US2802945A - Oscillator-type control circuit - Google Patents

Description

Aug, 13, 1957 H. T. SEELEY I CSCILLATOR-TYPE CONTROL CIRCUIT I Filed Novfl'Y, 1954 co/vrn 04 L 50 g 8 m2 5 #2 2 2 g /00 S 2 I 90 k l I so a m i k a V I #3 l l I l I I I I l I -a -a 4 -2 s -6 -4 -2 -4.7 CONTROL GRID VOLTS I CONTROL GRID VOLTS Ihventor: Harold T. Seeleg,

2,802,945 Patented Aug. 13, 1957 United States Patent Ofiice p 7 2,802,945 OSCILLATOR-TYPE coNTRoL cmcurr Harold T. Seeley, Havertown, Pa., assignor to General Electric Company, a corporation of New York .Application November 17, 1954, Serial No. 469,378

' 6 Claims. (Cl. 250--36) I .This invention relates to an electronic control arrangement and, moreparticularly, a control arrangement which utilizes an oscillator to eflect operation of an electroresponsive device.

Although various types of oscillators have been proposed heretofore for elfecting operation of an electroresponsive device such as a relay, prior arrangements of which I am aware have not provided the high degree of reliability required for applications such as protective relaying. Minor variations in the control voltage supplied to the elements of the oscillator tube have caused the tube to respond inconsistently to predetermined input signals. In a protective relaying system, where extreme accuracy is usually required, such a characteristic seriously impairs the reliability of the system.

It is,-therefore, an object of my invention to provide an oscillator which will consistently respond to input signals of a predetermined magnitude in spiteof any variations in the control voltage supplied to the oscillator.

A further object is to operate an electroresponsive device such as a relay from an oscillator, the startingpoint of which is insensitive to variations in the control voltage supplied to the oscillator. a

I In accordance with one form of my invention, I provide an electron tube having a cathode, a plate, and a control grid, the control grid and the platebeing coupled together in a manner to form an oscillator. Plate voltage is supplied by connecting the plate to one of the two terminals of a source of control voltage which is subject to variations. When a signal voltage varying beyond a predetermined starting value is applied between the control grid and the other terminal of the source, oscillations are initiated. This starting value is rendered. insensitive to variations in the control voltage by means of a voltage divider connected across said terminals and having a cathode-connected tap which is set at a predetermined percentage of the variable control voltage. By correctly selecting the value of this predetermined percentage, the bias applied to the cathode is varied in response to control voltage variations by an amount suflicient to compensate for any tendency of the varying control voltage to change the oscillation starting point.

, For a more complete understanding of my invention, reference may be had to the accompanying drawing in which Fig. 1 is a diagrammatic illustration of a control circuit embodying my invention, whereas Figs. 2 and 3 are graphical representations of certain characteristics of an electron tube of the type used in the circuit of Fig. 1.

1 Referring now to the drawing, the numeral is used to designate a device such as a protective relay which it is desired to operate instantaneously when the magnitude of a signal voltage varies beyond a predetermined value. Additionally, it is desired that the response of this device beinstantaneous even though the signal voltage varies only gradually. To obtain this desired response, I have provided an oscillator 11 having an input or grid circuit 12'to the terminals 13 and 14 of which the signal or inputvoltage is applied. Theoscillator comprises an electron tube illustrated in the drawing as a pentode 16 having a cathode 17, a plate or anode 18, a control grid 19, a screen grid 20 and a suppressor grid 21. The plate 18 is connected through an output or plate circuit 25 to the positive terminal 26 of a suitable source of unidirectional control voltage. A suitable transformer 27 is provided for coupling the output circuit 25 to the input circuit 12 in such a manner that oscillations will be produced when the input voltage, which preferably is unidirectional, exceeds or varies beyond a predetermined value. The frequency of these oscillations is determined largely by a capacitor 29 connected across the output winding 28 of the transformer 27 to form a tuned oscillation circuit, as is well known in the art. The usual grid leak resistor is shown at 29a connected in series with the control grid 19.

The screen grid 20 is shown connected directly to the positive terminal 26, whereas the suppressor grid 21 is connected in a conventional manner to the cathode 17. The cathode 17 is maintained ata voltage which is a predetermined fixed percentage of the total supply or control voltage between the positive terminal 26 and the negative terminal 30. This is accomplished by connecting the cathode to a tap 31 of a voltage-dividing resistor 32 connected across the terminals 26 and 30. In accordance with my invention, the setting of this tap 31 is selected in such a manner that the performance of the oscillator will be insensitive to variations in the control voltage existing between the terminals 26, 30. This latter feature will soon be explained in greater detail.

The circuit thus far described effects operation of the device 10 in the following manner: When the input or signal voltage applied to the terminals 13, 14 varies beyond a predetermined value, the oscillator 11 instantly begins to oscillate and thereby causes an abrupt increase in the effective current flowing through the output circuit 25. This abrupt increase in current :acts through an inductive coupling 34 to etfect instantaneous operation of the device 10 as desired. The increased efiective current continues to flow in the output circuit 25 until the signal voltage returns to a normal range of values and thereby stops oscillations of the oscillator 11.

A basic problem presented in operating an electroresponsive device from an oscillator of the type shown is that variations in the control voltage across the terminals 26, 30 tend to vary the oscillation starting point. This is an especially critical problem where a battery is used as a source for the control voltage since the voltage of a battery rated at, say, volts may vary between 100 and volts. Variations of this magnitude can materially effect the oscillation starting point and thereby cause inconsistent operation of the device it). Because of the particular setting chosen for the tap of my cathode-connected voltage divider, I am able to render the oscillator starting point insensitive to such variations in the control voltage. The manner in which this insensitivity is obtained may be better understood by referring to Fig. 2 wherein I have shown a family of curves representing certain tube characteristics for a typical pentode such as the type 5693. Each of the curves illustrates the manner in which, for a constant screen grid voltage, the plate current will vary in response to variations in the control grid voltage. Plate voltage variations have been ignored because in a well designed pentode, for normal voltage conditions, the plate current varies only slightly with plate potential.

For a predetermined coefiicient of coupling M between the input and output circuits, the starting point for oscillations will be determined by the slope of the curves shown in Fig. 2. Only when this slope equals a predetermined value, do oscillations begin. Thus, assuming that the control grid of the tube is initially biased below the oscillation starting point, then to initiate oscillations, it is necessary to increase this control grid voltage ina positive direction until a point on the curve is reached at *which the slope equals or exceeds the critb cal value; It has been found th at lthis criticalgvalue of slope is the same irrespective of thescreen -grid voltage. Assuming then that the value. of Mris constant, .the critical value of slope at which oscillations will begin is confoundon the other two curves. .Theselaredesignated respectively Ai andAz; .For different. values .of M, oscillations will start at diflerent sets of equal slopes.v I have designated another such set of these equal. slope values as B,.B1, and B2. After these points of equal slope have been determined, theymay, then beplotted in terms of asoaces he arrived at by experimentally varying the' tap setting in the actual tube circuit.

I am aware that voltage dividers have been used heretofore for obtaining varying degrees of bias on the control elements of electron tubes, but I am not aware of their use in accordance with this invention, i. e., in controlling the cathode voltage of an oscillator in such a manner that the oscillation startingpoint is insensitive to appreciable variations in the control voltage:

.It will now be evident to those skilled in'the art that the foregoing preferred embodiment of my invention including a pentode oscillator provided with inductive'feedback coupling is illustrative only, and that the invention control grid volts versus screen grid volts. Fig.3 shows these oscillation starting points so plotted, the curve A containing the points A, A1, and A2 and the curve B containing the points B, B1,. and B2. :Thus,.inl Fig. 3 each curve represents, for a fixed 'value of M, the oscillation starting points in terms of. screen grid volts versus control grid volts. With the curves of Fig. BestabIished, the tap setting of the voltage-dividing resistor 32 may then be determined. First, assume that the setting is arrangedto provide between the tap.31. andthe negative terminal 'a voltage drop of .10. times'th'e total volt: age between the terminals 26 and 30. With the cathode taken as a reference point of zero voltage and withra total of 100 volts between the terminals 26 and 30, the screen grid will have a voltage of 90 and the negative ten minal a voltage of 10. is the same as that used for establishing the cur've' B or Fig. 3, it can then be determined from .this curve that when the screen grid is at 90 volts, the control grid volt; age requiredto begin oscillations is 4.7 volts. This step is illustrated by the dotted lines-of Fig. 3. Since this value of control grid voltage is measuredswith re-, spect to the cathode, this means that theinputlvoltage between the control grid 19 and the negative terminal 30 must equal or exceed 4.7-(10) or +5.3 volts before oscillations will begin. Now, if for an identical tap setting this same series of calculations is performed for a total voltage of 140 volts across the terminals, 26,30, it will be foundthat an input voltage of 7.5 volts between the control grid and terminal 30 is required to initiate oscillations. Thus, it may be seen that, with a tap setting of .10, when the supply voltage varies between the extremes of l00and 140, the input voltage required to initiate oscillations may vary by about 40 percent. For protective relaying such a variation is obviously unac-. ceptable. calculations for a series of difiere'nt' tap settings, it is possible to determine the optimum tap setting required in order that this value of input voltage remain substantially constant as the control voltage varies between its extreme limits. For a curve such as B it has been found, by using the above calculations, that a tap setting of .052 will cause oscillations to start at an essentially constant input voltage (of 0.30 volts) even if the control voltage varies between 100 and 140 volts. Thus,

If the coupling coeflicient M However, by repeating the above described I ing an unidirectional control voltage which is subject'fto with the tap 31 so adjusted, the device 10 will consistently which the tap setting may be determined by calculai i tions, it i s to be understood that the same results can is equally applicable to other well-known oscillators having one or more than one grid between plate and cathode. Thus, while I have shown and described a particular embodiment of my invention, it will be obvious to those skilled in the art that.variouschangesandrmodifications may be made withoutideparting from.m'yin vention in its broader aspects and'I, therefore, intendlin the appended claims to cover all such changes and-modifications as fall within thetrue spiritand. scope ofjmy invention. 4 f ,7; What I claim as new and desire 'to secure 'by-Letters Patent of the United States is: 1 l

1. In combination, an electron tube havinga. cathode, a plate, and a control grid, means ,for coupling said plate and said control grid together in amanner to form an oscillator, a pairof terminals for supplying anunidirectional control voltage which is subject tovariations, means for connecting said plate to one of said terminals, means for applying between said control grid and the. other of said terminals a signal voltage which upon'varyin'g beyond a predetermined starting value is operableto ,initiate'and sustainoscillations and which upon returningto a nor mal range of values is operable to stop oscillations, and means. for maintaining said cathode at-a voltage-which is a predetermined percentage of saidvariable ,control voltage, said percentage being selected to vary the'cathode voltage in response to control voltage variations by an amount sufiicient'to maintains'aid predetermined'fstart ing value essentially constant inspite of said control voltage variations. so H .11 r;

2. In combination, an electronrtubehaving a cathode, a plate, and a controlgrid, meanswfor coupling theflcon trol grid and the plate together in a mannertojforni an oscillator, a pair of terminals forv supplying 'acon trol voltage which is subject to variations, means for, con: necting said plate to one of said terminals, means 'for applying between said control grid and the other of. said terminals a signal voltage which upon varying beyond a predetermined startingvalue is operable to initia'te"oscililations, a voltage divider connected across saiditerminals and having a voltage tap set at a predetermined percent;

age of said supply voltage, means for connecting, saidltap to said cathode, the setting of said tap being selectedjo vary the cathode voltage in response tocontrol voltage variations by an amount sufiicient to maintain saidpre':

: determined starting value essentially constant in spite of said control voltage variations. V v

3. In combination, an electron tube havin'g a, cam .de, a

plate, and. a control grid, a pair of terminals forjjsupplyi variations, an output circuit connecting said plateto e of said. terminals, means for coupling said output-' cireiiit to said control grid in a manner to form an, os"cillator', an electroresponsive device coupled to 'said output circuit and operable in response to the initiation ,off'oscillations, means for. applying between said control grid f and'ithe other of said terminals a signal voltage whichuponvary- I ing beyond. a predetermined starting,valuejisoperable ,to initiate and sustain oscillations, and means'fo'r'fendering said starting value insensitive tov control 1 voltagellaria' tions comprising means, for maintaining saidfca'tho'de' at a voltage which is a predetermined percentages ofl'salid variable control voltage, said percentage being selected to vary the cathode voltage in response to control voltage variations by an amount sufiicient to maintain said predetermined starting value essentially constant in spite of said control voltage variations.

4. In combination, an electron tube having a cathode, a plate, a screen grid and a control grid, means for coupling said plate and said control grid together in a manher to form an oscillator, a pair of terminals for supplying a control voltage which is subject to variations, means for connecting said plate and said screen grid to one of said terminals, means for applying between said control grid and the other of said terminals a signal voltage which upon varying beyond a predetermined starting value is operable to initiate oscillations and which upon returning to a normal range of values is operable to stop oscillations, means for maintaining said cathode at a voltage which is a predetermined percentage of said variable control voltage, the value of said percentage being selected to vary the cathode voltage in response to control voltage variations by an amount which compensates for variations in screen grid voltage, said compensation being effective to maintain said predetermined starting value essentially constant.

5. In combination, an electron tube having a cathode, a plate, a screen grid, and a control grid, means for coupling said plate and said control grid together in a manner to form an oscillator, a pair of terminals for supplying a control voltage which is subject to variations, means for connecting said plate and said screen grid to one of said terminals, means for applying to said control grid an input voltage measured with respect to the other of said terminals, which upon varying beyond a predetermined starting value is operable to initiate oscillations, and means for maintaining said cathode at a predetermined I 6 fixed percentage of said variable control voltage, said percentage being selected to vary the cathode voltage in response to control voltage variations by an amount sufficient to maintain said predetermined starting value essentially constant in spite of said control voltage variations.

6. In combination, an electron tube having a cathode, a plate, a screen grid, and a control grid, a pair of terminals for supplying a unidirectional control voltage which is subject to variations, means including an ouput circuit connecting said plate and said screen grid to one of said.

terminals, means for coupling said output circuit to said control grid in a manner to form an oscllator, an electroresponsive device coupled to said output circuit and operable in response to the initiation of oscillations, means for applying between said control grid and the other of said terminals a signal voltage which upon varying beyond a predetermined starting value is operable to initiate oscillations and which upon returning to a normal range of values is operable to stop oscillations, and means for maintaining said cathode at a voltage which is a predetermined percentage of said variable control voltage, said percentage being selected to vary the cathode voltage in response to control voltage variations by an amount which compensates for variations in screen grid voltage, said compensation being eifective to maintain said predetermined starting value essentially constant.

References Cited in the file of this patent UNITED STATES PATENTS 2,257,663 Albrecht Sept. 30, 1941 2,440,284 Levy Apr. 27, 1948 2,600,270 Saunders June 10, 1952

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