US2097297A - Mercury vapor discharge device - Google Patents

Description

Oct. 26, 1937. w. L. MEIER 2,097,297

MERCURY VAPOR DISCHARGE DEVICE Filed Sept. 22, 1933 FIG. 2

A.C. LOAD RELATIVE SHIELDING EFFECT DISTANCE 'BETWEEN GRIDS INVENTOR WILBER L. MEIER BY 20 30 4o 50 so 70 so 9 .w cw

. CONDENSED HG. TEMPERTURE IN 0 c. ATTORNEY Patented Oct. 26, 1937 PATENT OFFICE MERCURY VAPOR DISCHARGE DEVICE Wilber L. Meier, Arlington, N. 1., assignor to Radio Corporation of America, a corporation of Delaware Application September 22, 1933, Serial No. 690,514

6 Claims.

My invention relates to electron discharge devices, and more particularly to grid actuated hot cathode vapor tubes or vapor converters.

One commonly used vapor tube of this type has a highly evacuated bulb containing a small amount of mercury and enclosing a hot cathode, an anode, and an interposed grid electrode. In such a tube the grid can start the current flow, but cannot control or stop it, and regains control only if the current stops, as it will do when the voltage on the anode becomes zero or negative.

For a given anode voltage there is a definite and critical grid voltage at which the current flow starts, and the commercial tubes are usually so designed that this critical or breakdown grid voltage is a few volts positive.

Tubes of this kind as usually constructed for commercial purposes operate most satisfactorily and with the highest efficiency at temperatures of around 60 C. The most desirable operating temperature is within the temperature range of from 55 C. to 70 C. where these tubes have the peculiar characteristic of a double breakdown grid voltage, and may break down at either the designed breakdown positive grid voltage of several volts positive, or at a slightly negative grid voltage. The shift between these very different grid breakdown voltages is erratic and uncontrollable, is governed by various obscure factors, and is a serious disadvantage in some commercial applications of the tubes.

Tubes of the type described are used generally for various purposes and in various circuits. In one desirable application to self-starting converters for changing direct current to alternating current, the commercial tubes constructed in the usual way and having the double breakdown grid voltage characteristic frequently break down to pass current before the cathode has reached its full emission, so that not only is proper operation of the circuit interfered with, but the cathode of the tube may be destroyed. Another disadvantage of this type of tube as usually made is that the grid current at starting is comparatively high and increases with the life of the tube. This property makes diflicuit the design of an emcient circuit which will operate with the tube, and the increase during the life of the tube in the current taken by the grid at starting requires continual adjustment of the circuit constants in order to have the circuit operate properly.

An object of my invention is to provide a grid controlled electron discharge device of the type described in which the grid starting voltage is substantially constant for any operating temperature.

Another object of my invention is to provide an electron discharge device of the character described which has definite positive grid starting voltages over a wide range of anode voltages.

Still another object of my invention is to provide a device of the character described in which the current taken by the grid at starting is small to permit eificient and easily designed associated circuits and in which the grid current at starting remains substantially constant during the iii of the device.

To these ends and in accordance with my invention the single control grid heretofore used is replaced by two equi-potential perforated gridlike metal sheets or inserts mounted in parallel planes with the perforations in the grid-like inserts and the spacing between the inserts having definite and fixed relationships.

The novel features which I believe to be characteristic of my invention are set forth with particularity in the appended claims, but the invention itself will best be understood by reference to the following description taken in connection with the accompanying drawing in which:

Figure 1 is a perspective longitudinal section of an electron discharge device embodying my invention;

Figure 2 shows a schematic diagram of one circuit arrangement using electron discharge devices of the type shown in Figure 1.

Figure 3 is a chart showing the relationship of the grid starting volts and the condensed mercury temperatures for a single grid electron discharge device of the type described and for an electron discharge device embodying my invention;

Figure 4 is a diagram showing the relative shielding effect of the control grid with respect to the distance between the grid inserts in an electron discharge device embodying my invention.

Referring to the drawing the evacuated dome type envelope l0 has the usual stem and press II, and the usual base l2. The envelope contains a small amount of mercury I! to provide the mercury vapor which gives the tube its peculiar grid control characteristic. Supported within the envelope from the top is the diskshaped anode ll electrically connected to the cap IS on the outside of the envelope. The cathode Ii is preferably of the well known multicellular oxide coated type, heat shielded except for the open end from which the electrons emerge. and has a heater I 1 supplied with current in the usual manner by conductors I 8 extending through the press II. The cathode assembly is supported on the press II by side rods IS.

The grid or control electrode comprises two perforated sheet metal discs or grid inserts 20 and 21 electrically connected and mechanically supported in spaced parallel planes perpendicularly to the longitudinal axis of the tube by the perforated heat radiating metal collar or sleeve 22, which at one end overlaps the cathode i6 and at the other end overlaps the anode Hi. To provide a tube having a single breakdown voltage the spacing between the grid inserts 20 and 2| should be approximately equal to the diameter of the perforations in the grid inserts. For practical reasons this spacing is usually made slightly greater than the diameter of the perforations when the tube is assembled. The perforations in the grid inserts are in actual or eifective alignment which permits current flow from the cathode to the anode.

It has been found that for 110 volt operation a device of the character described operates most satisfactorily if the grid inserts are spaced about 100 mils. apart and each has about nine uniformly spaced circular perforations per square inch, each perforation being about mils. in diameter. The anode is about five millimeters from the top insert and the top of the cathode is about eleven millimeters from the bottom insert. As the perforations are made smaller the grid voltage necessary to cause breakdown of the tube increases sharply, which is undesirable as the correspondingly increased grid current reduces the efliciency of the associated circuits and makes their design very difflcult. As the perforations are made larger the tube loses its positive grid breakdown characteristic and breaks down at negative grid voltages. which is not desirable for commercial uses.

This double grid arrangement acts to shield the anode more effectively than the usual single grid and eliminates the double breakdown voltage characteristic of the single grid tube. The grid electrode structure is supported by grid side rods 23 secured to a collar 24 which is clamped on the stem I I. Theupper end of the cathode is tied to the grid by the glass bead spacers 25 between the cathode and the side rods 23.

In Figure 2 is shown one application of electron discharge devices embodying my invention as used in a, self-starting converter circuit. The transformer 26 has a primary 21 and a secondary 28 to which the load 29 is connected, the primary 21 of the transformer being connected to a D. C. supply line through a pair of electron discharge devices [0 and I0 each constructed as shown in Figure l.

The anodes M of tubes In and I0 are connected to opposite ends of the primary 21 of the load transformer, and are also connected "to each other through a commutating condenser 30 which is in shunt with the primary 21 of transformer 26. The cathodes it of the two tubes are connected by cathode leads 3| to the negative side of the D. 0. supply lines to which the heaters II of the two tubes are connected. The midpoint of the primary 21 of the transformer 26 is connected to the positive side of the D. 0. supply line through a switch 32 and an inductance 33 which prevents alternating voltages being impressed on the direct current supply lines.

The device is started by closing the switch 32, thereby energizing the cathodes l6 and impressing an operating voltage on the anodes ll of the two tubes through the inductance 33 and primary 21 of transformer 26. If the grid 22' to the negative side of the D. 0. supply line. The

resulting rush of current through the primary 21 will induce a volt ge in the secondary 28 and also build up a vol age difl'erence between the ends of the primary winding 21 which is in effect an autotransformer which impresses on the condenser 30 a charge of about twice the supply voltage, minus the drop through the inductance 33 and the drop through the tube III with the right hand plate of the condenser 30 positive and at the higher potential.

If now a breakdown voltage is applied to the grid of the tube I0 the breakdown of the tube I0 and resultant flow of current through it will in effect connect the right hand highly positive plate of the condenser 30 to the negative side of the supply line through the cathode lead 3| of the tube Ill. The voltage on the right hand plate of the condenser drops from the positive value to which it was charged down to a voltage substantially equal to the small drop through the self-sustaining discharge in the tube. The left hand plate of the condenser 30 sufiers instantaneously an equal drop on account of the high transient impedance of the transformer winding compared to that of the condenser, and since it was already at a very low positive potential this left hand plate of the condenser goes negative, and causes the anode M of the tube Ill to which it is connected to go negative momentarily, thus stopping the discharge through the tube In. The self-sustaining discharge in the tube Ill permits current to flow through the right half of the primary 21 and the condenser 30 will be reversely charged with the high positive potential on the left hand plate. Tube I again becomes conductive when a breakdown voltage is applied to its grid, whereupon the anode of tube Ill goes negative and the self-sustaining discharge in that tube stops. Alternately impressing breakdown voltages on the grids thus causes the current to flow alternately and in opposite directions through the primary 21 of transformer 26 thus inducing an alternating voltage in the secondary 28.

In order to impress a breakdown voltage on the grids of the two tubes alternately, use may be made of the grid voltage control circuit shown in Figure 2, and comprising condenser 3|, and a resistor 32, connected in series with the primary 33 of a transformer 34 which is in shunt with one-half of the primary 21 of transformer 26 and has a secondary 35 with its opposite ends connected to the grids 22 of the two tubes through 4:5' resistors 36. Normally, a su'flicient positive voltage to cause breakdown of the tube is impressed on the grids 22 of both tubes by a potentiometer 31 connected across the supply lines.

In a tube embodying my invention as the mercury temperature increases, the grid voltage necessary to produce breakdown decreases slightly up to some predetermined temperature and then has a substantially constant value. The

potentiometer is, therefore, so adjusted that the 75 voltage applied to the grid to cause the tube to break down is as small as possible and is such that for any particular operating conditions, on starting up the inverter, the cathode will reach full operating temperature and emission before the mercury reaches a temperature at which the applied grid voltage will cause breakdown of the tube. Thus no time delay devices are n to apply the anode voltage after the cathodes have heated up.

After the switch 22 is closed, due to slightly difi'erent characteristics inherent in tubes of the character described one of the tubes will break down before the other and pass current. Assuming that tube I breaks down and passes current, current will fiow through the left half of the primary 21 of transformer 26 as explained above.

Since the grid voltage apply ng circuit is connected across one-half of the primary 21 the voltage which is induced in this half of the primary as the current therethrough builds up to a maximum, is applied to the grid voltage applying circuit. This induces a voltage in the secondary 35 of the transformer 34 such that a negative voltage is applied to the control electrode 22 of tube It! thereby preventing breakdown of this tube at this instant. As the current'through the tube l0 approaches a maximum the induced voltage in the primary 21 decreases. Since this is the voltage reliedupon to energize the primary 33 of the transformer 34, the induced voltage in the secondary 35 also drops thus permitting the electrode 22 of tube III to become positive and break down to pass curent. when the tube ill passes current the left side of condenser 3| drops to a negative value as explained above to momentarily cause the anode ll to become negative, thus interrupting the flow of current through the tube ill. Since the induced voltage in primary 21 of transformer 26 is now reversed, the voltage applied to the primary 33 of the grid transformer 34 is also reversed applying a negative bias to control electrode 22 of tube Ill which will prevent this tube from breaking down at this instant. When the induced voltage in secondary 21 drops, electrode 22 of tube Hi again goes positive and causes tube In to break down repeating the operation described above. The current flowing alternately through. opposite halves of the primary 21 of the transformer 26 in this manner, induces an alternating voltage in the secondary 28; The circuit comprising the condenser 3|, resistor 32 and transformer 34 is so designed as to give the proper time and phase relation between the anode and grid voltages to bring about the operation described.

In Figure 3 the curves "0 and b show the grid breakdown voltages for different mercury temperatures for a single grid electron discharge device of the type described. These curves are only approximate and vary with different tubes and with the age of the tube, the curves a and b approaching each other as the tube ages and in some instances overlapping. Curve "0 shows the definite and substantially constant grid breakdown voltage for an electron discharge device embodying my invention. In asingle control grid tube in the region outlined by curve "a" very little or no grid breakdown voltage is necessary to cause the tube to break down. Hence with a single grid tube in the inverter circuit and at the normal operating temperature of around 60 C.,

the potentiometer which is depended upon to apply the breakdown grid voltage would be rendered useless to prevent breakdown of the tube tube to'the other would not take place and the tube which first broke down and passed current would cause a short circuit through that tube with the resulting instantaneous destruction of its cathode. In tubes having a single grid breakdown voltage characteristic such as the tubes embodying my invention the potentiometer can be properly adjusted so that the tubes will not break down until the cathode of the tube has reached its full emission state as pointed out above. Thus a tube embodying my invention provides the automatic time delay characteristic which makes a self-starting inverter circuit feasible.

In tubes having a positive grid breakdown voltage, a certain amount of grid current must flow before the tube will break down. A tube embodying my invention requires a smaller grid starting current than the conventional single grid tube and this current remains substantially constantduring the life of the tube which was not true of the single grid tubes. The smaller grid current results in greater efiiciency of the circuits used with the tubes since the power taken by the grid circuit is decreased. The constant grid current during the life of the tube makes it possible to determine the constants for a particular circuit without requiring change during the life of the tube. This also makes it possible to substitute tubes embodying my invention for each other in a circuit without making any necessary adjustments for the newly substituted tube.

The relative shielding effect of the double grid electron discharge device as the distance between the grid inserts is increased is shown graphically in Figure 4. The distance d is equivalent to the diameter of the perforations in the grid inserts. This curve indicates that as the distance between the grid inserts is increased the shielding of the anode becomes more eflective and the double breakdown grid voltage becomes less likely, this double voltage characteristic being practically eliminated when the distance between the grids is equal to the diameter of the perforations in the grids.

While I have indicated the preferred embodiments of my invention of which I am now aware and have also indicated only one specific application for which my invention may be employed, it will be apparent that my invention is by no means limited to the exact forms illustrated or the use indicated, but 'that many variations may be made in the particular structure used and the purpose for which it is employed withoutdeparting from the scope of my invention as set forth in the appended claims.

What I claim as new is:

1. An electron discharge device including an envelope containing an ionizable medium for supporting a self-sustaining discharge through said device and enclosing a thermionic cathode, an anode, and an equipotential control electrode interposed in the path of discharge between said cathode and anode and comprising a pair of parallel metal sheets. each having a plurality of circular perforations of uniform diameter and mounted with the perforations in efiective alignment, the spacing between said sheets being sub- 5 stantially equal to the diameter of said perforations.

2. An electron discharge device including an envelope enclosing a thermionic cathode and an anode, and containing an ionizable medium for supporting a self-sustaining discharge through said device, and a control electrode including a pair of discs electrically connected and each having a plurality of circular perforations, said discs being positioned in parallel planes with the per-- forations in eil'ective alignment, and spaced substantially equal to the diameter of said perforations.

3. An electron discharge device including an envelope containing an ionizable medium for supporting a self-sustaining discharge through said device, and enclosing a thermionic cathode, an anode, and a control electrode interposed between said cathode and anode and including a tubular member with its longitudinal axis in alignment with said cathode and anode, and a pair of parallel transverse discs inside of and intermediate the ends of said tubular member, each of said discs having a plurality of circular perforations, and said discs being spaced apart a distance substantially equal to the diameter of said perforations and with the perforations in said discs in effective alignment.

4. An electron discharge device including an envelope containing an ionizable medium for sup-,

porting a self-sustaining discharge through said device and enclosing a thermionic cathode, an

anode, and a. control electrode including a tubular member registry with said cathode and 4 said anode, and a pair of parallel transverse perforated discs intermediate the ends of said tubular member and spaced from each other substantially the diameter of the perforations in said discs and with the perforations in said discs in effective registry, said anode and said cathode being mounted at opposite ends of said tubular member.

5. An electron discharge device comprising a highly evacuated envelope containing mercury and enclosing a thermionic cathode, an anode, and a unipotentiai control electrode comprising a tubular member having a pair of parallel perforated metal sheets mounted with their perforations in registry to extend transversely of the discharge path between said cathode and anode. the spacing between said sheets being substantially equal to the diameter of the perforations in said sheets.

6. An electron discharge device comprising a highly evacuated envelope containing mercury and enclosing a. cylindrical oxide coated thermionic cathode, a flat circular anode in registry with the end of said cathode, and a unipotential control electrode comprising a perforated metal sleeve mounted to surround the discharge path between said cathode and said anode with one and overlapping said anode and the other end overlapping the end of said cathode, and a pair of parallel perforated metal discs set inside and transversely of said sleeve with a spacing substantially the sameias the diameter of the perforations in said discs and with the perforations in said discs in registry.

WILBER L. MEIER.

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