US2591825A - Gas tube circuit - Google Patents

Description

April 8, 1952 E. o. JOHNSON ETAL ,591,8 5

GAS TUBE CIRCUIT Filed Jan. 2. 1951 ATTORNEY Patented Apr. 8, 1 952 GAS TUBE CIRCUIT Edward Oscar Johnson and William Merle Webster, .Ir., Princeton, N. J., assignors to Radio Corporation of America, a corporation of Delaware Application January 2, 1951, Serial No. 203,952

9 Claims. (01. 315-349) This invention relates to improvements insystems for operating gas filled electrontubes, and particularly to an improved system for operating gas tubes with a source of voltage less than that required'to ionize the tube gas. L It has previously been proposed to operate a gas filled electron tube as an element-ina socalled inverter circuit wherein a unidirectional source voltage, less than that required toionize the tube gas, is converted to amplified alternating voltage by interrupting the current flow through an inductor in series with the tube and the voltage source (see e. g" Electronics, August, 1949, page 142). Ifa-circuit of this type is used with a gas tube having a thermionic emitter or cathode, a problem of starting the circuit operation is involved. That is, after the heater circuit is energized, one must waituntil the cathode reaches operating temperature before interrupting the inductor circuit to obtain the desired voltage step-up reaction.- Otherwise, the tube will not be. ready to operate when the inductive voltage surge is supplied thereto, and the circuit will not start functioning. i

It is a general object of the present invention to provide an improved system for operating a In accordance with the invention, the foregoing and other related objects and advantages are attained in, an arrangement wherein a, voltage pulse, generated by interrupting the current through an inductor,; is'stored to subsequently initiate circuit operation when the thermionic cathode of a gas tube has'been brought up to operating temperature. By utilizing a pair of gangedqswitches, the voltage pulse can be generated simultaneously with completion of the cathode heating circuit, thereby allowing an "operator to start the circuit with a single switch- .ing operation.

A more complete understanding of the invention can be had by reference to the following description of illustrative embodiment thereof, when considered in connection with the accompanying drawing,wherein: V

Fig. 1 is a schematic diagram of a gas tube inverter circuit illustrating one application of the principles of the. invention, 1 a

form of gas tube inverter system arranged in accordancewiththe invention.

Referring to Fig. 1 of the drawing, an inverter circuitarranged in accordance with the invention includes a gas tube,.shown as a gas tetrode II]. This tube, which may be a commercial type 2050. tube, for example, has the usual electron emitter or cathode I2 heated by a filament l3, a control electrode 14, ashieldelectrode l6, and a collector electrode or anode I8.

The anode I8 is connected to the cathode. I2 through a work circuit which includes the primary winding 20 of a transformer 22. The work circuit also includes a source of voltage 24 less than that required toionize the tube gas. The control electrode I4 is not used, and can be connected to any other of the tube electrodes as, for example, to the shield electrode 16.

A three-position single pole switch 26 is connected in the Work circuit between the transformer primary winding 20 and the voltage source 24. The same switch 26 also is connected in the filament circuit between the voltage source 24 and one side of the. filament l3. A capacitor 36 is connected in parallel with the tube 10' between the transformer primary winding 20 and thenegative side of the voltage source. 24.

When the movable contact 28 of the switch 26 is moved from the first fixed contact to the second fixed contact 32, current will begin to flow from thesource 24 to the filament l3, andthe latter eventually will bring the cathode l2 up. to operating temperature. However, as the voltage of the source 24 is less than that required to ionize the tube gas, substantially no current will flow in the work circuit.

In accordance withhthe invention, a starting circuit is provided in order to start current fiow through the tube Ill and through the work circuit as soon as the cathode I2 is heated. The starting circuit comprises an inductor 38, a recti fier 40, and a capacitor 42, arranged to be connected in series across the voltage source 24 I when. the switch 26 is actuated to complete the heater and work circuits; The rectifier 40 may be a crystal or copper oxide rectifier, or any equiv.- alent adapted for cold cathode" operation. A particularly suitable element with very high back resistance" is a neon bulb. The starting circuit further includes a second single pole three position switch 44 having a movable contact 46 mechanically coupled to the movable contact28 of the first switch 26. The second switch 44 is provided to by-pass therectifier 40 and capacitor 42 by connecting the inductor 38 directly across the voltage source 24 through a current limiting resistor 54. To this end, the second fixed contact 58 of the switch 44 is connected to the resistor 54, while the first and third contacts 48, 52 thereof are not connected to any circuit element.

When the movable contacts 28, 46 of the switches 26, 44 are moved downwardly from the first fixed contacts 30, 48 to the second fixed contacts 32, 50, current simultaneously will begin to flow through the inductor 38 and through the filament I3. Immediately, the contacts 28, 46 can be moved downwardly again to engage the third fixed contacts 34, 52. This action will leave the filament I3 connected across the source 24 to continue heating the cathode I2. However, the circuit through the inductor 38 and the resistor 54 will be opened.

As is well known, when an attempt is made to change the current through an inductor suddenly, an induced voltage will develop across the inductorof polarity such as to oppose the at-. tempted change. Thus, when the inductor circuitthrough the switch contacts 46, 58 is opened, there willbe applied to the series combination of the. rectifier 40 and. the capacitor 42 a resultant voltage equal to the sum of the source voltage and the voltage induced across the inductor 38.. This resultant voltage will send a current pulse through the rectifier 4.8 and the capacitor 42, charging the capacitorto a voltage which may be several times that of the source 24. Theoretically this voltageis where I is the initial, interrupted current; L the coil inductance; and C the condenser capacity. Theoretically, this can be thousands of volts but practically will never get over several hundred volts due to sparking at the switch, etc. It is readily apparent that the voltage stored in the capacitor 42 will be more than ample to ionize the tube gas.

In accordance with the present invention, the voltage stored in the capacitor 42 is to be used as a starting voltage to cause initial ionization of the gas in the tube I0. To this end, the capacitor 421is connected between the cathode I2- and an auxiliary tube electrode; inthis case the shield electrode I6.

As soon as the cathode I2 becomes heated, electrons will flow from the cathode I2ytothe shield, I6, discharging the capacitor 42 but also ionizing the tube gas.' Ionization of the tube 'gas will provide a highly conductiveplasma or mixture of ions and electrons between the cathode I2 and the anode I8 of the tube- Ill.

- It has'been found that such a conductive plas- 'ma will permit current flow between electrodes immersed. therein with a voltage drop between the electrodesof as little as 0.1 volt. Therefore, as soon as the ionizing current pulse fiows between the cathode I2 and the auxiliary electrode I6, it becomes possible'forcurrent to flow from the cathode I2 tothe anode I8 and through the transformer primary winding 20.

cause an induced voltage'to develop across the transformer primary winding 28.

This induced voltage will add to that of the source 24, the total building up to a value which will suddenly cause the tube to break down in a conventional discharge which generates a dense conducting plasma. A large current will fiow from the cathode I2 to the anode I8 and the anode potential will drop almost to cathode potential. This plasma will soon decay and the cathode current will be reduced resulting in another surge of induced voltage across the primary winding 28. The cycle continues to repeat in this fashion. The capacitor 36, although not absolutely essential, is preferably included since it comprises, with the transformer primary winding 28, an oscillatory circuit which stabilizes the circuit operation. The pulsating voltage developed across the transformer primary winding 28 will provide an alternating voltage across the secondary winding 23 which can be used for any desired purpose.

InFig. 2, there is shown an alternative, embodiment of the invention, wherein the inductor 38 of Fig. 1 has been eliminated. In the circuit of Fig. 2, the transformer primary winding serves both as the inductor to develop the required starting voltage, and as the inductor to sustain operation of the circuit after it has once been started.

In Fig. 2, the rectifier 48 and the capacitor 42 are connected in series with the transformer primary winding 28. Also, the resistor 54 is connected between the fixed switch contact and the transformer primary winding. The inductor 38 of Fig. 1 is eliminated. In all other respects, the circuits of Figs. 1 and 2 are identical.

When the movable switch contacts 46, 28 in the circuit of Fig. 2 are moved downwardly, engaging first the fixed contacts 50, 32 and then the fixed contacts 52, 34, there will be a current surge through the transformer primary winding 20 which will charge the capacitor 42. Simultaneously, the filament I3 will begin heating the cathode I2. As soon as the cathode I2 reaches operating temperature, an ionizing pulse of current will flow to the shield electrode I6. There- 'after, the circuit of Fig. 2 will function in the same manner as hasalready been described in connection with the circuit of Fig. 1.

It will, of course, be understood that the invention is not limited to useof a gas tube'of the specific type shown in Figs. 1 and 2. Allthat is required is a .gasxfilled'tubehaving; apair of electrodes (e; g. the. cathode I2 and the shield electrode I6 of the tube I8) cooperable: to pass ionizing current through .the tube gas,.and a pair ofelectrodes (e. g. the cathodeIZ' and the anode I6 of the tube I8) cooperable to: pass. current through the ionized gas.

With a slight change in tube structure, the rectifier 48 can be eliminated from the circuit of Fig. 2. Such an arrangement is. shown in Fig. 3.

It is known that electrons can be drawn from a cold metal surface, provided ahigh enough potential gradient is established at that surface. This phenomenon, frequently referred to as field emission, is utilized in the embodiment of the invention shown in Fig. 3.

In Fig. 3, the auxiliary electrode I6 is provided with a pointed projecting tab Ifia. The tab I6a extends towards the anode I8, and terminates a very short distance therefrom. The pointed or sharp edgesurface of the tab I6a makes it possible to establish at'the point or edge a high potential gradient such as will cause electrons to flow therefrom. The relatively close spacing between the anode l2 and the tab point also contributes to the same result.

The anode l2 preferably is provided with rolled edges in order that the anode will not present a point surface close to the tab I 6a. Otherwise, it is possible that field emission might take place from the anode.

The circuit external to the tube Illa in Fig. 3

is substantially the same as the circuit shown in Fig. 2 except for omission of the rectifier 40. In Fig. 3, the voltage developed at the anode 18a by inductive build-up will draw current from the tab lea, charging the capacitor 42. As soon as the cathode l2 becomes heated, the voltage stored on the capacitor 42 will initiate circuit action. Thereafter, the circuit shown in Fig. 3 will function in the same manner as that shown in Fig. 2.

It can be seen that the present invention provides a simple and efficient arrangement for initiating the operation of a gas tube inverter circuit by storing an inductively generated voltage sufiicient to ionize the tube gas.

What is claimed is:

1. In a gas tube voltage inverter system of the type comprising a gas tube having a plurality of electrodes including a thermionic cathode, and

a source of voltage less than that required to I ionize said tube gas, in combination, a circuit connecting one of said electrodes to said cathode through said voltage source, an inductor, a capacitor connecting said cathode to another of said electrodes, means connected to said voltage source and including said inductor for deriving from saidvoltage source a voltage of amplitude sufficient to ionize said tube gas, means connecting said voltage deriving means to said capacitorto store said derived voltage on said capacitor in proper polarity to draw ionizing electrons from said cathode to said another electrode upon heating of said cathode, and means including said voltage source for heating said cathode. 2. A system as defined in claim 1 wherein said .first'and last named means comprise mechanically coupled switches.

3. A system as defined in claim 1 wherein said second named means comprises a rectifier.

4. A system as defined in claim 1 including a second capacitor, and a transformer having a primary winding, said second capacitor being connected between said cathode and said one electrode, and said transformer windingbeing connected in said circuit.

5. A system as defined in claim 4 wherein said inductor comprises said transformer winding.

6. A gas tube inverter circuit comprising a gas filled electron tube having an anode, a cathode, and an auxiliary electrode, a source of voltage less than that required to ionize the tube gas, a transformer having a primary winding, a first circuit connecting said transformer winding and said voltage source in series between said anode and said cathode, a capacitor connected between said cathode and said auxiliary electrode, a rectifier, a second circuit connecting said transformer winding said rectifier and said capacitor in series across said voltage source, a third circuit connecting said transformer winding across said voltage source, a firstswitch in said first V trodes including an anode and cathode and an .trodes cooperable to pass 6 circuit, a second switch mechanically coupled to said first switch and connected in said third circuit, and means comprising said switches for closing said first circuit and simultaneously closing and opening said third circuit.

7. A gas tube inverter circuit comprising a gas filled electron tube having an anode, a cathode, a filamentary heater for said cathode, and an auxiliary electrode, a source of voltage less than that required to ionize the tube gas, a transformer having a primary winding, a first circuit connecting said voltage source to said heater, a second circuit connecting said transformer winding and said voltage source in series between said anode and said cathode, a first switch common to said circuits, a capacitor connected between said cathode and said auxiliary electrode, an inductor, a rectifier, a third circuit connecting said inductor, said capacitor and said rectifier in series across said voltage source, a current limiting resistor, and a second switch mechanically coupled to said first switch and operable upon closure of said first switch to close and open a series circuit through said resistor, said inductor and said voltage source.

8. In a gas tube inverter circuit including a gas filled electron tube having a first pair of elecionizing current through the tube gas and a second pair of electrodes cooperable to pass current through the ionized gas, in combination, a source of voltage less than that required to ionize the tube gas, a first circuit connecting said second pair of electrodes through said source, said circuit including means responsive to interruption of current flow therethrough to develop a voltage which, when added to said source voltage, is suf ficient to ionize said tube gas, a second circuit connecting said first pair of electrodes through said source and including means responsive to interruption of current flow therethrough to develop a voltage which, when added to said source voltage, is sufiicient to ionize said tube gas, means to close said first circuit, means to close and open said second circuit substantially simultaneously with closing of said first circuit, a capacitor connecting said first pair of electrodes, and means to store in said capacitor said voltage developed in said second circuit.

9. In an electrical inverter circuit, in combination, a gas filled electron tube having elecauxiliary electrode, a source of voltage less than the ionizing voltage of the tube gas, a work circuit including said anode, said cathode and said source, an inductor, a capacitor, a second circuit connecting said source, said inductor and said capacitor in series, said capacitor also being connected between said auxiliary electrode and another of said electrodes, a switch connected to complete a circuit through said source and said inductor upon closure of said switch, and a rectifier connected in said second circuit to store on said capacitor voltage developed across said in-- ductor by opening said switch.

EDWARD OSCAR JOHNSON.

WILLIAM MERLE WEBSTER, JR.

No references cited.

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