US2434084A - Protective device - Google Patents

Description

Jan. 6, 1948. I Wl A, STEWART 2,434,084

PROTECTIVE DEVICE Fild Nov. 3o, 1945 C752 250,000Jl (74 /w Fier/Hf@ L ,Z l i I .500555 T-MT -i- Patented Jan. 6, 1.948v

by mesne assignments, to Philco Corporation, Philadelphia, Pa., a corporation `of Pennsylvani'a Application November .30, 1943, Serial No. 512,383

claims. (c1. 25o-27) This invention relates to a novel means for protecting electrical devices and circuits from the deleterious effects of arcing or spark-over. More particularly, the invention relatesto a cir-'- cuit arrangement employing a parallel resistance-capacitance combination, serially connected between a high voltage source and a device supplied thereby for swiftly clearing the system of undesired or fortuitous electric arcs. The invention has been found particularly useful as a means for protecting space discharge devices, and particularly high voltage vacuum tubes, 'from damage resulting from inter-electrode sparkover. The present invention is generally related to that disclosed in a copending application of Richard G. Clapp7 Serial No. 508,731, led November 2, 1943 now Patent Number 2,428,616 issued October 7, 194'7. This invention, however, is characterized by the employment of the parallel resistance-capacitance combination above mentioned.

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 tri-` ode 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 may occur due to residualgas, etc., ultimately destroying the tube, or at least unduly shortening its life.

In consequence of this difliculty, 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 microsecond, and the pulses may occur at the rate of 1000 per second, there being an interval of about 1000 microseconds between pulses. Thus it will be seen that during, say, 99.9% of the time no spark-over 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, the en` suing arc will be quickly extinguished by the normal termination of the said period'. Consequently, when plate pulsing is employed, sparkover 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 cathodeanode circuit of the tube is permanently con-v nected to the high voltage source, the tube bef ing Vturned on and off at the desired rate by the application of a suitable pulse- Yor switchingsignal to the. control grid. Since the arc within the tube, e. g. between plate and cathode, cannot be` terminated by the signal appliedr to the control grid, the arc may persist until theu tube is destroyed,A or at least until its emission is seriously reduced. In practice, such spark-over usually occurs intermittently, gradually gaining in frequency of occurrence 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-electrode arcs within such a short time of their institutiori that no appreciable damagel results therefrom. Through the practice vof this mvention the use (Sigrid-pulsed high voltage space discharge devices isrnade feasible, and the advantages of grid-pulsed `operation of radio frequency equipment are made available for practoable utilization. n y

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.

It is a further object of the invention to provide an arc-.quenching `means whose presence shall have no. substantial effect upon the normal operation of the system whichv it is employed.

It is another object of the invention tovpr'ovide anl arc-,quenching circuit having novel means for reducing ,theV arc-producing voltage to such a low level that the arc is no longer sustained.

It is still another object of the invention to` provide a novel arc-quenching means in the form of a parallel resistance-capacitance circuit serially connectedv between the` high voltage source and the device supplied thereby.

Other objects and features of the invention will appear hereinafter. e

In the accompanying drawing:

LFiel. 1 is a schematic diagram of av grid-pulsed R. FQ oscillator circuit vconstructed in accordance With the prior art;-

Fig. 2v is a similar diagram of a grid-pulsed RF. oscillator embodying the present invention;

Fig. 3 is a graphic nillustration of the operation of the circuit of Fig. 2; and

Fig. 4 is a diagrammatic illustration of an alternative form of oscillator which may be substituted for the oscillator shown in Figs. 1 and 2,.Y

It is believed that thek present invention will best be understood if reference is first made to Fig. 1,v which illustrates a grid-pulsed ultra high frequency oscillatorV circuit of a type preceding the present invention. The source l of ultra high frequency (U. H. F.) or radio frequency (R. F.) oscillations is here shown to consist of a triode oscillator mounted in a suitable oscillator cavity 2. element 3, a grid 4, and a plate 5. The triode may be of a type known as a lighthouse tube (e. g. type 446B, 464A, etc.) in which the cathode, grid, and plate electrodes are arranged in closely adjacent parallel planes, and are generally circular or disk-like in form. Inside the cavity 2 there may be provided a suitable grid cylinder` Ii which is physically supported by the peripheral portion of the grid 4, and electrically connected thereto. A plate choke 'I is usually associated with the central conductor or plate pillar 8 which extends inside the cavity 2 to the plate 5. The cathode 3 is electrically connected to the base of the cavity as shown. The oscillator I provides a U. H. F. oscillation, the frequency of which depends upon the design and dimensions of the cavity. An output signal may be derived by inserting a suitable probe 9, capacitive, inductive, or conductive, into the cavity in a manner well understood in the art.

Plate voltage may be supplied to the cavity oscillator from any suitable high voltage source, such for example as the high voltage rectifier I0. rlhe necessary ltering may be provided, for eX- ample, by a suitable iilter section comprising the series resistor II and the shunt condenser I2.

The cavity oscillator I may be switched periodically from an inactive or non-oscillating condition to an operative or oscillating condition by the application to the control grid 4 of a suitable switching signal, so as to provide, in the output of the oscillator, an interrupted carrier wave signal. The switching signal may be derived from a suitable pulse source, so designated in the drawing. In a typical form of the system the oscillator may be switched on-and-oi at the rate of 1000 times per second, each operative period being of the order of one microsecond.

In systems of the character described, it is usual to employ plate voltages of an unusually high order considering the size and spacing of the elements and tube electrodes involved. It is not uncommon to employ plate voltages of the order of 3000 volts, even though the electrode spacing in the triode is very materially less than that which is customary in tubes ordinarily operated at no more than 250 volts. In consequence, as already indicated, internal spark-over or arcing is frequently encountered, the arc occurring between the plate and grid, or between the plate, grid, and cathode. In the event of an arc, and in the absence of means for extinguishing the arc,

all of the electrical energy stored in the filter condenser I2 is discharged through the tube. If the filter section employs circuit components adequate properly to filter the output of the rectier I0, e. g. a shunt output filter condenser of 0.1 microfarad, there will ordinarily be sufficient electrical energy in the output condenser to damage the tube rather seriously. Continued repetition of such arcing may either destroy the tube, or render it otherwise useless.

In accordance with the present invention, as illustrated in Fig. 2, a current-limiting resistor I3 is connected between the output of the lter (i. e. the juncture of resistor II and condenser I2) and the plate pillar 8. Further, a condenser I4 is connected in shunt relation with the resistor I3, thus forming a parallel resistor-capacitance circuit serially included in the high voltage connection between the voltage source and 'Ihe triode comprises a cathode 1 the anode of the oscillator tube. The values of the resistance I3 and the capacitance I4 are such that the voltage applied to theV tube is normally maintained substantially constant, but decreases to a low value in the event of arcing within the tube. In the device of Fig. 2, a resistor I3 of 22,000 ohms may be utilized, together with a condenser I4 of 20,000 micromicrofarads, for one speciiic type of vacuum tube oscillator. It will be understood, however, that the values of these elements will be varied according to the specic use to which the invention is to be applied in any instance.

The operation of the system of Fig. 2 produced by the incorporation of the present invention may best be understood by reference to the graphical illustration of Fig. 3. Of course, it will be understood that this figure cannot accurately show the relative time intervals mentioned above. In the upper portion of Fig. 3, 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 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 pulse source supplies a control voltage which is sufficiently negative to bias the oscillator below plate current cut-off. During the active period, the pulse source supplies a square pulse I5 which drives the oscillator grid suiiiciently positive to permit oscillation during what is referred to as the oscillation period. As mentioned above, this period may have a duration of one microsecond.

The lower graph of Fig. 3 represents the voltage applied to the plate or anode of the oscillator tube. From Fig. 2, it will be apparent that initially, and prior to the application of a pulse to the control grid of the oscillator, the voltage on the oscillator plate will be substantially equal to the voltage of the supply source, since there is no current flow and hence no voltage drop between the high voltage source and the plate. That is, there is no potential difference between the two plates of the condenser I4, and no charge exists thereacross. When a controlling pulse, such as shown at I5 in Fig. 3, is applied to the control grid of the oscillator, the oscillator tube is rendered operative and plate current flow is initiated. During the pulse interval, the condenser I4 is suddenly charged by virtue of its inclusion in a completed circuit with the high voltage source, the oscillator tube in its conductive or operating condition presenting low impedance to the charging current. The condenser I4, in its uncharged state, presents an even lower impedance to the now of current, and consequently plate current is supplied to the oscillator at almost the normal output voltage of the power supply system. If the condenser I4 is chosen suiiioiently large this condition will remain throughout the pulse period. The slight plate voltage drop which is occasioned by the condenser i4 is illustrated in the lower graph by the falling portion I6 of the plate voltage characteristic. When the controlling pulse interval terminates, the plate current flow (and hence also the condenser charging process) is interrupted, and the condenser I4 discharges through the resistor I3, causing the voltage at the plate terminal 30 to return to its full value as indicated at I1 in Fig. 3.

When tube spark-over or internal arcing occurs it usually is initiated by the applied pulse signal. Assume, for example, that the pulse I5a s. (Fig. 3) initiates such a condition; There will follow an initial rush of current through the tube asthe condenser I4 is suddenly charged. Since, in the spark-over condition, the triode oscillator has a v'ery'loxivV impedance, the condenser charges toa hig'hvoltage almost instantaneously, resulting in the sudden fall in plate voltage indicated in Fig. 3 by the reference numeral I8. As the condenser I4 approaches a fully charged condition the fall in plate voltage proceeds less rapidly as indicated by the portion Ia of the plate current characteristic. When the plate voltage has fallen sufciently the arc is no longer sustained. In Fig. 3 this occurs at the point |81) at which time the are is said to be quenched The condenser I4 then discharges through the resistor I3, the plate voltage returning along an exponential path I9, the speed of return depending upon the time constant of the circuit I3-I4. It should be noted that the resistor I3 is of such high value that it cannot, of itself, supply suflicient current to the tube to sustain the arc.

Care should be exercised in selecting the condenser I4 so that it is large enough to prevent too great a plate voltage drop I6 during normal operation (a drop in plate voltage is usually allowable), but not so large that the current required to charge it in the event of spark-over is of a magnitude su'lcient to damage the tube before the arc is quenched.

While Fig. 2 illustrates the invention as applied to a cavity-type oscillator, the invention may be applied to any 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. 4. As shown, the oscillator comprises a triode 20, a split tank coil 2I--22, a tank tuning condenser 23, and a grid condenser-grid leak combination 2li-2E. A source 26 of pulse signals may be connected in the grid circuit of the oscillator as shown, an R. F. bypass condenser 21 being connected in shunt with the pulse source to provide an R. F. signal path thereacross. An impedance 28, which may be a resistor or an R. F. choke, is connected between the triode plate and the high voltage terminal 29. If it is desired to substitute the oscillator of Fig. 4 for that of Fig. 2, the latter may be disconnected and the terminal 29 of Fig. 4 may be connected to the terminal 30 of Fig. 2.

It will be apparent to those skilled in the art that the invention is not limited to a specic circuit nor to the particular values of the circuit constants given herein, but is capable of various applications and modifications such as fall within the scope of the appended claims. In the claims the term arcing is to be construed as suinciently broad to include the phenomenon of spark-over.

I claim:

l. In an electrical circuit, an intermittently operable load device to which a predetermined voltage is applied during the operating periods thereof, said device being subject to internal arcing, a voltage supply source connected to said device, and a parallel resistance-capacitance circuit serially connected between said source and said device, the values of resistance and capacitance being such that during normal operation of said device the voltage applied thereto is substantially unaffected but, in the event of an arc greets-rwitliin the device; said voltage decreasesf abrl'iptly:v

to* a value vbelo'v'vthat required to' sustain -1 said" are.

2. In an eletrical'circuit,- an intermittently" operableV electron tube' tothe anode of which a predetermined voltage is applied during the operating periods 'thereo'said tube being subject to internal? arcinga voltage' supply source' connected to the anode of said tube, a capacitori'serially connected between said source and said anode, said capacitor having such a capacitance value that during normal operation of said tube the anode voltage is substantially unaffected by the presence of the capacitor but, in theevent of an arc within the tube, said voltage is decreased abruptly by the capacitor to a value below that required to sustain said arc, and a resistor in parallel with said capacitor for discharging the capacitor, said resistor having such a resistance value that it cannot deleteriously affect the operation of said tube nor interfere with the arc-quenching action of said capacitor.

3. In an electrical circuit, an electron tube having a control electrode and also having an anode to which a predetermined voltage is applied during operation, said tube being subject to internal arcing, a source of pulse signals connected to said control electrode for operating said tube intermittently, a voltage supply source connected to the anode of said tube, and a parallel resistancecapacitance circuit serially connected between said source and said anode, the values of resistance and capacitance being such that during ncrmal operation of said tube the anode voltage is substantially unaffected but, in the event of an arc within the tube, said voltage decreases abruptly to a value below that required to sustain said arc.

4. A system for transmitting high frequency pulse signals, comprising a vacuum tube having an anode and a control electrode, an anode voltage supply source connected to said anode, said tube being subject to internal arcing, a source of pulse signals connected to said control electrode for operating said tube intermittently, and means including parallel-connected resistance and capacitance elements serially connected between said source and said anode for preventing damage to said tube in the event of arcing therein, the resistance and capacitance values of said elementsbeing such that during normal operation of said tube the anode voltage is substantially unaiected but, in the event of an arc within the tube, said voltage decreases abruptly to a value below that required to sustain said arc.

5. A system for transmitting ultra-high frequency pulse signals, comprising an ultra-high frequency oscillator including a vacuum tube having cathode, grid and anode electrodes, said electrodes having such close spacing that the tube is subject to internal arcing, an anode voltage supply source connected to said anode, a source of pulse signals connected to said grid for operating said oscillator intermittently, a capacitor serially connected between said source and said anode, said capacitor having such a capacitance value that during normal operation of said tube the anode voltage is substantially unaffected by the presence of the capacitor but, in the event of an arc within the tube, said voltage is decreased abruptly by the capacitor to a value below that required to sustain said arc, and a resistor in parallel with said capacitor for discharging the capacitor, said resistor having such a resistance Value that it cannot deleteriously affect the operation of said tube no1` interfere with the :airc-v quenching action of said capacitor.

WILLIAM A. STEWART.

REFERENCES CITED The following references are of record in the le of this patent:

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