US2404919A - Electronic switching device and circuit therefor - Google Patents

Description

July 30, 1946. w. P. QVERBECK 2,404,919

ELECTRONIC SWITCHING DEVICE AND CIRCUIT THEREFOR Filed May 1, 1940 4 0 Z #18 UV Ov +26v 6v +67v OUTPUT DEVlCE.

-170v 0v 0v +ll5v +67v j? 4 o o o O 6 D 8 10 0 7 U Wibzeas g flank #5W 5 o o g @mwha QK/NMJ/ U o 0 0 cc Patented July 30, 1946 UNITED STATES PATENT OFFICE ELECTRONCIC SWITCHING DEVICE AND IBCUH. THEREFOR Wilcox P. Overbeck, Waltharn, Mara, assignor to Research poration of New York Application May 1, 1940, Serial No. 332,854 3 claims (01. 250-27) 1 The present invention relates to. electronic switching devices and circuits therefor, and more particularly, to devices and circuits offa type used for switching, triggering and control applications. One object ofthe invention is to provide an electronic switching device and a switching circult which will operate at extremely high speed.

' tronic switching device illustrated in one of its stable-conditions; Fig. 2 is a diagram similar to Fig. 1 but illustrating the other stable condi- Qtion-f Fig. 8 is a diagram of the preferred form of switching circuit embodying the present inventiom'andFig. 4 is a diagrammatic view of a modified form of tube.

The tube illustrated in Fig. 1 comprises a cathode lwhichis either directly or indirectly heated, three grids 6, 8 and Ill, and an anode l2. As in conventional tube construction the three grids I terms.

The control and screen grids are aligned, that is, wound with the same pitch and spacing to define electron beams, as distinguished from the usual construction in which these grids are wound with opposite pitch and different spacings to obtain greater uniformity of distribution. The suppressor grid Ill is also pitched in the same direction as the other grids and differs from the conventional suppressor grid in that the spacing of the convolutions is smaller. As shown in the drawings the suppressor grid spacing is about half that of the other grids so that a suppressor grid wire is present within the limits of the electron' beam defined by the first two grids.

The tube is enclosed in the conventional vessel which is exhausted to an extremely high vacuum.

In the use of the tube in a switching circuit, as illustrated in Fig. 3, a positive potential is applied to the anode and a positive potential is Corporation, New York, N. Y., a coralso applied to'the screen grid through a resistor. Before'describing the circuit of Fig. 3, the two principal conditions of operation of the tube will be described. These are illustrated by the electron paths of Figs. 1 and 2. In Fig. 1 the suppressor grid is at zeropotential and the anode circuit is conducting. The flow illustrated in Fig. 1 is due principally to the inertia of the electrons and the velocity to which they are accelerated by the screen grid, as-well as to the attraction of the positive anode potential. A few of the electo it after passing between its wires.

are both stable.

trons flow directly to the screen grid or return 7 For the purpose of reliable switching action, I have found that one requirement is that the number of electrons which continue to the anode should be as large as possible and that the number which strike the screen grid shall be as small as possible. This is accomplished by the alignment of the screengrid wires with those of the control grid, so that the electron flow is in the nature of a directed beam, which, in the condition of Fig. 1, passes through the screen grid with only relatively slight diversion of electrons from the beam to the screen grid. This directing effect may be enhanced by the application of a negative potential to the 1 control grid,'which will tend to restrict the electrons to narrower beams and thus to diminish the attractive effect of the positive screen grid potential. The use of a negative control grid potential, however, reduces the total number of electrons passing through the tube. An optimum value of negative control grid potential may be determined in any given instance by experiment, but for purposes of this description the application of zero potential to the control grid will be assumed.

Under conditions shown in Fig. l, the suppressor grid is at or near cathode potential and exerts little or no influence on the electron beams.

The condition shown in Fig. 2 is one under which the anode is at positive potential, the screen grid is also at a positive but lower potential than in Fig. 1, but the suppressor grid is at a negative potential with respect tothe cathode. Under this condition electrons which get through the screen grid are repelled by the negative suppressor grid potential and cannot pass to the anode so that they are obliged to return to the screen grid which now carries a substantial current. In this condition the anode circuit of the tube is non-conducting.

The two conditions illustrated by Figs. 1 and 2 Furthermore, a shift ma be made from one stable condition to the other tive pulse to the suppressor.

acoaeie merely by changing the suppressor grid potential. This change of otential need only be momentary, as by the application of a rapid pulse. A shift from the condition of Fig. 1 to that of Fig. 2 may be effected by the application of a pulse of negative potential to the suppressor, and the reverse shift may be effected by the application of a posi- The influence of variations in suppressor grid potential on the electron paths depends on the disposition of the suppressor with respect to the other electrodes and on the spacing of the convolutions of the suppressor. In the construction shown in Figs. 1 and 2, the suppressor is disposed close to the screen grid and therefore in a region of high electron velocity, so that the close spacing of the suppressor convolutions is desirable to exert the necessary control on the electron paths.

A modified tube construction is shown in Fig. 4, in which the suppressor grid is disposed farther outward, approximately as far from the screen grid as the latter is from the cathode. In this modified form, the suppressor exert a large influence on the electron paths because it is in a region of low electron velocity. In order that it may not interfere with the flow of electrons in the anode-conducting condition, it is preferably wound with its convolutions in alignment with those of the other grids. Thi modified tube operates on the same principles as that of Fig. l, but the construction of Fig. 1 is ordinarily to be preferred because it makes for a device of smaller size.

A switch or trigger circuit involving the use of the tube described above is shown in Fig. 3. The control grid is at zero potential, the screen grid 8 is excited through a resistor M from a positive voltage source and the anode is connected to a source of positive potential through an output device l6 which may be a relay for actuating any desired equipment. The suppressor grid is connected through a high resistance l8 to a source of negative potential. Typical values of the several'potentials are indicated in Fig. 3.

Since the screen grid is excited through the resistor M the current flowing therethrough produces a voltage drop. In the condition of Fig. 1, when the anode circuit is conducting, the screen grid current is small and the positive potential applied to the screen grid is relatively high. When the anode current is cut oil, as illustrated in Fig. 2, the accompanying increase in screen grid current increases the voltage drop through the resistor I4 and reduces the positive potential of the screen grid.

These potential variations of the screen grid are fed back to the suppressor grid through a coupling circuit including a resistor 20 and condenser 22 in parallel thereto. The purpose of the condenser is to cause the voltage variations of the screen grid to reach the suppressor grid more rapidly. Without this condenser th transmission of voltage variations would be slower because of the capacitance between the suppressor grid and other elements of the tube, which would require the accumulation of the charge through the resistor l8 and would introduce a small time lag. The condenser 22 thus improves the speed of operation of the circuit.

An input circuit 24, including a condenser 26, is connected to the suppressor grid. Pulse of short duration, either positive or negative with respect to the cathode, may be applied to the input circuit to effect the switching operation of the tube. A second input circuit 28 including a condenser 30 may be connected to the control grid for a purpose to be later described, in which 1case a resistor 32 is included in the control grid and.

The operation of the circuit shown in Fig. 3 is described as follows: Assuming the tube to be in the conducting condition of Fig. 1, that is, the condition in which current is flowing to the anode, the current to the screen grid and the drop through the resistor M are relatively small so that the screen grid is at its highest positive po-' tential. The values of the resistors l4, I8 and 20 are such that the suppressor grid is maintained at a slightly positive potential, preferably a fraction of a volt, with respect to the cathode. This positive potential is so small as not to affect the description heretofore given, wherein zero potential of the suppressor was assumed, the principal purpose of the positive potential being to maintain stability and to prevent the suppressor from going negative under any slight fluctuations which might occur accidentally in the system. The positive potential at the suppressor, and consequently the suppressor grid current, are necessarily small since any tendency to increase the positive potential would produce a compensating voltage drop through the resistor 20. The circuit will, therefore, remain stable in this condition with the suppressor grid only slightly positive, with the screen grid at its maximum positive potential (for example, 56 volts) and with current flowing to the anode.

For a tube designed for operation at the electrode potentials indicated in Fig. 3 it has been found that the satisfactory value of the resistors are as follows: Resistor M, 100,000 ohms; resistor 20, 500,000 ohms; resistor l8, 2 megohms; and resistor 32, 100,000 ohms.

If a pulse of negative potential is applied to the input circuit 24 of sufflcient value to swing the suppressor grid negative, the non-conducting condition of Fig, 2 is immediatel assumed. This is brought about by the fact that the first effect of the negative potential on the suppressor grid is to repel the electrons flowing toward th anode and to cause them to flow to the screen grid. The screen grid current immediately increases and through the drop in resistor l4 causes an immediate reduction of the screen grid potential.

This variation of potential is transmitted through condenser 22 to the suppressor grid, which then assumes a stable negative value even after the pulse has terminated. The negative potential assumed by the suppressor grid is determined by the constants of the tube and its associated circuits and is independent of the magnitude of the pulse by which the transfer was initiated. In the typical example illustrated by Fig. 2, the suppressor grid is at a negative potential of 8 volts and the screen grid is at a positive potential of 28 volts. The anode circuit is now non-conducting, and the system will remain in that condition until its stability is upset by the application of a positive potential to the suppressor.

A return to the initial conducting condition .a

I t whereby the two st 0011411510118 heretofore described are suillciently different so that there'is no possibility of swin in from one condition to the other under any circumstances other than the application of pulses of sufllcient magnitude t6 the input circuit. To shift from one stable condition to the other, it is necessary to apply a pulse greater than some minimum voltage. which minimum may be considered as approximately equal to one-half the swing of suppressor grid potential between its stable values.

. In actual operation. the value of the minimum pulse which will effect the shift is not of great importance, since to insure reliability it is preferred to apply a pulse of a magnitude at least as great as the swing of suppressor potential, that pressor grid potentials will be changed in the positive direction and, therefore, the tube will swing to its Fig. 1 or conducting condition exactly as if a positive potential had been supplied to the suppressor grid. In any case. the stable condition persists even after the termination of the pulse which causes it.

As illustrated in Fig. 3, the input circuits 24 and 28 are bothlncluded. Either one may be omitted,

ifitisdesiredtooperatethetubeby'pulsesfrom one source only. But with two input circuits it is possible to control the tube from either. This ability for selective operation by pulses to both input circuits is of considerable importance in certain practical applications, particularly in a type of electronic counting circuit described in my co-pending application, Ber. No. 332,853, ill of even date herewith.

The advantages of the tube constructed according to the present invention may be explained by comparison with existing devices used for switching and triggeringappllcations. In the past most electronic switching systems have been conflned to the use of gaseous discharge tubes of the thyratron type. The speed of operation of these tubes has been limited by the fact that the Alternative methods of control consist in applying initiating pulses either to the screen grid or the control grid. The application of pulses to the screen grid diflers very little from the application of pulsesto the suppressor grid, except that such pulses, if confined to the screen grid alone, would ordinarily need to be of greater magnitude than if applied to the suppressor grid. In fact, it will be noted that the system of Fig. 3 in which the condenser 22 is provided serves to apply initiating pulses both to the suppressor grid and screen grid. Although the foregoing explanation has been based on the effect of the pulse on the suppressor grid only, the transmission of the pulse through the condenser 22 to the screen grid enhances the effect and contributes to reliability of operation with minimum time delay.

The second alternative above mentioned, namely, the application of initiating pulses to the control grid, makes use of the second input circuit 28. The operation is similar to that previously described, except that a positive pulse applied to the control grid will cut off a previously flowing anode current and a negative pulse will cause the anode circuit to become conducting.

To explain the operation of the system under the action of pulses applied to the control grid assume first that the tube is initially in the conducting condition of Fig. 1. A momentary positive pulse applied through condenser to the control grid will momentarily increase the number of electrons flowing both to the anode and to the screen grid. This momentary increase of screen grid current increases the voltage drop through the resistor i4 and reduces the screen grid potential. This reduction of screen grid potential is transmitted through the resistor 20 and condenser 22 to the suppressor grid to cause the latter to become negative. The result is therefore the same as if a negative pulse had been applied to the suppressor grid and the circuit immediately shifts to the Fi 2 condition in which no current flows to the anode. If now a negative pulse is applied to the input circuit 28, the total number of electrons flowing in the tube will be reduced, the screen grid current will be decreased, both the screen grid and the supinitiating pulses, particularly those required to shut off the flow of current, must be of sumciently long duration to allow de-ionization of the gas. Although attempts have-been made .to obtain higher speed operation with so-called hard tubes,

the provision for directing the electron beams,

and in the greater influence of the suppressor grid. It has been found that a pentode of conventional design may be used in the circuit of Fig. 3 and it is intended that the present invention comprehend the use or such a tube except where the appended claims are limited to the preferred tube construction. But the tube of the present invention is in all cases to be preferred, since it offers important advantages over the conventional type for the following reasons:

Since the grid wires in the conventional pentodes are not in alignment the ratio of screen grid current to total electron current in the anode-conducting condition would be relatively high. This means that in the transfer from the a conducting condition to the non-conducting condition, the relative increase in screen grid current would be fairly small. Therefore, the difference in screen grid voltage for the two conditions of operation would be much smaller than if the preferred form of tube were used. Consequently, the two conditions of the circuit with the conventional tube would be more nearly alike and the values of the various circuit constants would have to be chosen with great care to discriminate properly between them; furthermore the system might be unreliable, because of the possibility of upsetting one of the conditions of operation by accidental causes. 1

The tube of the present invention owes its advantages in a large part to the fact that the electron beam is directed past a positive electrode (the screen grid) and the electron paths are largely prevented from terminating thereon, except when they are required to do so under the influence of a negative potential applied to the suppressor. In the anode-conducting condition (Fig. 1) the screen" grid current constitutes a small proportion of the total electron current, while in the non-conducting condition (Fig. 2) the screen grid current is the total electron current. There is, therefore, ample discrimination between the two conditions of stability to insure that an unintended shift from one condition to the other cannot occur. It should be understood that the invention is not limited to the precise construction herein described, but that other arrangements within the scope oi the appended claims are to be considered within the purview of the invention.

Having thus described .theinvention, I claim:

1. An electronic switching circuit comprising a tube having a cathode, a control grid, 8. screen grid, a suppressor grid, and an anode, the control grid and screen grid having wires in alignment to determine electronic beams, an anode circuit, a screen grid circuit including a source of positive potential and a, resistor, a coupling circuit between the screen grid and the suppressor grid, and an input circuit for applying pulses to the control grid to effect changes in the screen and suppressor grid potentials. r

8 2. An electronic switching circuit comprising a tube having a cathode, a control grid, a screen a grid, and an input circuit for applying a pulse to the control grid and thereby to shift the screen and suppressor grid potentials from one stable condition to the other.

3. An electronic switching circuit comprising a tube having a cathode, a control grid, a screen grid, a suppressor grid, and an anode, the control grid and screen grid having wires in alignment to determine electronic beams, an anode circuit, a screen grid circuit including a source oi. positive potential and a resistor, a coupling circuit between the screen grid and the suppressor grid to sustain at each either of two stable conditions caused by changes of potential at the suppressor grid, and input circuits for applying pulses to the suppressor grid and control grid.

WILCOX P. OVERBECK.

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