US2280670A - Cathode ray tube circuit - Google Patents

Description

April 21, 1942. s. c. SPIELMAN 2,280,670

CATHODE RAY TUBE CIRCUITS Filed March 21, 1940 DE F' L C Tl/YG CIRCUITS @rye Imercial A. C. sources.

CATHODE RAY TUBE CIRCUIT Sterling C. Spielman, Philadelphia, Pa., assignor' to Philco u i:

o and Television Corporation,

Philadelphia, Pa., a corporation of Delaware Application March 2l, 1940, Serial No. 325,241

(Ci. Z50-27) l2 Claims.

This invention relates to protective circuits for cathode ray tubes and particularly to improved circuits for preventing damage to the uorescent screens thereof. The invention is particularly adapted for use in cathode ray te'levision receiving circuits and will be explained with reference to these circuits; however it will be understood that the invention is not restricted to use in television.

The usual cathode ray tube comprises at least;

an electron emitting cathode, an anode adapted to be supplied with a relatively high positive potential for accelerating the emitted electrons to cause them to impinge upon a fluorescent screen,

and a grid which is usually interposed between' the cathode and the anode to which. may be applied a varying potential to control the intensity of the cathode ray beam. In addition, some means are usually provided for defiecting the cathode ray beam so as tocause it to impinge upon various portions ofthe fluorescent screen. For example, in a television receiver there are provided horizontal and vertical deflecting cir-A cuits which are operated so as to-cause the beam to scan the screen in a series of vertically spaced horizontal lines. The high positive potential for supplying the cathode ray tube anode may be obtained by the rectification of power from corn- Generally, power for the operation of the deectlng circuits will be obtained from the same source by a separate rectier circuit. In general, the voltage required `for the cathode ray tube anode supply will be considerably higher than that required to supply the tubes employed in the deecting circuits. This' being the case, it is observed that the high voltage rectier circuit is characterized in that the voltage developed thereby persists for some time -the high time constant of the filter used in connection with the rectifier which cannot conveniently be reduced. The result is that the cathode ray beam persists after the defiecting circuits have ceased functioning. When this occurs, the electrons continue to impinge upon a particular portion of the screen area until the anode voltage decays to a point at which it becomes ineffective to accelerate the electrons sufficiently to cause them to reach the screen. 'I'he continued impingement of electrons upon a small area of the screen will cause this area to deteriorate, this eiect being commonly referred to as burning.

The principal object of the present invention is to provide means whereby this difliculty may be avoided by automatically cutting ofi the electron beam as soon as the rectiiiers are disconnected from their source of power. By the inventionI means are provided for achieving this end with a minimum of additional apparatus and without appreciably modifying the circuits already used. The invention will now be explained in detail with reference to the drawing in which v Fig. l is a diagrammatic illustration of the invention applied to a television receiver; and

Fig. 2 is a similar illustration of another embodiment of the invention.

Referring to Fig. l, the parts of the receiver which need not be considered in detail for the purposes of this disclosure are represented by the block labeled Receiver circuits. These may include the radio frequency amplifier supplied with signal from the antenna I, a converter stage,- an intermediate frequency amplier, a second detector, and a video frequency amplifier and synchronizing signal separator. The detected video signal from these circuits may be supplied to the grid of a final amplifier tube 2, the output circuit of which may be coupled in the manner shown, or in any other suitable way, to the grid of the cathode ray tube 3. The circuit comprising the resistors 4 and 5 and the diode 6 serves to level the signal supplied to the grid of tube 2 and thereby restores the D. C. component lost in previous stages. The separated synchronizing signals from the separating stage may be applied as shownto control the operation of the deiiecting circuits, which may be of the conventional form. Voltage for the plate and screen circuits of tube 2 is obtained from the rectiiler-lter circuit comprising the diodes l, the condensers 8 and 9, the choke I Il, and the condenser II. The plates of the diodes, it will be noted, are connected to the secondary of the transformer I2, the primary of which is connested by means of a switch I 3 to the A. C. source which may be a 60 cycle line. This rectifieriilter circuit also provides voltage through the connection It to the tubes employed in the deiiecting circuits. Suitable bias for the cathode ray tube 3 may be obtained from the potentiometer I5. Further, it will be-noted that the bypass condenser' I 6 provided for this potentiometer, instead of being connected to ground, may be connected as shown across the plate filter I'i for the purpose of neutralizing any disturbances engendered in the power supply and appearing across the plate filter. Anode voltage for the tube 3 is obtained from the rectifier comprising the diode I8, the condensers I9 and 20, and the resistors 2i and 22 deriving power through the transformer 23 whose primary is connected through the switch I3 to the A. C. source.

Now it will be noted that upon the removal of power from the rectifier circuits by the opening of the switch I3 the condenser 20 in the cathode ray tube power supply iilter will commence to discharge through the bleeder resistor 22.

making this time constant smaller than that of the nlter circuit associated with the power supply which supplies the denecting circuits, to insure cutting oi! the cathode ray beam before the denecting circuits cease to function when A. C. power is removed from the rectifier circuits. However in order to do so, it becomes necessary to make resistor 22 of very low resistancel and high power-dissipating capability, which is inconvenient and expensive.

According to the present invention, there is provided a novel and simple means for cutting on the cathode ray beam, which eliminates the necessity of modifying the time constant as above-mentioned. By this invention, there is provided a condenser which is charged while the receiver is in operation. When power is removed from the rectiilers, this condenser is connected to one of the electrodes of *he cathode ray tube in such a manner that the voitae across the condenser is eiective to cut oft the cathode ray beam. The condenser may be permitted to discharge at a rate such that the condenser voltage is suillcient at all times to keep the beam cut on until the anode voltage has fallen to such an extent that the beam can no longer exist.

In the embodiment of Fig. 1, the condenser 2l l serves this purpose. It will be noted that this condenser is permanently connected to the main power supply through the resistor 25, and to the cathode of tube 3 through the resistor 26. l There is also provided a switch 21 in the bias connection to the cathode of tube 3, and this switch is ganged to the power switch I3. When the switch I3 is closed, switch 21 will likewise be closed and the bias on the tube 3 will depend upon the voltage drop across a portion of the potenti` ometer resistance I5. At the same time, the condenser 24 will be charged through the resistor 25. When power is removed iromvthe rectifiers by opening switch I3, switch 21 vwill also be opened. Current in the resistor 25 will cease to now and the cathode of the tube 3 will assume the potential of the condenser 24. The condenser will, of course, discharge through the various resistive paths shunting it. In general, however, the rate of 'discharge will be determined by the magnitude of resistor 25 which should be erge enough to maintainthe desired bias upon the tube 3 to cut off the cathode ray beam until the anode potential has been sufiiciently reduced. 5g

The following vames or circuit elements are gigesi:"lggss '023 exemplary of those which might be used but it Resistor megohms" o 47 will be understood that they are not to be re- Resistor u ohms" 300 ended as placing any limitation upon the 1n- Resst.. l33:53:5:353335531d.- 5000 venuon: 60 Condenser 39 microfarads 40 Resistor 2a nhme 200,000 Condenser 20 do-- 0.06 Resistor I5 do 47,000 Resistor 22 -i .megohms 15 Resist g It will be apparent that'other embodiments oi' Resistor n 65 the invention are possible and the invention is to (30mn-W u --mcrofamdsn 4 be regarded as umited muy by the scope of the Condenser s do 8 appended claims Condenser 2l do- .03 I claim. Resistor 22 megohms.. 30

Referring now to Fig. 2, there is shown another embodiment of the invention in which a different arrangement of the circuit is used to eiTect the charging of the biasing condenser and -to connect it to an element o1' the cathode ray tube when power is removed. In this circuit, a

dinerent mode of coupling is employed between the ampliiier tube 2 and the cathode ray tube 3. The load impedance for tube 2 comprises the resistors .23 and 3l and the inductance 3I shunting' the latter. The plate supply is from the rectifiers 1. as before. An A. C. coupling including the condenser 32, the peaking coil 33, and the damping resistor 34 is provided between the plate of tube 2 and the grid ot the cathode ray tube 3. 0! course it is essential also to transmit the D. C. component to the grid of the picture tube. In the present instance, this is done by two connections, one of which supplies the D. C.

component developed across the resistorv to the cathode of the picture tube. while the other connection is from the screen grid of tube 2 via the potentiometer 35 to the grid of the picture tube. It will be noted that the resistor 35 is' by-passed by the condenser 31 for A. C. and also that provision is made for controlling gain by the inclusion of the potentiometer 38 and condenser 33 whereby the degeneration of the tube 2 may be varied. The resistor is large by comparison with the resistor 35 and its eii'ect is unimportant except when switch 4I is open. Therest of the circuit is similar to that of Fig. 1 and need not again be discussed.

Upon the closing of the power switch I3, switch 4IA which is ganged thereto is also closed so as to include the resistor 35 in the circuit. The condenser 39 will be charged by the flow of current in the tube 2. It has been found that the potential assumed by thisv condenser is sufcient, when applied to the cathode of a picture tube, to bias off the cathode ray beam therein. This obtains when the switch 4I is opened upon the removal of power from the circuit. At this time, the current in the resistor 42 is greatly reduced and the potential applied'to the cathode of tube 3 through the connection 43' is substantially that across the condenser39. Resistor 4I which is provided to discharge the condenser should be sumciently large to do so at a relatively slow rate compared to that of the discharge of condenser 20 through the bleeder 22 in the anode supply to tube 3. Thus, condenser 33 serves not only in conjunction with potentiometer 33 to regulate degeneration in tube 2, but also serves to develop the required voltage for cutting oi! the cathode ray beam in accordance with the in-` vention.

The following values are given as exemplary in the circuit of Fig. 2 but are not to be regarded as restrictive of the invention:

1. In a cathode 4ray'tube system, a cathode ray tube having an electron beam-emitting means,

a beam-sustaining electrode and a control element, means for supplying a beam-sustaining potential'to said electrode, a space discharge device arranged to receive control energy and supply the same to said control element, means in the cathode circuit of said space discharge device for controlling the output thereof, said lastnamed means including a condenser across which e voltage is established, and means for applying said condenser voltage to said cathode ray tube power; means for adapting power from said source to provide anode voltage for said tube, said means being characterized in that the voltage provided thereby persists for a time after the disconnection of said means from said source; a switch for connecting said means to said source; a condenser; means for charging said condenser whenever said means is connected to said source for developing across 'said condenser a voltage which, when applied to one of the electrodes of said cathode ray tube, is effective to cut o said cathode ray beam; means for applying a relatively small fraction of said condenser voltage to said electrode when said power adapting means is connected to said source; and means for applying a larger fraction of said condenser voltage to said electrode when said power adapting means is disconnected from said source.

3. In a cathode ray tube circuit; a cathode ray tube comprising a cathode, an anode, and a control grid adapted to cooperate in the formation and control of a cathode ray beam; a source of power; means for adapting power from said source to provide anode voltage for said tube, said means lbeing characterized in that the voltagi provided thereby persists for a time after the disconnection of said means from said source; a switch for connecting said means to said source; a condenser; means for charging said condenser whenever said means is connected to said source for developing across said condenser a voltage which, when applied to one of the electrodes of vsaid cathode ray tube, is effective materially to reduce the intensity of said cathode ray beam; means for applying a relatively small fraction of said condenser voltage to said electrode when said power adapting means is connected to said source; and means for applying a larger fraction of said condenser voltage to said electrode when said power adapting means is disconnected from said source.

4. In a cathode ray tube circuit; a cathode ray tube comprising a cathode, an anode, and a control grid adapted to cooperate in the formation and control of a cathode ray beam; a source of power; means for adapting power from said source to provide anode voltage for said tube,

said means being characterized in that the volt-v age provided thereby persists for a time after the disconnection of said means from said source; a switch for connecting said means to said source; a condenser; means for charging said condenser whenever said power adapting means is connected to said source for developing across said condenser a voltage which, when applied to one of the electrodes of said cathode ray tube, is effective materially to reduce the intensity of said cathode ray beam; a voltage divider connected to said condenser; means for connecting said electrode to a point on said voltage divider when said power adapting means is connected to said source to apply a relatively small fraction of the voltage across. said condenser to said electrode; and means for connecting said electrode to a different point on said voltage divider when said power adapting means is disconnected from said source to apply a larger fraction of said condenser voltage to said electrode.

5. In a cathode ray tube circuit; a cathode ray tube comprising a cathode, an anode, and a control grid adapted to cooperate in the formation and control of a cathode ray beam; a source of power; means for adapting power from said source to provide anode voltage for said tube, said means being characterized in that said anode voltage decays slowly upon the disconnection of said means-from said source; a switch for connecting said means to said source; a condenser:

means for charging said condenser whenever said power adapting means is connected to said source for developing across said condenser a voltage which, when applied to one of the electrodes of said cathode ray tube, is effective materially to reduce the intensity of said cathode ray beam; means for applying a relatively small fraction of said condenser voltage to said electrode when said power adapting means is connectedv to said source; means for applying a larger fraction of said condenser voltage to said electrode when said power adapting means is disconnected from said source; and means for discharging said con denser at a rate such as to maintain said cathode ray beam at substantially reduced intensity during the decay oi said anode voltage.

b. In a cathode ray tube circuit; a cathode ray tube comprising a cathode, an anode, and a control grid adapted to cooperate in the formation and control of a cathode ray beam; a source of alternating current; means for rectifying said alternating current to provide anode voltage for said cathode ray tube, said means including a illter circuit having large time constant; a switch ior connecting said means to said source; a condenser; means for utilizing a portion of the rectiiied current to charge said condenser whenever said rectifying means is connected to said source, for developing across said condenser a voltage which, when applied to one of the electrodes of said cathode ray tube, is eiective materially to reduce the intensity of said cathode ray beam; means for applying a relatively small fraction of said condenser voltage to said electrode when said rectifying means is connected to said source; means for applying a larger fraction of said condenser voltage to said electrode when said rectifying means is disconnected from said source; and means for discharging said condenser at a rate such as to maintain said cathode ray beam at substantially reduced intensity during the decay of said anode voltage.

7. In a cathode ray tube circuit, a cathode ray tube having an electron beam-emitting means and a beam-sustaining electrode, means for supplying a beam-sustaining potential to said electrode, a condenser, means for charging said condenser whenever said potential-supply means is operative, thereby to develop a voltage across said condenser, means for applying a relatively small fraction of said condenser voltage to said tube to maintain said electron beam at a desired operating intensity when said potential-supply means is operative, and means for applying a larger fraction of said condenser voltage to said tube to substantially reduce the intensity of said beam when said potential-supply means is rendered inoperative, thereby to render ine'ective any residual potential remaining on said electrode.

8. In a cathode ray tube circuit, a cathoderay tube having an electron beam-emitting means and a beam-sustaining electrode, means for supplying a beam-sustaining potential to said electrode, a. condenser, means for charging said condenser whenever said potential-supply means ls operative, thereby to develop a voltage across said condenser, means for applying a relatively small fraction of said condenser voltage to said tube to maintain said electron beam at a desired operating intensity when said potential-supply means is operative. means for applying a larger fraction o! said condenser voltage to said tube to substantially reduce the intensity ofsaid beam when said potential-supply means is rendered inoperative, thereby to render ineffective any residual potential remaining on said electrode, and means for discharging said condenser at a rate appreciably less than the rate at which said residual potential is reduced.

9. In a cathode ray tube system, a cathode ray tube having an electron beam-emitting means, a beam-sustaining electrode and a control element, means for supplying a beam-sustaining potential to said electrode, a space discharge device arranged to receive control energy and supply the same to said control element, means in the cathode circuit of said space discharge device for controlling the output thereof, said last-'named means including a condenser across which a voltage is established, means for applying a relatively small fraction of said condenser voltage to said tube when said potential-supply means is operative, and means for applying a larger fraction of said condenser voltage to said tube when said potential-supply means is rendered inoperative, thereby to render ineffective any residual potential remaining on said electrode.

10. In a cathode ray tube circuit, a cathode ray tube having an electron beam-emitting means and a beam-sustaining electrode, means for supfraction of the voltage thereacross to said cathode whenever saidpotential-supply means is operatlve, and means for modifying the impedance o! said divider whenever said potential-supply means is inoperative, to thereby apply a larger voltage to said cathode.

11. In a cathode ray tube circuit, a cathode ray tube having an electron' beam-emitting means and a beam-sustaining electrode, means for sul plying a beam-sustaining potential to said electrode, switching means for controlling said potential-supply means, a condenser, means for charging said condenser when said potential-supply means is operative, thereby to develop a voltage across said condenser, connections i'or applying said condenser voltage to said tube to control the electron beam, and switching means operable conjointly with said -first-mentioned switching means to modify the effectiveness of said connections and thereby control the application of said condenser voltage to said tube.

12. In a cathode ray tube circuit, a cathode ray tube having an electron beam-emitting means, a beam-sustaining electrode and a beam-controlling electrode, means for supplying a beam-sustaining voltage to said beam-sustaining electrode, said last-named means comprising a source of power, a rectifying device and illter means, means for supplying a beam-controlling voltage to said beam-controlling electrode, said last-named means comprising a source of power, a rectifying device and iilter means, said last-named filter means normally having a timev constant smaller than the time constant of said rst-named illter means, means for applying a relatively small fraction of the voltage developed by said controlling voltage supply means to said beam-controlling means during normal operation, and means for modifying the constitution of said last-named lter means in response to the disconnection ot said rst-named power source from said iirstnamed rectifying device, to thereby apply a relatively larger fraction of the voltage developed by said controlling voltage supply means to said beam-controlling electrode and to render the time constant of said last-named i'llter means greater than the time constant ofsaid firstnamed lter means.

STERLING C. SPIELMAN.

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