijsbertus Wilhelm Mulder, Eindhoven, Netherlands, assignors to N. V. Philips Gloeilampenfabrieken, Eindhoven, Netherlands Original application August 6, 1934, Serial No.

August 7, 1933 2 Claims.

The present invention which is a division of our copendingr U. S. patent application Ser. No. 738,758, led August 6, 1934, relates to incandescible-cathode, gas-iilled rectifier tubes, and 5 more particularly to tubes for rectiiying alternating current of medium voltages, i. e., voltages having a peak value of about 250 volts to about 1000 volts and higher, and for current of several ampres up to 100 amps. and higher. 1 Although incandescible-cathode gas-iilled rectier tubes have been Widely used for various applications and for various voltages, commercially successful tubes of this type could be made for only two voltage ranges, namely, for low voltages up to about 150 volts, and for high voltages above about 1000 volts, whereas for intermediate voltages no satisfactory tubes of this type could be made.v The reasons therefor will be explained below. c

In designing commercial rectier tubes the following main three factors must be considered. The tube must have a satisfactory long life, i. e. of the order of one thousand hours and more; must have a high efliciency, i. e., a low internal voltage-drop as compared to the voltage of the alternating current source to be rectified; .and no back discharge should occur in operation, i. e., the tube must withstand the highest inverse voltage which may occur across its electrodes. during operation.

For low voltages there is no difculty in providing a tube which meets the above requirements. To provide a low voltage drop, the gaseous filling of such tubes consists solely or mainly of an inert gas, for instance 4of argon, and the pressure of the gas filling can be quite high withback as long as the voltage to be rectiiied has a low value, if the tube is to stand higher voltages the pressure of the gaseous iilling has to be reduced. This, in the case of tubes having an inert gas filling, results in an objectionable phenomenon, namely, the gradual disappearance oi the gas iilling which is due to the disintegration of the electrodes, especially of the cathode. This disintegration increases with increasing voltages and decreasing gas pressures, whereby thedisintegrated particles of the electrodes occlude the gas 55 within the tube, and deposit on the walls and Divided and this application January 17, 1938, Serial No. 185,456. In

Germany (Cl. Z-27.5)

metal parts of the tube. Such disappearance of the gas filling not only lowers the efficiency of the tubes, but further reduces the protective action of the gas filling. This in turn further increases the rate at which the disintegration of the electrodes takes place and still further accelerates the disappearance of gas. The result is that the usefulness of such a tube is destroyed in a compartively short time.

In low-voltage tubes, the disintegration of the electrodes, because of the low voltages and the protective action of the high pressure gas iilling, is small and the rate of the disappearance of the gas is slow. Also, because of the high pressure of the gas lling, and thus the large quantity of gas in the tube, the amount of gas which becomes occluded by the disintegrated particles is only a small part of the initial gas iilling of the tube. Thus, while in low-voltage, high-gaspressure tubes the disappearance of the gas iilling is negligible, in higher-voltage, low-gas-pressure tubes this phenomenon has been detrimental to the tube. It has been found furthermore that as long as the initial pressure of the gas filling is of the order of several mm. of Hg., the disappearance of the gas during the normal life of the tube is not fatal to the tube; however, with initial gas pressures below a few millimeters the disappearance of the gas materially reduces the life of the tube.

To avoid the disappearance of the gaseous lilling in tubes operating on voltages which require gas pressures below a millimeter to prevent breakdown of the tube, mercury vapor has been resorted to as the gaseous filling of the tube, whereby an excess of mercury, in liquid form, is provided in the tube; the liquid mercury serving as a reservoir to replace the mercury which disappears because of its occlusion by the disintegrated particles oi the electrodes.

The required pressure of the saturated mercury vapor in the tube is obtained by the proper temperature of the mercury, Which in turn, as is in a tube designed for quite high voltages, for instance, for voltages abo-ve 1000 volts, as the internal voltage drop in percentage of the voltage of the alternating current source is not excessive, and also because these tubes are usually designed for small currents, whereby the loss in the tube is not a serious matter.

However, for the intermediate voltages with which the present invention is concerned, namely, above a peak voltage of about 250 volts and up to about 1000 volts, the high voltage drop in the tube is highly objectionable, especially if the tube is for large currents, as this results in a large internal energy loss and a large amount oi heat developed in the tube. It such tubes for intermediate voltages are lled only with an inert gas, the disappearance of the gas filling-because of its initially low pressure-would result in a rapid deterioration of the tube.

On the other hand, low-pressure mercury vapor, as stated provides for too high a voltage drop, and ii the pressure of mercury vapor is increased to about a millimeter or several millimeters, back discharge is likely to occur and at the same time the pressure of mercury at higher pressures is quite sensitive to temperature variations caused, for instance, by changes in arnbient temperature. Thus a rectifier having a gaseous illing consisting solely of mercury vapor at a pressure of the order of several millimeters, is likely to be unstable in operation.

Because or the above difficulties, prior to the present application. gasiilled incandescent cathode rectiiier tubes were not commercially available for rectiiying alternating voltages of about 250 volts to about 1000 volts (peak values) except in cases or certain special. applications where the high voltage drop of the tube was of no consequence.

In an endeavor to provide such tubes, for which there is evidently a great for various purposes, it has already been proposed to use a reservoir of liquid mercury for maintaining a constant mercury vapor pressure, and at the same time, to reduce the voltage drop of the tube and stabilize pressure by adding an amount ci inert gas of such a pressure as has been iound necessary to avoid the disappearance of the gas. However, such tubes which contained mercury vapor and an inert gas, for instance argon, of a pressure of several millimeters, when used Yfor rectifying alternating voltages of a peak value of 250 volts or more, did not give satisfactory results.

This was due to the fact, that in such tubes, the usual conditions of operation, back discharges could not be avoided. in the operation of such tubes for recti'fying alternating currents oi peak value of about 250 volts, voltages much higher than 250 volts can occur. For instance, when using a single three-phase rectier rect-iiying three-phase alternating current having an eective phase voltage of 220 volts and a phase peal; voltage slightly above 300 volts, a peah voltage of 537 volts will occur between the and we have found that under certain conditions a value douhle this voltage may occur in the tube. The difficulty is further increased if the rcctier is used for the rectification of currents of substantial value, especially in tie case oi the high eflicicncy oxide cathode tubes. lin such case because of the large currents, the anode as unes a comparatively high temperature and part or" the electron-emitting material, for in stance barium oxide, which disintegrated 4from the cathode, deposits on the anodes and causes a marked electron emission of the same to thereby promote back discharge. Even if initially the rcotiiier tubes might operate satisfactorily under such conditions, back discharge will set in after awhile.

Thus problem oi devising satisfactory gasnlled incandescent cathode rectier tubes for mediuin voltages remained unsolved.

Applicants have found the very surprising phenomenon that in the presence of mercury Vapor a much smaller quantity of inert gas, or in other words, an inert gas filling of much lower pressure, can be preserved within the tube than has been assumed heretofore, Thus, while argon disirom the tube, if used alone at a pressure of l millimeter, when argon is used in the presence of mercury vapor it does not disappear from the tube, even if the initial pressure of the argon is of the order of tenths or hundredths of millimeters. We do not know the exact reason for such behavior, but believe that a protective action by the mercury-vapor takes place, the indications being that the disintegrating particles of the electrodes occlude only mercury vapor, instead of occluding both the mercury vapor and the inert gas.

More particularly we have found that gas- .illled rectifier tubes having a gaseous lling com- Yising an inert gas, preferably argon or neon at pressure ci' between 0.01 and 0.5 mm. of Hg in case of argon, and from 0.02 to 1 mm. of Hg il@ 4in the case of neon, together with saturated mercury vaporthe pressure of the mercury vapor being preferably the same order of magnitude as the pressure of the inert gas-will retain their initial gas pressure in operation. Especially tubes comprising argon at a pressure of 0.2 to 0.4 or neon at a pressure of 0.3 to 0.8 mm. of Hg, give an all-round satisfactory performance at medium voltages. More particularly the disintegration of the electrodes is prevented, the tubes have a satisfactory life and efficiency and the danger of bach discharge is overcome.

To prevent formation of amalgam, due to the presence of mercury, the anodes of such tubes are made oi or provided with a surface coating of a material which does not form an amalgam with mercury. Preferably the anodes are made of polished graphite, of metal coated with graphite, or a chromium or chromium oxide coating may be used.

To prevent back discharge, further measures are also taken. For instance the supports and lead wires may be air-tightly enclosed with reiractory insulating material, for instance, with steatite. The cathode may also be partly enclosed by a shield to eliminate or at least minimize the deposition on the anode ol disintegrating cathode particles.

In order that the invention may be more clearly understood and readily carried into effect we shall describe same in more detail with reference to the accompanying drawing in which,

Figure l is a side view partly in section, of a three-phase rectier tube embodying the invention, and

Fig. 2 is a section along line 2-2 of Fig. 1.

The rectifier tube illustrated in Figures 1. and 2 is designed for the rectification of a three-phase alternating current having an effective phase voltage of 220 volts, and thus a peak voltage somewhat above 300 volts. The illustrated tube is designed for currents ci about 6 amperes and is shown in reduced scale, the length of the conas shown at I3.

tainer tube being about |7' and its diameter being about 3%.

The tube comprises a container i of vitreous material. Disposed within the container and slightly below the center thereof, is a filainentary cathode 2, preferably an oxide cathode, provided with a suitable oxide coating of high electron emissive properties such as a barium oxide coating.

Cathode 2 is supported by supports' 4 and 3, which are secured, preferably welded to chromeiron discs 5, to the outer face of which discs are welded lead wires fi, the chrome iron discs being air-tightly fused to the glass.

Surrounding cathode 2 is an open-ended cylin drical screen whi may be electrically connected to one end oi' the cathode, for instance by a conductor il connecting screen 'l with support 4. The screen has among others the advantage of reducing the danger of disintegrating cathode particles reaching thel anodes and the glass wall of the tube.

Arranged above cathode 2 are three anodes 9, Ill and I I of similar construction, each being connected to a conductive rod I2, which may be threaded at its end and screwed into the anode, Anode rod' I2 is surrounded by a suitable insulating tube I4, for instance of steatite, which at its upper end I5 is. sealed to a glass tube I6 which preferably forms an integral extension of the glass wall of container I.

Rod I2 has its upper end welded to the inner face of a chrome-iron disc Il, which is fused into container l. Secured to the outer face of chrome-iron disc Il is a terminal connection 29 for the anode II, which is electrically connected thereto through rod I2 and chrome-iron disc Il.

The anode is provided on its upper portion with an annular bore I8, which extends inwardly into an annular bore I9 of slightly smaller diameter. The lower end of insulating tube I4 snugly nts into bore IS, whereby rod I2 is protectedly enclosed by the tubes I4 and i6. At the same time the annular gap formed around tube I4 at bore I8 offers a narrow high-resistant path between the contacting point of tube I4 with the anode and the discharge space. This provision reduces the danger of a glow-discharge being formed at said point.

Surrounding anodes 9, I0 and Illis a cylindrical housing 20, which is closed at the top and open at the bottom. The space within housing 20 is subdivided into three parts by three radial and preferably imperforated partition walls 2I (see Fig. 2), each part housing one of the anodes. Opposite the central portions of the anodes the wall of housing 20 is broken away and replaced by metal gauze screens 22, 23 and 24 respectively. Such arrangement permits the discharge to take place between the cathode and the anode not only through the open bottom of housing 20, but also through the gauze 22, 23 and 24, the latter also facilitating the heat dissipation of the anodes. On the other hand the non-perforated partitions 2| and the non-perforated portions of the housing 20, effectively prevent a discharge between the anodes.

If desired, a suitable potential may be applied to housing Eil by connecting the same to a suitable potential by means of a wire Ztl hermetically led through the container wall at 2B.

A mica disc 2l provided in the upper portion of the tube,-serves to protect the seals from being excessively heated from the discharge space.

It will be noted that the anodes have a sectorshaped cross-section so as to more completely fill the space formed within housing 20. As stated, We prefer to make the anodes of graphite, which is preferably polished on its surface opposing the cathode, or to provide a metal anode which is coated with either graphite or with another substance which does not produce an amalgam with mercury, as chromium or chromium oxide. l l

The gaseous filling of the tube may consist, for instance of neon at a pressure of 0.5 mm. of Hg; a drop of liquid mercury 28 being provided for the generation of mercury vapor, preferably at a pressure of the same order of magnitude as the neon. The desired mercury vapor pressure is obtained by the mercury assuming in operation the temperature giving this pressure, which as is known, can be obtained by the proper disposition of the liquid mercury with regard to the cathode and the discharge.

While we have described our invention in connection with specific applications and embodiments, we do not wish to be limited thereto but desire the appended claims to be construed as broadly as permissible in View of the prior art.

What we claim is:

l. A rectifier tube for rectifying alternating voltages of a peak value of at least 250 volts comprising, an envelope, an incandescible cathode, a cylindrical-shaped screen of conductive material forming a plurality of cavities having sectorshaped cross sections, an anode within each of said cavities and having a sector-shaped crosssection, said screen separating said anodes from each other and substantially enclosing each anode while allowing access of the main discharge thereto, and a gaseous lling comprising saturated mercury and an inert gas.

2. A rectier tube for rectifying alternating voltages of a peak value of at least 250 volts cornprising, an envelope, an incendescible cathode, a screen of conductive material provided with a plurality of cavities each having an opening toward said cathode and an opening at the peripheral surface of the screen, a metallic network covering said latter opening, an anode within each of said cavities, said screen separating said anodes from each other while allowing access of the main discharge thereto, and a gaseous lling comprising an inert gas and saturated mercury vapor.

JOHANNES BRUIJNES.

DANIEL MARIE DUINKER.

JOHANNES GIJSBERTUS WILHELM MULDER.

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