US3103610A

US3103610A - Antffiloom networks for use with dis

Info

Publication number: US3103610A
Authority: US
Publication date: 1963-09-10
Anticipated expiration: 1980-09-10

Description

Sept. 10, 1963 ANTIBLOOM NETw'oRKs L B GANGAWERE OF THE CATHODE RAY TUBE TYPE Filed Sept. 20, 1960 FOR USE WITH DISPLAY TUBES IN VEN TOR. AZ 0Y0 B. GANG/[WERE A FOP/V5 Y5 United States Patent ANTInLooM NETwoiu-rs FOR USE WITH nrs- PLAY TUBES on THE CATHODE RAY TUBE TYPE Lloyd B. Gangawere, Baltimore, Md., assignor, by mesue This invention rel-ates to protective circuits and more particularlyto antib'l-ocm networks for cathode ray tube (CRT) type circuits or storage tube type circuits to protect these tubes from excessively high conduction during transient periods of the application of high voltage to change the tube circuit from the standby condition to the operate condition.

In circuits employing cathode ray tubes for the display of video information by intensity modulation of the electron stream, one problem must be solved both in the interest of appearance and of long display tube life. That problem is the transient state between the standby condition where only filament and biasing voltages are applied to tube circuits for maintaining these tubes in a standby condition and the operate condition at which time all other operating voltages are applied to the tubes; If the operating voltage is applied to the cathode of the electron gun, a Very usual method for CRTs and the only practical method 'for storage tubes at the present state-ofltheart, and the video signals applied to the control grid of this electron gun through a capacitor, the result is a very large current flow to the screen of the display tube, limited only by the current capacity of the high voltage power supply, until the video coupling capacitor is charged to the potential of the cathode voltage through the display tube grid resistor and through the grid. This time may be as long as several seconds depending on the size of the grid resistor and the video coupling capacitor. The results of this transient state produces an unpleasant flash of light on the screen of the display tube or CRT known to those skilled in the art as blooming. This transient state may also cause ultimate, if not immediate, destruction of the cathode in the CRT, and ultimate, if not immediate, destruction of the storage properties of the storage grid in the case where a storage tube is used. Another result is, that it is quite possible that the grid clamping diode may be destroyed because of excessive inverse voltage developed across the CRT resistor.

Several solutions are possible, one of which may be in grounding the cathode and in operating the second anodeat a positive potential. This solution requires that the deflection plates of the tube be operated at the same high potential and therefore requires four high voltage coupling capacitors for each electron gun and precludes the possibility of direct coupling for the. display of very low frequency or direct current deflection signals. This solution is not practical for other electrical reasons for a storage tube. Another solution may be to cathodecouple the video signals to the CRT, leaving the grid free of large capacities. This method requires a low impedance source of negative going video signals. This may be \a satisfactory solution in some cases but has certain requirements which may not be desirable such as high power requirements for the video output stage and a high capacity, high voltage coupling capacitor, or a video output transformer, usually large and expensive. Still another solution may be in the use of a self-biased diode, of the vacuum tube or semiconductor type, across the grid and cathode of the CRT. This has a disadvantage of requiring a very high resistance (on the order of to 20 megohms) and a very large capacitor (at least ten times the value of the CRT grid coupling capacitor). In the case of using a semiconductor diode, it may increase the input capacity of the CRT from 2 to 5 times its normal input capacity depending on the choice of diodes and how high the back biasing voltage becomes. For video display, this capacity may not be tolerable.

Although there are probably other solutions, this in.- vention discloses possible protectivenetwork of using a diode to protect a CRT or storage tube as well as the biasing circuitry and clamping diodes of such CRT or storage tube circuits. The diode is coupled across the grid and cathode of the tube to be protected through a portion of the biasing circuitry so that high current, which normally would pass through the-biasing circuitry or cathode-(grid circuitry of the CRT to charge the video coupling capacitor, would be bypassed through the diode protective network. In the diode the filament is energized through a separate and isolated filament circuit which energizes the diode filament in the standby condition, but upon application of: high voltage to the CRT this filament circuit is broken. By this means the bypass conduction of high voltage charging the video coupling capacitor exists only during the cooling period or the diode filament. This controlled bypass of the protective network .preventsdamaging currents from passing through the biasing circuitry or the. grid-tocathode circuitry of a CRT or storage tube during the transient period from standby to operate condition. It is therefore a general object of this invention to provide protective circuitry to bypass high transient currents produced by high voltages across theelectrodes of a cathode ray tube or storage tube during the transient periods from standby to operate conditions.

These and other objects and the attendant advantages, features, and uses may become more apparent to those skilled in the art as this description proceeds when considered along with the accompanying FIGURE of drawing illustrating one form of a protective nework using a diode.

Referring more particularly to the figure, there is illustrated for the purpose of describing this invention a portion of a cathode ray tube 10 with the grid 11 and cathode 12 shown therein. Although this invention may be equally applicable to storage tubes, the grid and cathode elements of a cathode ray tube are illustrated herein for the purpose of convenience and to avoid any complication of .multiplying the embodiments to describe the invention. The grid 11 of the CRT 10 is coupled to a video signal source at the terminal 13 through a video coupling capacitor 14. The cathode 12 of CRT 10 is coupled to a common conductor'15 to. which voltage maybe switched by means of the switching means 16 having the switched positions of off, standby, and on, whereby high voltage from a high voltage source 17 may be applied to the cathode and other CRT circuitry. The grid 11 of CRT 10 is biased by a biasing resistor 18 and a portion of the resistance in a potentiometer 19 through the adjustable tap 20 thereof. The resistance element of potentiometer 19 is coupled in series with a fixed resistance 21 across a biasing voltage 22. The biasing voltage 22 is switched on and off through a switch means 16b which is mechanically coupled to the principal switch 16. Clamping diode 23 is coupled in parallel with the biasing resistance 18 to clamp voltages between the grid and cathode 11 and 12 of the CRT 10. The junction of the clamping diode 23 and biasing resistance 18 is coupled through a capacitor 24 to the common conductor 15. One end of the resistance element of the potentiometer 19 is coupled to the common conductor 15 and a voltage regulator tube 25 is coupled in parallel with the potentiometer 19 to provide a regulated supply of the biasing voltage 22 to the grid 11 of the cathode ray tube and to the anode of the clamping diode 23. The above described circuitry is that usually and normally found in general for display tube circuits such as CRTs. If switch 16 is switched to on from the standby position in which biasing potentials have been supplied to the cathode ray tube circuit, a high voltage would immediately be applied to the cathode 12 of CRT 10 and to the biasing circuits by way of the common conductor means 15. The video coupling capacitor 14 would immediately charge to the high voltage applied plus the biasing voltage and in so doing high currents would have to flow across the grid and cathode 11 and 12 of the CRT and also through the grid biasing resistor 13 and a portion of the potentiometer 19 to charge the capacitor 14-. This eventually, and sometimes immediately, destroys the CRT or its biasing circuitry and sometimes the clamping diode 23. Accordingly, a protective network is most desirable to increase the reliability of such CRT or display tube circuits.

In the illustrated form of this invention a diode 30 is anode coupled to the grid 11 of the CRT 10 and is cathode-coupled to the junction of the potentiometer 19 and fixed resistance 21 of the biasing circuitry. The filament 31 of the diode 30 is coupled in circuit relation with the secondary 32 of a filament transformer 33 the primary 34 of which may be coupled to an alternating current source through a switch 160. The switch Me is mechanically coupled to Work as a compound switch with the switch components 16 and 16b, the standby position of which is coupled through the primary 34 from the alternating current source 35. The diode 3% should be a high conductance diode and the filament transformer 33 must have its secondary insulated from the high voltage used in the system. The filament voltage source 35 must be controlled independently of other filament voltages in the CRT system. For each additional CRT electron gun in the system only an additional diode is required since all diodes can be operated from the same filament transformer. A separate bias voltage supply is shown for the circuit in the figure although it is to be understood that this bias may be derived from the high voltage supply if desired.

In the operation of the circuit shown in the figure let it be assumed that switch 16 is thrown to the standby posit-ion at which time the filament supply voltage 3-5 is supplied to the filament 31 of the diode 30 by way of the filament transformer 33 and that all other filament voltages (not shown) are supplied to the CRT and related circuitry as well understood by those skilled in the art. When it is desired to place the CRT system into the operative position at which time it would provide the display of targetsor other objects on the screen of the CRT (not shown), switch 16 should be thrown to the on position at which time high voltage from the source 17 is applied to the circuit. If a video signal were applied at terminal 13 at this time, and there was no protective network, CRT 10 would go to full conduction since the bias voltage on CRT 10 would be zero. It is to be noted at this time that switch section 160 breaks the filament voltage circuit to the filament 31 in the diode 30 and this filament starts to cool. The video coupling capacitor 14 is already charged to the bias supply voltage and now charges to the high voltage potential of 17 by way of the cathode and anode of the diode 30 which diode 30 will at the same time hold a constant difference of potential between the grid 11 and cathode 12 of the CRT 10 keeping it cut ofi. This difference of potential is very nearly equal to the bias voltage since the drop between the anode and cathode of the diode 30 is very small. The time required for charging the video coupling capacitor 14 will vary with circuit values and the current capabilities of the high voltage power supply 17 but this time will be on the order of one millisecond. When the filament 31 of the diode 3b is sufficiently cool, conduction will cease and this diode will be cut ofi so that normal circuit voltage conditions are restored through the action of the clamping diode 23 whereby normal CRT 19 operation may be maintained for the purpose intended. The CRT 10 and its related biasing circuit and clamping diode circuit have thereby been protected from high transient currents which would normally flow therethrough to charge the video coupling capacitor 14. The anode and cathode of the protective diode 30 obviously will produce some capacity in the CRT circuit which may vary from 2 to 10 rnicro-microfarads depending on the diode chosen. This value represents from 5 to 10 percent of the normal capacitance of the CRT circuit and is not serious. The time to recover normal circuit functions will vary depending on the thermal inertia of the cathode of tube 30. In most cases, this recovery time can be tolerated in view of its protective characteristics.

While many modifications and changes may be made in the constructional details and features of this invention without departing from the spirit and scope of this invention, it is to be understood that I desire to be limited in my invention only by the scope of the appended claims.

I claim:

'1. An antibloom network for use with standby radar recording display tube circuits having a cathode and a grid with the cathode in a switch circuit to high voltage to switch said recording display tube from a standby condition to an operative recording display condition and with the grid coupled to a biasing voltage and branch coupled through a coupling capacitor to a signal source, the invention which comprises:

a diode having an anode coupled to the grid of said recording display tube, having a cathode coupled through a resistance to the cathode of said recording display tube to provide a conduction path parallel to said recording display tube grid and cathode, and having a filament; and

filament control means coupled to said filament and to a filament supply voltage for controlling the supply of voltage to said filament and thereby conduction of said diode a predetermined interval of time after switching high voltage to said recording display tube cathode and thereafter control said diode to nonconduction whereby blooming of said display tube is prevented by virtue of bypassing high currents around said recording display tube cathode and grid through said diode to said coupling capacitor for said predetermined interval of time until said coupling capacitor is fully charged to said high voltage.

2. An antibloom network as set forth in claim 1 wherein said filament control means is a transformer having the secondary therof providing said coupling to said diode filament and a primary switchable in said coupling to said filament supply voltage with the switch thereof actuable in alternate closing and opening sequence with the high voltage opening and closing sequence, respectively, whereby said predetermined interval of time of diode conduction is determinative from filament cooling.

References Cited in the file of this patent UNITED STATES PATENTS 2,457,112 Abercrombie et al Dec. 28, 1948 2,756,378 Scott July 24, 1956 FOREIGN PATENTS 1,055,699 Germany Apr. 23, 1959

Claims

1. AN ANTIBLOOM NETWORK FOR USE WITH STANDBY RADAR RECORDING DISPLAY TUBE CIRCUITS HAVING A CATHODE AND A GRID WITH THE CATHODE IN A SWITCH CIRCUIT TO HIGH VOLTAGE TO SWITCH SAID RECORDING DISPLAY TUBE FROM A STANDBY CONDITION TO AN OPERATIVE RECORDING DISPLAY CONDITION AND WITH THE GRID COUPLED TO A BIASING VOLTAGE AND BRANCH COUPLED THROUGH A COUPLING CAPACITOR TO A SIGNAL SOURCE, THE INVENTION WHICH COMPRISES: A DIODE HAVING AN ANODE COUPLED TO THE GRID OF SAID RECORDING DISPLAY TUBE, HAVING A CATHODE COUPLED THROUGH A RESISTANCE TO THE CATHODE OF SAID RECORDING DISPLAY TUBE TO PROVIDE A CONDUCTION PATH PARALLEL TO SAID RECORDING DISPLAY TUBE GRID AND CATHODE, AND HAVING A FILAMENT; AND FILAMENT CONTROL MEANS COUPLED TO SAID FILAMENT AND TO A FILAMENT SUPPLY VOLTAGE FOR CONTROLLING THE SUPPLY OF VOLTAGE TO SAID FILAMENT AND THEREBY CONDUCTION OF SAID DIODE A PREDETERMINED INTERVAL OF TIME AFTER SWITCHING HIGH VOLTAGE TO SAID RECORDING DISPLAY TUBE CATHODE AND THEREAFTER CONTROL SAID DIODE TO NONCONDUCTION WHEREBY BLOOMING OF SAID DISPLAY TUBE IS PREVENTED BY VIRTUE OF BYPASSING HIGH CURRENTS AROUND SAID RECORDING DISPLAY TUBE CATHODE AND GRID THROUGH SAID DIODE TO SAID COUPLING CAPACITOR FOR SAID PREDETERMINED INTERVAL OF TIME UNTIL SAID COUPLING CAPACITOR IS FULLY CHARGED TO SAID HIGH VOLTAGE.