US2428616A - Electrical protective device - Google Patents

Description

Oct. 7, 1947. R. G. CLAPP A 2,428,616 A 5 l ELECTRICAL PROTECTIVE DEVICE Filed Nov. 2, 1945 2 sheets-sheet 1 WV 5 ,Pfff/m5? H v l ,Z /7

f aba); L y FF i0 `6` P' 007/507' J-l/C 250,000 f1. 70,000fl 07 51019507" @a wmf OCL 7, 1947. R. G. cLAPP ELECTRICAL PROTECTIVE DEVICE Filed Nov. v2, 19,43

2 Sheets-'Sheet 2 www# .that during, say 99.9%

Patented Oct. 7, 1947 2,428,616 ELECTRICAL PROTECTIVE DEVICE Richard G. Clapp,

Haverford, Pa., assignor, by

mesne assignments, to Philco Corporation, Philadelphia, Pa., a corporation of Pennsyl- Vania Application November 2, 1943, Serial No. 508,731 4 claims. (c1. 25o-27) This invention relates to a method and means for protecting electrical devices and circuits from the deleterious eiects of arcing or spark-over. More particularly, the invention relates to a circuit arrangement for utilizing a delay line as a means for swiftly clearing high voltage electrical systems of undesired or fortuitous electric arcs. The present invention has been found particularly useful as a means for protecting space discharge devices, and particularly high voltage vacuum tubes, from damage resulting from interelectrode spark-over. y

In the construction of light-weight, portable, radio detecting and ranging equipment (Radar), it has been found expedient to utilize, as the source of high frequency pulse signals (interrupted continuous waves) a small oscillator triode having its electrodes very closely spaced so as to decrease electron transit time as much as possible, and thus permit use of the device at very high frequencies. In order to derive substantial amounts of power from such tubes, it is customary to operate them at very high plate voltages, so high in fact, that arcing within the tube frequently occurs, ultimately destroying the tube, or at least unduly shortening its life.

In consequence of this difficulty it has been customary to operate these tubes in accordance with the known plate pulsing method, wherein the high voltage plate potential is applied to the tube only during the brief pulse intervals during which carrier energy is actually generated. In conventional Radar equipment the pulse duration may be of the order of one microsecc-nd, and the pulses may occur at the rate rof 1000y per second, there being an interval of about 1000 microseconds between pulses. Thus it will be seen of the time no sparkover can occur since the tube is not then subjected to operating plate potentials. Moreover, in the event that spark-over should occur during one of the short pulse periods,rthe ensuing arc will be quickly extinguished by the normal termination of the said period. Consequently, when plate pulsing is employed, spark-over has not proved to be a serious problem.

In the grid-pulsed mode of operation, however, spark-over and arcing have given substantial trouble, since in this system the cathode-anode circuit of the tube is permanently connected to the high voltage source, the tube being turned on and off at the desired rate by the application of a suitable pulse.- or switching-signal to the control grid. Since the arc within the tube, e. g. between plate and cathode, cannot be trlnlated rmay be substituted for by the signal applied to the control grid, the arc may persist until the tube is destroyed, or at least until its emission is seriously reduced. In .practice, such spark-over usually occurs intermittently, gradually gaining in frequency until the tube is either physically destroyed or otherwise rendered useless, usually by destruction of the cathode.

In accordance with the present invention, means are provided for extinguishing inter-electrede arcs within such a short time of their institution that no appreciable damage results therefrom. Through the practice of this invention the use of grid-pulsed high voltage space discharge devices is made feasible, and the advantages of grid pulsed operation made available for practicable utilization.

It is a principal object of the present invention to provide an electric circuit means which, in response to the occurrence of an undesired electric arc, or spark-over, shall act automatically to quench or extinguish the arc.

Itis a further object of the invention to provide an arc quenching means whose presence shall have no substantial effectl upon the normal operation of the system in which it is employed.

It is another object of the invention to provide an arc quenching device in which means are provided for delaying the operation of the device so that it shall exert no substantial effect upon pulse signals of predetermined quenching device is normally subjected.

It is Ystill another object of the invention to provide an arc quenching circuit having novel means for reducing the arc-.producing voltage to such a low level thatthe 'arc is no longer sustained.

Other objects and features ofthe invention will become apparent hereinafter. The invention itself will bestbe understood by reference to the following drawings, in which Figure 1 is a schematic diagram of a gridpulsed RF oscillator circuit constructed in accordance with the prior art;

Figure 2 is a schematic diagram of an improved grid-pulsed RF oscillator having certain of the features of the present invention;

Fig. 3'is a schematic diagram of a preferred embodiment of the invention;

Fig. 4 is a diagram used in explaining the operation of the circuit of Fig. 3; and

Fig. 5 is an alternative form of oscillatorwhich the oscillator of Figs. l,

It isbelieved that the present invention will duration to which the grid, and cathode.

in the absence of means forextinguishing theatre,

bestr be understood if reference is rst made to Fig. 1, which illustrates a grid-pulsed ultra high frequency oscillator circuit of a type preceding the present invention. The source I of ultra high frequency (UHF) or radio frequency (RF) oscillations is hereshonm to consist Aof..a triode oscillator mounted in fa suitable oscillator cavity 2. The triode comprises a cathode elementt, a grid 4, and a plate 5. VThe triode may be of a type known as a lighthouse tube (e. g. type 464-A) in which the cathode, grid, and plate Yelectrodes are arranged in closely adjacentparallel planes, and are generally circulargor disk-like in form. Inside the cavity 2 there maybe provideda suitable grid cylinder 6 which is physically supported by the peripheral portion of the grid 4, and electrically connected thereto. A plate 4choke 'I is (e. g. 1 inicrosecond) Where a type 464-A tube is used, with 3000 volts on the plate, the con- 'denser' may have to supply about 0.7 ampere at substantially rated voltage for about one micro-V second. Itis inevitable, of course, that, *during the periodof oscillation, i.. e. during the'load I denser by the triode.

usually associated with the central conductor or plate pillar 8 which runs down the cavity 2 Y to the plate 5. .The cathode 43.is electrically connected to the baseV of thecavity as shown. The

Y oscillator I provides a UHF oscillation, the frei quency of which depends upon the design and dimensions of the cavity.. An output Vsignal Vmay be derived by inserting asuitable 'probe' S, capacitive, inductive, or conductive, into the cavity in a manner well understood in the art.

Plate voltage maybe Asupplied to the cavity oscillator from any suitablethigh voltage source,

...such -for example as the high voltage rectifier I0.

The necessary'ltering may be provided, for eX- ample, by a suitable filter section comprisingY the series resistor II and theshunt condenser I2.

The cavity oscillator-.I kmay be switched periodically from an inactive onnon-oscillating condition to an operative or oscillating condition by the application to the control grid `4 of a suitable generated carrier is apt to occur. Between pulses period, the voltage across the condenser will vary (i. e'., fall) as energyY is taken from the con- However, so long as the voltageis not permitted to drop more than, say 5%, no undesired results will be experienced. Ii the voltage varies too greatly during the oscillating period undesired frequency modulation of the the condenser is, recharged to rated Y voltage through the current limiting 'resistor I3.

In the device of Fig. 2 a series resistor I3 of 70,000 ohms may be utilized, together with a shunt condenser Il! of l0,1003 ymicrolaradl In the event Vthat la' spark or arc should occur,

Y the'energy in the condenser I4 will discharge switching'signal, so as to provide, in the output of the oscillator, an V,interrupted .carrier wave signal. The switching signal may be derived from a suitable pulsesource, sodesignated in the drawing. In a typical form of theQsystem-theoscillator may be switched on-and-o'ii.V at the. rate of 1000 times persecond, eachoperative .period being of the order of onemicrosecond.

In systems Aof the vcharacter described it is usual to employ'i'plate voltages Lof an unusually .high i order considering the .size `aidspaeing off the `ele- V ments and tube electrodesinvolved. VIt islnotsuricommon to employ plate voltages of the order of 3000volts, -even though .-the yelectrode spacing in the triode is very materially less than-that which is customary in tubes'ordinarily operated In consequence, as,

at no more Vthan 250 volts.Y already indicated, internal spark-over `or arcing is frequently encountered, the arc occurring'be- "tweenthe -plate and grid, v=or between the plate, .In the event of an arc,rand

limiting resistor I 3 is connectedhetween .theout-Y put of the ri'ilter (i. e. the juncture of resistor vII Y K fand condenser-I2) vand the plate pillar il.^ In addi-F.V

non a relatively small condenser I4. is Yt .r1r1ected ybination IS-I.

through the triode, and this "discharge will beaccornpanied by a rapid Vfall( in condenser voltage. Xhcn this voltage has fallen suiciently, the arc is no longer sustained. Once the arc is extinguished normal conditions are rapidly restored, the voltage on Ithe condenser returning i to normal afteran interval kdetermined by the V time constant of the resistor-condenser com- It will benoted that the arc in the triode cannot be sustained directly by the voltage across lter condenser I2 because the series current-limiting resistor I3 limits the instantaneous current which can be derived from thatsource to a very `low value.

With regard to vthe Vcircuits oir'Figs. 1 and 2,

and with particular reference tothe circuit constants thereindicate'd, it is to be noted that Vby resorting to the present invention Vit has been possible Vto reduce the size `of .the condenser immediately in shuntwiththe oscillator by a factor ci 1/53. It therefore Vfollows. that the lelectrical energy available in the system'ofr Fig. 2, in the event of an arc, is only about 3l/3% Yof thatpresent in the circuitof Fig. 1. g

While the embodiment of ,'Fig. 2 represents fa substantialadvance over thesystem of Fig.V 1it leaves much to be desiredin systems Where it vis important or essential that vthegplate,voltagebe maintained invariable throughout the oscillation period. In the preferred embodiment, illustrated inY Fig. 3, a construction is provided whichfcombines the protective features of Fig. 2, together withtheV advantage of a plate :current source whose voltage remains at apredetermined, sub-Y stantially fixed level throughout theoscillation period. In this embodiment the current limiting l resistor i3 of Fig. 2 ispretainedfbut in place of the condenser I4 there isgprovided a` suitable delay line, so designated in the drawing, whose remote end is .preferably open circuitedas A.iif

lustrated, orterminated in .animpedance whichY is high compared to the characteristic impedance of the line. Y comprising series .inductance andshuntcapacitance elements, and the-simplest Ylines of lthisv character 'are suitable. In -the design'iof the .Preferably thegline .is one articial line, resort may be had to conventional line theory, the principal requirements being that the lines characteristic impedance be preferably as purely resistive as design considerations permit. Preferably also the characteristic impedance is kept reasonably low for a reason which will appear hereinafter. In one physical embodiment of the invention a characteristic impedance of 150 ohms was found satisfactory. In this particular embodiment the line consisted of three identical bridged-T sections, each section comprising a series inductance of 17 microhenries bridged by a condenser of 125 micromicrofarads, with a 0.001 microfarad shunt condenser connected between the midpoint of the inductance and the other side of the line, in this instance ground. This particular line was chosen because it provided a practically constant delay over most of the frequency band occupied by the pulse signals applied to the input circuit of the oscillator.

While the circuit of Fig. 3 has an important attribute not possessed by that of Fig. 2, to be explained hereinafter, the circuit of Fig. 3 protects the oscillator triode from damage due to spark-over in much the same way las that of Fig. 2 in that in the event of spark-over the only energy available to pass through the tube in any appreciable quantity is that available on the three shunt condensers I5, IE5, and I1. These condensers should be only of such magnitude that they are enabled to supply the normal requirements of the oscillator during the oscillation period, their initial charge being restored during the relatively long inactive period between pulses. It may be noted that in Fig. 3 the sum of the capacities of the three shunt condensers I5, Iii, and I1 is equal to the capacity of the single shunt condenser Il! of Fig. 2.

The peculiar operation of the delay line system, which distinguishes it from the operation of Fig. 2, may best be explained by referring to the diagram of Fig. 4. In the upper portion of the drawing there is shown a graph of the grid signal voltage. This voltage, which is in the form of a switching-signal, is derived from the pulse source I8, and applied to the control grid of the oscillator triode to control the oscillation thereof. During the inactive period, which may comprise 99.9% of a complete operating cycle, the source I8 (Fig. 3) supplies an output voltage which is suiciently negative to bias the oscillator below plate current cutoif. During the oscillation period the source I8 supplies a square pulse I9 (Fig. 4) which drives the oscillator grid sufficiently positive to permit oscillation during what is referred to as the oscillation period. In Fig. 4, for purposes of illustration, this period is of 1 microsecond duration.

Although the artificial delay line is physically very short, the oscillation period of one microsecond expires before the reflected plate current pulse returns to the input end, or simultaneously therewith, and consequently, although the far end of the line is open-circuited, the impedance seen looking into the line, during the short oscillation period, is the characteristic, or surge, impedance of the line. Since the line is now effectively the source of plate current (due to the charge residing in the condensers I5, I6, and I'l) the plate current source appears to have an internal impedance equal to the characteristic impedance of the line. In the physical embodiment hereinbefore discussed, this impedance amounted 6. to 150 ohms, and since, during the oscillation period the triode draws 0.7 ampere, the plate voltage is diminished by about 150 0.7 volts, i. e. by approximately volts, during this period. This dro-p is indicated in Fig. 4 by the portion 20 olf the plate voltage graph 2i. Since the characteristic impedance of the delay line is substantially pure resistance the voltage drops almost instantaneously, and then remains constant at the level 22 during the oscillation period. Because of this the oscillator triode is enabled to operate at constant plate Voltage, and consequently undesired frequency modulation effects are largely avoided.

From the foregoing it will be evident that, by choosing a delay line having not too high a characteristic impedance, no substantial drop in plate voltage is effected by operation oif the oscillator. In the example under consideration the drop of 100 volts, from an initial value of 3000 volts, 'is not regarded as substantial. This is particularly true since the operating plate voltage level of 2900 remains fixed throughout the oscillation period.

Preferably the delay line is of such length that at the conclusion of the oscillation period, or shortly thereafter, the pulse of plate voltage, which occurred across the input terminals of the line at the start of the oscillation period, returns-after reflection from the open-circuited end of the lineto the input terminal of the line. Since the return pulse is in the same polarity as the initiating pulse it also produces an effective lowering of the plate voltage. If the oscillation period terminates at precisely the instant that the reflected pulse returns (as shown in Fig. 4), and if the losses in the delay line are small, as is preferred, the returning pulse will approach, in magnitude, twice the magnitude of the initiating pulse. In the case under consideration the initiating pulse (i. e. the plate voltage drop) had an amplitude of approximately 100 volts, represented by the voltage drop 20, and consequently the voltage drop produced by the reflected pulse is represented by the difference between the normal voltage level and that represented at 23. This new voltage level persists without substantial change for the duration of the original pulse, i. e. 1 microsecond, after which the voltage returns to normal along a path 24 which approximates the discharge curve of a condenser, ibut which may vary therefrom depending upon the characteristics of the particular line employed.

When it is desired that the front ofthe reiiected pulse shall return at precisely the instant that the oscillation period terminates, as illustrated in Fig. 4, the delay line should b'e of such length that its delay time (the time required for an impulse to travel from one end of the line to the other) is substantially equal to one-half the oscillation period.

Let us now assume that, at the instant designated 25, an arc occurs between the anode and cathode of the oscillator tube of Fig. 3. The Sudden imposition of a heavy load (which may approach a short circuit in magnitude) upon. the effective source of plate current, i. e. the delay line, causes a sudden and very considerable drop in plate voltage, as indicated by the falling portion 25 of the plate voltage graph. A new voltage level 21 is then maintained until the instant 28 at which time the reflected pulse returns from the end of the line and drives the plate voltage still farther down, reaching the new level designated 29. This level may be of the order of zero zaza-foie Volts, or it may -belabove or evenbelowfzero, Athe precise magnitude of' the'reflected pulsefdepend` ingupon a number of factors such as the. plate impedance of ther tube duringthe arc, the designandvlosses' of the delay line, and possibly other factors. In any event the plate voltage in a properlyconstructed system will fall so low that the arc can no longer bemaintained, with the result that the tube is speedily cleared of the Yundesired condition. Shortly after the voltage level 29vis reached, theV voltage again returns to' normal along a path 30' which approximates the discharge curve of a condenser, but which may vary therefrom depending upon the particular circuit constants employed.

In the schematic diagrams of Figs. 1 to 3- the vacuum t-ube oscillator illustrated is one known as a cavity type oscillator. The invention is perfectly general in its application, however, and it may be associated with any conventional oscillator circuit, Whether low frequency'or ultra high frequency, and whether the control grid of the oscillator tube is supplied with a modulation frequency or not. A conventional oscillator circuit of` the Hartley type'is illustrated in Fig. 5. As shown; the oscillator comprises a triode 3|, a split tank coil 32-33, a tank tuning condenser 3.4, and agrid condenser-grid leak combination 35,-35. A source I8 of pulse signals may be connected in the grid circuit of the oscillator as .Y shown, an RF bypass condenser` 3-1 being con` nected in shunt with the-pulse source to provide an RF signal path thereacross; 38, which may be a resistor or anRF choke, is connectedY between the triode plate and the high* voltage terminal 39. If it isdesired to substitute the oscillator of Fig. 5` for any of the oscillatorsV of Figs. 1, 2, and 3, the-latter maybe disconnected, and the terminal 39 of Fig; 5 connected to the high voltageterminal 40 of the said figures.

The present invention is not limited to the speciiic circuits and electricalv constants disclosed in the accompanying drawings, and it will be evident tothose skilled in the art that Ythe broad underlying principles disclosed herein admit of considerable latitude in the construction of theprotective networks contemplated by the invention.

I claim:

1. In a pulse-operated vacuum tube circuit embodying a vacuum tube and a high Voltage plate supply source therefor, said vacuum tube having cathode and anode electrodes and a control electrode, a protectivecircuit for said vacuum tube comprising a resistor'connected between'one i terminal of said source and one of the electrodes of said vacuum tube, aV connection between the other terminal of said source and the other electrodeof said vacuum tube, means for applying control pulses to said control electrode to render said vacuum tube operative during each pulse period, and a delay line having its input terminals connected in shunt with said vacuum tube, and

` its output terminals open-circuited, the shunt capacitance of said delay line beingof such magnitude as to Venable'it to store electrostatically An impedancev Y electrode to render said vacuum tube operative only that electrical energy required to maintain l said vacuum tube in an operating condition throughout the pulse period, said resistor being of suflicient magnitudeto limit `to a very loW value the instantaneous current which can be derived wherein the delay time kofthe line is substantially equal to one-halfthesaidpulse period., Y

3. In a pulse-operated oscillator circuitk ern: bodying a resonant circuit, a vacuumtuberhaving anlanode, a cathode and a controllelectrode, and a. high voltage platey supply source therefor hav.- ingv positive and negative terminals., Vsaid vacuum tube being subjectto fortuitous.,internaljarcing, an arcextinguishing circuitcomprising a resistor connectedbetween the positive terminal, of said source and the anode of said vacuum tube, aconnection between, the negative terminal of said source and they, cathode of said vacuum` tube, means for applying controlV pulsesto said control during each pulse period, andaV delay linercomprising series inductance and shunt capacitance having its input terminals connectedjinshunt with the anode-cathode circuit of said vacuum tube and whose output terminals are substantially open-circuited, the shunt Vcapacity of said delay line being of substantially no greater magnitude than that required to enable the line to store electrostatically only that electrical energy required to maintain the oscillator Vin serviceable operatingcondition throughout the pulse period, saidresistorbeing of suicient magnitude to limit the instantaneous currentwhich can Vbe derived directlyfrom said sourcetoa value lessthan that required to sustain ,an arcbetween the electrodes of said vacuum tube.

4. In a pulse-operated electron ytube circuit,`

an electron Ytube having cathode, anode and control electrodes, ai high voltage anode supply source, connections ,between said source andA said cathode and anode, meansforeifecting intermittent pulse eXcitationrof said control electrode, aV

resistor serially included in` one of` said power supply connections and having a magnitude'suiiicient to limit to a low value the current :dow from said source to said tube in the'event of a sparkover within the tube, and a delay-line, including capacitance and inductance, having itsV input terminals eiectively connected to said anode and cathode, and, through said resistor,` to said source, the-shunt capacitance of said delay line -being of such magnitude as to renable'- it to vstore electrostatically only that electrical energy required to maintain said vacuum tube in an operating con-v Y dition throughout the pulse period. Y

Y RICHARD G. CLAPP.

REFERENCES crriiny The followingY referencesfare of record in the Y le of this patent: Y Y

UNITED STATES PATENTS" Number Name Y i ,Date

2,270,799 Gulliks'en- Jan.,2019`42 2,129,646 Bouwers.Y Sept. 131938 1,978,461 Hoover` etV al. Y Oct.. ,30, 1934 2,206,710, Tonks V 'July 2, V1940 FOREIGN BATENTS,

' v379,957 Great Britain Sept. 8; 1932 497,147 *zGreat Britain Dec. 9,-:1938

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