US2297547A - Television power supply - Google Patents

Description

Sept. 29, 1942,

Patented Sept. 29, 1942 UNlTED STATES PATENT OCFICE TELEVISION POWER SUPPLY Dudley E. Foster, South Orange, N. J., and John A. Rankin, Port Washington, N. Y., assignors to Radio Corporation of America, a corporation of Delaware 1 Claim.

This invention relates to an improvement in television receivers and more particularly in the power supply used in such receivers.

In a television receiver a cathode ray tube is used in which a beam of electrons is generated, the electron beam being directed against a fluorescent, luminescent or electron responsive target surface in order to produce light as a result of the electronic stream bombardment. The electron beam is deflected in substantially mutually perpendicular directions in order to scan the target surface and the intensity of the electron beam is altered in accordance with the received picture signal series. The produced variations in the intensity of the cathode ray beam, therefore, together with the scanning operation, produce the television light image at the electron responsive target. The light image produced at the target surface may be viewed directly or may be enlarged by projection through a series of lenses upon a viewing screen.

In order to produce a television picture having the desired light intensity it is necessary to use an electron beam of considerable velocity and it has been found that velocities and intensities which are necessary to produce the desired results are suiiicient to permanently damage the target surface unless the cathode ray beam is continuously deflected. Should the cathode ray beam remain lsubstantially stationary at any particular point on the target surface permanent injury would result and the surface of the target would be spoiled. Y

The possibility of permanently damaging the target surface of the cathode ray receiving tube is particularly great when the receiver is initially turned on, since frequently the cathode ray tube and its power supply are in an operating condition before the deflection generators associated therewith have been sufficiently heated to produce the desired beam deflections. Under these conditions a cathode ray beam will be produced and will be directed against a relatively small area on the target surface for an appreciable length of time prior to the operation of the deflection generators and as a consequence the target surface will be burned over that particular area.

In order to avoid such an operation in conventional receivers, users of the receivers have been cautioned to move the beam intensity control knob to minimum beam position when the receiver is turned on and to leave the control knob in that position for a time suicient for the deflection generators to begin operation. The

intensity of the cathode ray beam may then be increased by the manipulation of the control knob to a value necessary for proper operation of the tube and receiver. Such a precaution, of course, is annoying to the operator of the receiver and should such a procedure be overlooked, it is quite possible that the television receiving tube would be irreparably damaged.

One purpose of the present invention, therefore, resides in an improvement in the power supply associated with the television receiving tube such that production of a cathode ray beam will be prevented for a length of time sufficient to permit the defiection generators to begin operation.

Another purpose of the present invention resides in the provision of means associated with the power supply of a television receiving tube whereby the receiver may be turned on without the necessity of previously adjusting the position of the beam intensity control member.

A still further purpose of the present invention resides in the provision of means associated with the po-wer supply -of a television receiving tube whereby the entire television receiver may be brought to proper operating temperatures and conditions before a cathode ray beam is permitted to be developed in the television receiving tube.

Still other purposes and advantages of the present invention will become more apparent to those skilled in the art from the following specification, particularly when considered with the single figure wherein a power supply for a television receiver is shown incorporating the present invention to accomplish the purposes set forth above.

In the drawing, a television receiving tube I0 is shown having a cathode I2, a heater element I4 therefor, a screen grid electrode IG, a rst accelerating anode i8, and a second accelerating anode 20. When proper potentials are applied to these electrodes, a focused cathode ray beam is produced and projected against the target surface 22 at the enlarged end of the television receiving tube IB. For deflecting the cathode ray beam in mutually perpendicular directions dcflecting coils 24 and 26 are provided which when energized with the proper voltage variations cause vertical and horizontal deections of the cathode rai7 beam respectively. Although electromagnetic deflecting means are shown in the drawing, it is to be understood, however, that electrostatic deflection 0r a combination of these types of deiiections may be used. The

to the cathode, the potential being somewhatdess.

than the potential on the second accelerating anode. Furthermore, the screenfgridelectrode:I6

is maintained positive but1loy-al1lesseramount.as

is well known to those skilled in the-art.

The cathode.,Y ray tube also includes a control electrode 28 by means of whichthe vintensityof the cathode ray beam may be controlled and this control electrode is normally maintained negativel .with respect.to the cathode.l2. In .order .to :modulate the `intensity .of .the cathode ray. .beam signals are .applied'asbetween the.. control .electrodeZv and.the..cathode l2. In order to maintain. the; proper. biased .relationship .between the control electrode..and...the cathode of thecathode `ray tube, the. cathode. may.be..normallyl operated ata. slight .Gand adjustable) .positive .potential withrespectl to. ground..and..thecontrol. electrode 4may. .be operated. at. substantially :ground .potential.

Furthermore, the` potentialoftherst accelerating anodeis adjustable .with respect to the potential .of `the .secondraccelerating anode Yin order toproduce the. proper-focal. condition ofthe cathode ray beam.

.Forproducing .the desired potentials which L .mustbeapplied..to '..the cathodenay tube, the .power is fappliedto .the terminals 30 which .are .connected to the .primary winding;32..of. thepower transformer' -34..by .way .of=..the. control switchA 36. The power .applied by the terminals 30 ,is normally derived. from o the i60 cycle, ..110 .volt commercial supply system. The .transformer V34 f. has .two .secondary .windings38- and .46, the former-.being for.. the purpose .of .supply-ing .the heating .current tolthe` cathode 42 of thehalf-wave rectifier. tube .44. vThe.other-..secondary.winding 40 which sup- .plieshigh voltageis connected .between ground and the anode 46 of therectier tube. When .the rectifier tubeis energized,..pulsating.direct current may. be. .derived between. cathode 42 `.and thegrounded endl of .the .winding 4.-with the ycathode positive ...in .polarity with .respect to ground.

In .order `tot produce Lsubstantially uni-directionalcurrent fromlthe pulsating current :a .filter circuit is provided which includes condensers48 and -a. resistance .58. Due .to .the electrostatic capacityof the condensersfd and. theimpedance .of 'the resista-nce 50,.the. potential available. at the output of thelteris. substantially constant. It is' lpossible to. use a. resistance.50` in .place of a .conventional iinductance [choke -inasmuch yas .the .power requiredfto. operate .-.the`. television receiving tube. is relativelyllow,v the .current 'demand being of .the order of -afewmilliamperes .Connected between the output of the .lter supply-and .ground is .aV high resistance..52 which is not .entire1y..necessary..in the..circuit..but when -includedassists inimproving the regulation of the rectifier. The output terminal of. thefilter supply is highly. positivewithrespect to ground .and this terminal is connected directly .tothe second .accelerating anode. 2G. lIn. order to have available thenecessary positive., potentials of lowerv inten- .grid electrode is connected to the junction of resistances 58 and 66, the values of which are so chosen in proportion to the other resistance mem- '.bers.and Lpotentimeters to produce the desired .potentialfor thescreen grid electrode for nor- Ymal-operation of the cathode ray tube. The cathode-of the'icathoderay tube is connected to the `adjustable -terminal -of the potentimeter 62 in order to aord a manual control means for varying-'the average intensity of the cathode ray beam in orderto .control thebackground level..of .the

received -television .picture in .a lmanner .Well

known to those skilledin'jthe art.

The drawing also showsa televisiony receiver 64 which includes. means for receivngboth the .television.l pictureand sound signals and `.for developingv therefromaudio signals whichare. applied. .to .thespeaker 66 :and the detected4 picturesignals which rare. applied between .the-4 control electrode 28 an'dcathode. l2 .overrconductorsll The television receiver 64..also :includes deilection generators which .respondto `received synchrOniZingsignals and whichgenerate .voltage variations. necessary to .produce the required.. rate of change of. electromagnetic `v.ux at the .deflection .yoke (includingco'ils- 24.. and-,26) for deflect- .ingthe cathode raybeamf in mutually perpendicular. directions. The .deflecting coils .24v and.26

.arel therefore. connectedto fthetelevision receiver `(iflby way of .conductors10..and.l2.respectively.

The television receivenshown schematically at 64 also includes apowersupplyfiunit which islnec- .essary to supply the. 4required operati-ng'potentials to thetubes included ;in.the receiver. A'Ihe .-power input of thereceiver-:for thepower .-supplyL contained therein isthereforeshownas connectedzin .parallel .-with f tha-primary -:32 of transformer 34 .in 4.order-that the singlecontrol switch. 36- -may .controlthe operationfof the television receiver .aswell astheapplication ofthe-:potentials to the .power -supply v:unit: -for :the` television receiving tube.

With the system as so far described, when the .-powersupply switchv-36 is closed=it:.-is-fpessible vfor the 'televisionreceiving tube' yI 0 to-beginroperation before `the .-televisionreceiver 64Y has `begun to .operatefand-fasffa consequencethecathode rayf-beam-v. produced in4 the television; receiving vtube-.would .be vdirected againstrthe target surface v22^-with considerable 4intensityand velocity before th'e deecting coils24 rand 26 4#would be energized to produce-a dellection ofnthe beam. Such `an -undesirableoperation -could be prevented-as described abovebymanipulationof the beam intens-ity'controljpotentiometer62'so as to increase the bias .onthe-cathode: ray tube :to` a pointfwhere no beam .would'rbei generated but,'as stated above, the purpose of the presentinvention is toavoid the necessitytofsuchfa'dnanipulation and toeavoid 1 the possibilityzof:permanently damaging the .target screenitinthe. absenceof sucha manipulation.

Before acathode Yray Abeam L-canibefgenerated inv a receiving tube,. it is .'-necessaryt .thatf all 'of the' electrodes of the)` gun;structurefhaveapplied thereto approximately the correct operating potentials. If, for example, the potential of the first accelerating electrode I8 has applied thereto a potential considerably lower than that necessary for proper operation of the tube no cathode ray beam will be produced. In accordance with this invention, therefore, there is provided a means wherebyr proper operating potentials are not applied to all of the electrodes of the cathode ray tube until after sufficient time has elapsed to permit the receiver 64 to assume its normal operating temperature conditions and potentials. In order to prevent the application of the correct potentials to the electrodes of the cathode ray tube for a brief period of time immediately following a closure of the power supply switch 36 a condenserY 'l5 is provided. This condenser is connected between ground or the negative source of high potential and a point along the potentiometer or voltage divider which is connected between the cathode 42 of the rectifier tube 44 and ground. As shown in the drawing this condenser l5 is preferably connected between ground and the junction of resistance 54 and potentiometer 56 so that the screen grid electrode I6 and the rst accelerating electrode I8 are not supplied with their proper operating potentials until after the lapse of a predetermined length of time.

The delay in the application of the proper operating potentials to the electrodes I6 and I8 of the cathode ray tube is a result of the time necessary for the condenser 15 to be charged through resistance 54 to a potential corresponding to the static potential of the junction point between resistance 54 and potentiometer 56. When th'e television receiving set is in an inoperative condition (switch 36 open) the condenser 15 will have no charge thereon since any previous existing charge will be dissipated through resistclosure of switch 36 a certain rather bri-ef time delay is necessary to permit electrons to ow through the rectier tube. After the expiration of this time, however, the rectier tube will operate and a positive potential will appear at the output end of the filter resistance 50. Since the I' condenser l5 is relatively large and inasmuch' as the resistance 54 is quite high, the junction of the resistance 54 and the potentiometer 56 will be at substantially ground potential and the potential of this point will increase in a positive direction at a relatively slow rate. During this time no cathode ray beam can be produced in the television receiving tube since the condenser 'l5 prevents normal application of proper operating potentials to the electrodes I6 and I8. After the elapse of a predetermined time interval, as determined by the size of the resistance 54 and the capacity of the condenser 15, a cathode ray beam will be produced but by this time the television receiver 64 will have reached proper operating condition and deflection energy will be present in the deflecting coils 24 and 26. Accordingly, through the use of condenser l5 it is possible to turn on the television receiving set without the necessity of first adjusting the background control potentiometer 62 and without any possibility of the production of a cathode ray beam in the receiving beam prior to normal operation of the television receiver 64.

Since the amount of current required to operate the television receiving tube i6 is relatively low, the aggregate resistance of the voltage divider connected across the high voltage power supply may be quite high. rIhe total resistance included in the voltage divider is normally chosen such that the current ow therethrough will be of the order of one or two milliamperes. Furthermore, since the voltage applied to the first accelerating anode I8 is of the order of onethird to one-sixth of the potential applied to the second accelerating anode 26, the value of the resistance 54 is quite high and materially in excess of the combined resistance of the remaining elements of the voltage divider. In view of the fact that a very high resistance is included between the ungrounded plate of the condenser 15 and the positive terminal of the power supply, the charging rate of the condenser 'l5 is relatively slow and the size of the condenser 15 need not be particularly large and may well be of a value which can be conveniently and economically included in a television receiver. As stated above. the size of the condenser l5 determines the time which must expire before proper operating potentials are applied to the electrodes of the cathode ray tube and accordingly the condenser 15 is chosen in accordance with the length of time that it is desired to prevent production of the cathode ray beam. This time interval should, of course, considerably exceed the length of time required for the television receiver 64 to begin full operation.

From the above, it may be seen, therefore, that a new and improved power supply for a television receiver has been developed wherein any possibility of permanently damaging the target electrode in a television receiving tube by reason of a delay in the operation of the television receiver has been substantially entirely eliminated. Furthermore, it may be seen that through the exercise of the present invention, such a safeguard can be very conveniently and economically included in a television receiver.

It should be understood that the present invention is applicable to either a direct viewing tube or to a projection tube and although a tube of the direct viewing type is shown in the drawing, it is very desirable to apply the described invention to projection tubes since the beam velocities of such tubes and the intensity of the cathode ray beam are relatively high and consequently the target screen could be permanently ruined in a very short space of time.

Various alterations and modifications of the present invention may become apparent to those skilled in the art and it is desirable that any and all such modifications and alterations be considered within the perview of the present invention except as limited by the hereinafter appended claim.

We claim:

.A power supply for a television receiver including a rectifier and a filter network for supplying a source of direct current potential. a voltage divider connected across said source of potential including a plurality of resistor elements, a cathode ray tube including a cathode, a first accelerating anode and a second accelerating anode` means for connecting said second accelerating anode to the positive potential end of idlhe voltage divider, .meansifor connecting ,the frst :accelerating :anode to one of :the vresistor elementsi of the fvoltag'e f divider means Vfor connecting the cathode tohthevoltagetdvider near -the negative endthereof.; and aondenser directly y*connected between `the negative end of the voltage dividerandafpoint along the voltage ydivider near rwhere the rst, accelerating anode fisfgconnected -Wherebyrwhen-a, source of potential anode will; gradually increasein; ya .positivefzdirecytion :at a ratei determinedgbyjthefsize ffofithe `condenser ;and the; resistance of 1 the'yoltage divider through lwhich it js-A charged.

.DUDLEY rE. ,-FOS'PER. JOHN A.:RANKIN.

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