US2596956A - Electronic relay circuit - Google Patents

Description

May 13,V 1952 L.. G. Nn-:RMAN

ELECTRONIC RELAY CIRCUIT Filed June l0, 1946 j/uf QW Patented May 13, 1952 UNITED STATES PATENT GF'FICE yELEC'IRONIC RELAY CIRCUIT .Leonard G. .Nierman, Chicago, Ill., assignor-tothe United States of America as representedby the United States'Atomic Energyv Commission Application Junel'o, 1946,1seriarNo. 675,558

This invention relates to `an i improved :directvo'ltageactuated-electronic relay circuit for fsafety and control applications. In .many Iindustrial applications, itis desirable to .actuate an velectrical devicesuchas an alarm or-anautomatic shut-oit by v`a detector such vas a photocelLwhich detector does not supply suicient current for the operationofan ordinarycommercially made relay such as the solenoid type. .Oneapproach to the problem has been the use of a direct-current amplifier, the last stage of which is used `to drive the relay. Such a system isvopen to both the objection of the instability of a direct-current amplier, which is well-known, and of the uncertainty of the exact tripping point of therelay, which'is a function, in inexpensive relays, ofthe clean conditionof the moving parts, and of age. It is desirable, therefore, to have an electronic system such that the current through the relay coil, upon the detectors reaching the predetermined output level representing a Value, for example, of light, abruptly and immediately rises from a value far less than that necessary to actuate the relay to avalue far greater than that necessary to -actuate the relay, or vice versa. In'thismanner, the eflectsof drifts in the operating point of the relay and of drifts in the currentsupplied to the relay by the electronic system are minimized. Y

One device for accomplishing thepurpose last steted-usesa. grid-controlled gas discharge tube, commonly a thyratron. This device is unsuit-l able in many cases because of the relative instability of the ring point vof such tubes and'because, in operating "from a high-impedance detector, the grid-to-cathcde current of such a tube is excessive. Another device for accomplishing the lpurpose is the Vvacuum-tube vtrigger vpair circuit. Such a circuit is relatively-stable'in its operating point, Ybut the circuits of this nature heretofore in use are so insensitive that in applications requiring high sensitivity it is necessary to precede Athe trigger pair with one or more stages of direct-voltageamplification. The cascading in this manner of direct-voltagestages requires -a plate-voltage power supplyof very high Voltage or separate power supplies for the ampli- 'erstages and the trigger pair.

The .diiculties yin employing the circuits heretofore rin use .are encountered in valarm and control vdevices yused to monitor and control the operating level of ka .neutronic chain reactor. In 'suchfa case, 4the circuit :must beactuated `by an extremely high-impedance .device such fas .an ionization chamber, Aand yet must .be fsumciently sensitive and :reliable VtoA assurefsafetyffand proper operation.

One object of @the :present linvention `is -to .provide .a .highly sensitive, .yet-,simple :stable and reliable directevoltage :actuated electronic relay circuit.

4Another obj ect .of `the vInesent `invention lis lto provide a sensitive trigger Apaircircuit.

yAfurther object is to` provide `a :simple yet accurate control circuit Ifor maintaining such quantities as light or .operation `level of a neutronic chain reactor constant.

`Other aims and objects lwillappear ir'omfthe description below and the attached drawing, 1in which:

Figure `1 isa schematiccircuit diag-ram of-an ionization chamberiand electronic .rela-y circuit used with a safetydevice for alneutronic `chain reactor; and

Figure 2 is anrelemental family-of characteristic curves of a typical pentode vacuum 'tube with the plot of alinear resistancesuperimposed.

Referring-now to 'Figure 1 of the drawing, tubes 2 and '4 are connected as a form of `trigger pair,

' which is in turn one form of a trigger circuit.

A trigger circuit may be defined as a circuit V.in which the direct output voltage of current changes abruptly from one stable value to vanotherstable value at a critical value of resistance or voltage Aappearing vat an input` point land changes back abruptlytoapproximately its orig-.inal'value at another critical'value'of resistance or voltage appearing at Aether'the same oranother input point. A trigger vpair is apair of electronic tubes, usually vacuum tubes, `soiinterconnected lthat there Vare values 'of vcurrent'in both'of the tubes'at which equilibrium cannot be obtained. One tubeissubstantially fully conducting and the other substantially cutoff. By an appropriate means, either'theapplicationfof a voltage signal-or thevariation of va resi-stance, the vconditions of full rconduction "and substantial cut-off in the respective tubes-may be labrupt-- ly reversed, the Vcircuit being 'saidlto have been triggered. 'The circuit may be triggered vback 4to the `original condition ofconduction by another-- voltage signal at the sariieor another inputpo'int, or :again byvariationfof a resistance.

"In'F'igure 1, the-cathodes 6 of triode vacuum tubes 2 and 4 are ,connected directly together and :to the negative side of the 'plate `voltage -supply 8; The Apositive sidefof the-platelvoltage supply 8 is connected to the fplate `rof tube l through Kplate load resistor 12 and ito 'Itheuplate Irl Aof tube 4 "through the energizing coil 116 o'f a solenoid relay I8 and a resistance 20, indicated by dotted lines in the drawing because such resistance 28 is preferably the direct-current resistance of the coil I6 itself, since current through the coil i6 is thus maximized when tube 4 is conducting. The plates l0 and I4 of tubes 2 and 4 are connected respectively to the grids 22 and 24 of the opposite tubes 4 and 2 through resistors 26 and 28. Grid 22 is connected, through resistor 30, to the negative side of the grid bias supply 32.

Grid 24 is connected directly to the plate 34 of pentode vacuum tube 36, of which the cathode 38 is directly connected to the negative side of the grid bias supply 32. The screen-grid 40 is connected directly to the cathodes 6 of the pair, grid bias supply 32 thus serving as the source of screen-grid voltage for tube 36. Suppressorgrid 42 is directly connected to cathode 38. The control-grid 44 is connected to input terminal 46 and biased negatively with respect to cathode 38 by grid bias supply 48, the negative terminal of which is connected to input terminal 50.

In Figure 1, the circuit as illustrated is used for control of the power output level of a neutronic chain reactor. Ionization chamber 52 is in series with voltage supply 54 and resistor 56, which is connected across input terminals 46 and 53. The ionization chamber 52 is filled with a. neutron-sensitive gas such as boron-trifluoride. It is placed in a neutronic chain reactor. The neutron flux through the ionization chamber 52, as is well known in the art, causes neutron absorption in the boron contained in the gas lling, accompanied by the emission of alpha particles. The alpha particles induce ionization current through the chamber 52 and thus produce a voltage across the resistor 56. This voltage is impressed across input terminals 46 and 58. The voltage so impressed is proportional to the neutron flux through the chamber 52 and thus to the power level of the neutronic reactor.

In explaining the operation of the circuit, it will be desirable to first analyze the operation when a variable resistor, hereinafter called the triggering resistor, is substituted for vacuum tube 3S between grid 24 and the negative side of grid bias supply 32, thus constituting a trigger pair circuit heretofore in use. The action of the circuit depends upon the fact that current ows through only one tube at a time. Suppose, for the purpose of explanation, that equal currents could flow in both tubes 2 and 4. Then any small increase in plate voltage or decrease of negative grid voltage of tube 2 increases the plate current in tube 2 and thus increases the voltage drop in load resistor l2. Because of the voltage divider consisting of resistors 26 and 33, this makes the grid 22 of tube 4 more negative, which decreases the plate current of tube 4 and the vvoltage drop across load resistor 23 and thus makes the grid 24-of tube 2 less nega-- tive. The action is cumulative and results in an abrupt increase ci plate current in tube 2 and substantial cessation of plate current in tube 4. Because of the well-known fact that extremely small voltage variations are always present in electrical circuits, it may be seen that there are values of plate current in both tubes in which equilibrium cannot be reached. The circuit thus has two stable states, one in which tube 2 is highly conducting and tube 4 substantially cut off, and one in which the reverse is true. If tube 2 is conducting and tube 4 nonconducting, the circuit may be triggered to the opposite state by decreasing the resistance of the triggering resistor, there being a critical value of the triggering resistor at which transfer takes place. If tube 2 is non-conducting and tube 4 conducting, the circuit may be triggered to the opposite state by increasing the resistance of the triggering resistor, there being another critical value of the triggering resistor at which transfer takes place. The critical value in the latter case is higher than that in he former. Thus for extremely high values of the triggering resistor, tube 4 is necessarily non-conducting, and for extremely low values of the triggering resistor, tube 4 is necessarily conducting. But there is a range of values of the triggering resistor in which the existing state of equilibrium is determined not only by the momentary value of the triggering resistor, but also by the direction in which it is being varied.

In the present invention, a. grid-controlled vacuum tube 36 is substituted for the triggering resistor. The direct-current plate resistance o! tube 36 is the ratio of plate voltage to plate current. For any given value of plate voltage, the plate current may be increased by making the control-grid 44 less negative with respect to the cathode 38, thus decreasing the resistance. In a tube 36 of high transconductance, such as for example, a GSJ'I, this resistance may be .4. varied over a wide range by relatively small variation of the grid voltage. Thus the tube 36 may be used to trigger the circuit from one stable condition to another by small decrements and increments of the voltage between grid 44 and .r cathode 38.

In the embodiment illustrated, grid bias supply 48 may be of such magnitude that when the neutron flux in the chain reactor reaches a value exceeding safe operation, the value of resistance of tube 36 is the critical value for triggering the circuit from the condition in which tube 4 is non-conducting to that in which tube 4 is conducting, thus actuating relay I8, to the terminals 60 of which may be connected a safety device for shutting down operation and sounding an alarm. Alternatively, the bias suply 48 may be of such magnitude that when the neutron ux reaches a value representing the maximum of a range of desired operation, the value of resistance of tube 36 is the critical value for operating the relay as above described, the relay terminals 60 being connected to a device for lowering the level of operation. In such a case, the latter Acontinues to operate until the neutron iiux, and thus the voltage appearing across resistor 56, falls to a value which makes the value of the resistance of tube 36 the critical value for triggering the circuit back to its original condition. In this way the level of power operation may be held within narrow limits.

The use of pentode tube 36 serves another function in addition to providing a resistance element whose resistance is variable by a voltage. In trigger circuits, such as that of Figure 1, which are adapted to be triggered by the variation o! e. resistance, the triggering is due to the change of voltage across the resistance caused by the variation of the value of the resistance. One aspect of the present invention lies in the discovery that because the triggering action itself acts to change further the voltage across the resistance, such voltage change being in the same direction as the voltage change causing the triggering, the triggering action may be improved by employing as the variable resistance an element whose resistance increases with increasing voltage, and decreases with decreasing voltage.

In Figure 2 is shown the shape of typical curves of plate current as ordinate plotted against plate voltage as abscissa with the screen grid at a potential positive with respect to cathode, and the suppressor grid at cathode potential, for two values of control-grid Voltage of a pentode vacuum tube. Curve |00 represents the characteristic for a value of control-grid potential less negative with respect to that of the cathode than the value of control-grid potential represented by curve |02. Suppose that the circuit of Figure 1 is in the stable condition wherein tube 2 is conducting and tube 4 is non-conducting. Suppose that the grid potential of tube 36 is at the value represented by curve |02 and is gradually raised to that of curve |00. Suppose further that when the grid potential reaches this value, the value of plate voltage being B, the plate current is A, and the resistance of the tube is B/A, the critical resistance at which the circuit triggers to the other stable state. The straight line |04 is a plot of current against voltage in a linear resistance of value B/A, superimposed on the characteristic curves |00 and |02. All points on curves |00 and |02 which lie above line |04 thus represent direct-current plate resistance values smaller than B/A, and points which lie below represent direct-current plate resistance values greater than B/A. From the analysis above, it is apparent that when the circuit commences to trigger, the voltage across tube 36 begins to fall. From Figure 2 it is apparent that the resistance of tube 36 is thereby further reduced and continues to fall throughout the triggering action, which is thus aided. It may be shown that triggering in the reverse direction is similarly aided. The trigger pair is thus made very sensitive and the effect of drifts in the characteristics of tubes 2 and 4 on the critical triggering points is thus minimized. In addition, the time taken for the triggering action to occur is thus reduced. Although this latter effect is usually not important in relay applications, such as the one illustrated in the drawing, there are applications of such a trigger pair, such as in scaling circuits, where it is required that the transition from one state to another take place in times of the order of a fraction of a microsecond.

Applications of the teachings of the invention other than the one described above and illustrated in the drawing will be readily apparent to persons skilled in the art. As one instance, the pentode tube may be used in trigger circuits other than the triode trigger pair described. As another instance, the advantage which is shown above to be obtained by the use of a resistance element whose resistance increases with increasing voltage may be obtained from other than a pentode vacuum tube, resistors of such characteristics being commercially available. Further, a plurality of such resistance elements may be used advantageously in some applications; for example, resistor 30 of Figure 1 may also be replaced by a pentode vacuum tube or other resistance element having the above-described characteristics; the invention teaches that advantage is to be gained in the trigger circuit by use of such a resistance element even where it is not required that the resistance be variable by a voltage such as the voltage between the grid 44 and the cathode 38 of Figure 1. Nor is the teaching of the invention limited to trigger circuits activated by varying direct voltages. It may be employed, for instance to provide scaling trigger pairs of increased sensitivity and shorter response time. Furthermore, if an alternating voltage'such as a sine-wave is impressed between terminals 46 and 50 of Figure 1, the output voltage taken at either of the plates of the trigger pair will consist of square pulses of the frequency of the input signal and of a duration variable by varying the value of the bias supply 43, the circuit thus serving as a pulse generator. Similarly, many other applications will be found for the teachings of the present invention by persons familiar with the prior art regarding trigger circuits and electronic relays.

In addition, a voltage divider such as that of Figure 1 consisting of resistor 28 and tube 36, wherein the voltage across the latter resistance element increases at a greater rate than the rate of increase of a voltage impressed across the voltage divider, will likewise be readily applied to other than the device described.

What is claimed is:

An electronic control circuit comprising, in combination: means to generate a voltage varying in response to a condition to be controlled; a trigger circuit including a pair of vacuum tubes. said tubes having plate circuits connected in parallel, both oi said plate circuits containing resistors and one of said plate circuits being connected in series with the eld coil of a relay, the control grid of the vacuum tube in the first of said circuits being connected to the plate of the tube in the second of said circuits through a resistor, and the control grid of the second trigger tube being connected to the plate of the first trigger tube through a resistor; and a third vacuum tube having a plate directly connected to the control grid of the rst trigger tube, said third vacuum tube having a cathode connected to the control grid of the second of said trigger tubes through a resistor, the means to generate a voltage being connected between the grid and cathode of said third vacuum tube, and said third vacuum tube being biased to be conducting, whereby variations in the potential of the control grid relative to the cathode will be effective to trigger the trigger circuit.

LEONARD G. NIERMAN.

REFERENCE S CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,873,786 Ranger Aug. 23, 1932 1,934,322 Osbon Nov. '7, 1933 2,050,059 Koch Aug. 4, 1936 2,260,933 Cooper Oct. 28, 1941 2,401,396 Wolfner June 4, 1946

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