US2152821A - Cathode ray tube apparatus - Google Patents

Description

April 4, 1939- K. SCHLESINGER CATHODE RAY TUBE APPARATUS Wren/0m Filed Feb. 9, 1935 Patented Apr. 4, 1939 PATENT OFFICE CATHODE RAY TUBE APPARATUS Kurt Schlesinger, Berlin, Germany, assignor to Radioaktiengesellschaft D. S. Loewe, Berlin- Steglitz, Germany Application February 9, 1935, Serial No. 5,870 In Germany February 12, 1934 3 Claims.

The invention relates to oscillographs or television apparatus using cathode ray tubes the cathode ray of which is kept in movement by a relaxation oscillator connected to the deflecting plates or coils of the tubes. It is well known that the fluorescent screen of the tube is damaged when the ray hits for a longer time the same point of the screen.

It is the object of the invention to avoid the burning of the stationary cathode ray into the luminous screen by preventing that the ray is not switched on until the relaxation oscillator has already commenced to move the ray and to switch out the ray when the oscillator ceases to work.

This is illustrated in the accompanying drawing, in which Fig. 1 shows a simple embodiment of the invention, whilst Figs. 2 and 3 show more complicated connections according to the invention.

In Fig. 1, the relaxation condenser 3 is charged from the battery I having a relatively high potential (approximately 1000 volts) through the medium of a resistance 2, and is discharged through the medium of a gas-filled grid-controlled tube having a hot cathode and grid. The anode of the tube is connected to an amplifier supplying the deflecting potentials respectively currents, whilst the grid, in the case of a television receiver, receives the synchronizing impulses. The maximum charging potential of the condenser 3, if the hot cathode is in operation, amounts to not more than 20 to 40 volts depending on the value of the adjusted grid bias of the tube 4. Until the cathode has become hot, however, the tube does not ignite until considerably higher potentials have been reached. In certain forms which operate with an extremely small pressure, for example those filled wtih. mercury vapour, an ignition does not take place at all in the cold condition, as the requisite vapour pressure only develops as the cathode gets hot.

In the meantime, the cathode ray may already be developed and burn the screen. The

same happens when, by any reason, the relaxation oscillations cease before the cathode ray is switched off.

In order to overcome this danger a glow lamp 5 in series with the electromagnet 6 of a relay switch i, 8 is provided in parallel to the tube 4.

This glow lamp 5 preferably contains cold electrodes, is filled with a rare gas of proper kind and pressure and is so dimensioned that its ignition potential is higher than the anode potential of the normally operating tube 4. On

the other hand the ignition potential of the safety lamp 5 should be lower than the ignition potential of the unheated tube 4. These conditions may readily be fulfilled, for example, by filling the lamp 5 with neon of 15 mm. pressure, whilst the tube 4 is filled with argon or mercury vapour at a pressure of approximately 10- to 10- mm. The ignition potential of 5 is then of the order of 150 volts, whilst the ignition potential of 4 if.filled with mercury vapour is not 10 reached by the battery I at all, and if filled with argon is of the order of several hundred volts.

In consequence the glow lamp 5 lights first shortly after the switching on. Its terminal potential is approximately volts and does not surmount this normal cathode fall owing to the large surface of its electrodes. Immediately the lamp 4 has become hot, the potential on the same drops to approximately 20-40 volts, so that the glow lamp 5 is extinguished, as its quenching 20 potential is not much less than 100 volts.

The relay 6 interrupts the circuit at l and 8 for such time as a discharge current traverses the auxiliary glow lamp 5. In this circuit there may be connected, for example, the anode potential of 25 the television tube or the grid potential of the television tube, in such fashion that the moving image point does not appear until the contact between I and 8 has been closed. In this way the television tube, even if the same has a shorter heating period than the connected tilting apparatus, is prevented from burning the screen with a stationary spot of light.

This method of mechanically switching on may be replaced by an electrical method principally making use of safety glow lamps the particular dimensions being adapted to the special purpose.

Two forms of embodiment of such an arrangement according to the invention are illustrated by way of example in Figs. 2 and 3. 40

' In Fig. 2, the line and frame relaxation oscillators of a television receiver are, according to Fig. 1, marked with 2, 3, 4 and 2, 3', 4'. The tubes 4 and 4' are shunted by glow lamps 5 and 5', which are, instead to a relay, connected to a 45 common resistance 6. They accordingly produce at this resistance 6 a bias, which is positive against earth, up to the moment when the tubes 4 and 4 have become hot and the glow lamps 5 and 5, therefore are extinguished. The high 50 tension supply apparatus of the television tube, which includes the one-way rectifier tube It the condensers 9 and I0 and the steadying resistance H, is coupled galvanically with this resistance 6 through the medium of a resistance l2. The 5 cathode l3 of the television tube is connected with the point l4, its control grid 15 with the cathode of the tube. The bias is adjusted finely by the potentiometer tapping H.

The operation of the arrangement is as follows: so long as the glow lamps 5 and 5' are burning, i. e., the tilting apparatus is not in operation, the cathode is is positively biassed against the control grid I5 of the television tube, which grid remains negative. This bias is certainly but a fraction of the potential at 6, but the latter may be selected in such amount that-the bias sufii ces to keep the image field dark. Thus, for example, the resistance I2 may be selected at 10 ohmsand the potentiometer H at 3x10 ohms. Of the latter merely one-tenth is required for adjusting the bias of l5. There accordingly then occurs at the cathode I3 exactly one-quarter of the bias of the resistance 6 against the control grid I5. 1f the resistance 6 is selected at 100,000 ohms, there is readily obtained at the same a bias of volts, as the feeding of the relaxation apparatus with the potential l8 takes place with a potential of approximately 1,000 volts. There is accordingly obtained a blocking potential of 25 volts, which is fully sufficient to darken the television tube. Immediately the tubes 5 and 5' are extinguished, the potential at 6 collapses automatically, and the television tube is released.

Another form of embodiment of the invention is shown in Fig. 3. In this there is employed a special glow lamp 19. The lamp has two anodes 20, 20, which are connected with the relaxation condensers. it also possesses a cathode 2i and a third electrode 22. Now the conditions of discharge are so unfavourable between this third electrode and the cathode 2| that the discharge is not-initiated upon full operating potential of the television tube. This may be obtained in several known ways, for example by placing the auxiliary cathode 22 so that it extends into the dark space in front of the cathode 21. On the other hand the anodes 2 0, 20, ignite against the cathode 2| as soon as the apparatus is switched on. Immediately this discharge has commenced, which ensures that the relaxation condensers cannot be charged higher, the anode rays pass through the tubular cathode 2|, and causes electrons to be emitted from the auxiliary cathode 22, which produce a discharge between 2i and 22. By reason of the resistance 23 according to the invention the cathode 13 of the television tube is then raised to an abnormally high positive potential in relation to the control grid l5 of the same ray, so that the tube is blocked. Immediately the tubes 4 and 4' have become hot the discharges between 26, 20 and 2| cease, and in this way the discharge between 22 and 21 is also caused to disappear. The television tube is thus able to perform emission. The resistance 23 is so chosen that no appreciable drop in potential takes place at the same during the operation of the television tube. With the usual emissions of approximately 500 microamperes for the television tube the glow lamp H! is so calculated that a current of a few milliamperes flows through the glow lamp. The resistance 23 is then of the order of several times 10,000 ohms. It is bridged by a condenser 24, which amounts to approximately 1 mf., and passes emission peaks in the television tube occurring during the normal operation.

I claim:

1,. ,ln a relaxation oscillation circuit comprising a direct current source, a grid-controlled gasfilled thermionic tube, a condenser and a charging resistance, a'safety device consisting of a glow-discharge lamp in series with a relay, both in parallel to said tube, the igniting potential of said lamp being higher than the anode potential of the normally operating tube.

2. In a cathode ray tube system comprising a cathode ray tube having cathode, control grid an .fl'node, a-direct current source and two relaxation oscillators according to claim 1, and a high-voltage direct current source connected to cathode and control grid of said cathode ray tube, a safety device for-suppressing the cathode ray as long as said relaxation oscillators are not working, said device consisting of two glow discharge lamps, each connected to the anode of one of said thermionic tubes, and their common second terminal connected to an earthed resist ance, said common terminal connected by a high resistance to the cathode of said cathode ray tube, the control grid o f said cathode ray tube being connected to said high voltage source at a Doint more negative than said cathode.

3. In a cathode ray tube system comprising a cathode ray tube having cathode, control grid andanode, adirect current sourceand two relaxation oscillators according to claim 1, and a high voltage direct current source connected to said cathode-andcontrol grid of said cathode ray tube, the positive terminal of said high voltage source being earthed, a safety device for suppressing the cathode rayas long as said relaxation oscillators are not working, said device consisting of a glow discharge lamp having .four electrodes, one of said electrodes connected to the anode of one ofsaid thermionic tubes, another of said electrodes connected to the anode of the other of said thermionic tubes, a third electrode connected to earth, and the fourth electrode connected to the cathode of said cathode ray tube, a resistance having one terminal connected to the cathode of said cathode ray tube and another terminal connected to said high voltage source, and a condenser shunted across said resistance, the control grid of said cathode ray tube being connected to said high voltage source at a point more negative than said cathode.

' KURT SCHLESI-NGER.

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