US2554994A - Electronic switching circuit - Google Patents

Description

y 29, 9 B. R. LESTER 7 2,554,994

ELECTRONIC SWITCHING CIRCUIT.

Filed Feb. 16, 1949 FEEDBACK RECYCLlNG SWITCH Inventor": Burton R. Lester-,

b mamw His Attorney.

Patented May 29', 1951 UNITED STATES ATENT *O' FFFCE ELECTRONIC SWITCHING CIRCUIT .JBurtonLR. .Lester, .(Jamillus, N. -Y., -ass.ignor to .GeneraLElectric Company, a. corporation of 7 New York Application February 16, 1949, Serial -N o. 76;? 26

timing circuits utilizing electronic switching devices in cascade. 'More specifically, it relates to acircuit for coupling a trigger pulse into-amultivibrator of the above mentioned type.

It is an object of my invention to provide a new a-nd improved network for coupling-atrigger 'pulse to the input circuit of a multivibrator.

Another object of my invention is to provide an improved multivibrator of the type having two'con'ditionsof stability-which is suitable for useas a switching device in an electrical counting: system.

For additional objects and advantages and for a better understanding of the invention, attention is now directed to-the following description and accompanying drawing, and also to the appended claims inwhich the features of v the invention believed to be'-novelare -moreparticularlypointed out.

Thesin'gle figure of the-drawing is a schematic diagram illustrating an embodiment of my invention constituting a multivibrator circuit havtwo conditions of stability, which is suitable for useasa binary switching element in an-electronic counter chain or system.

Referring to the drawing, there is--shown,- an

"electron discharge device! of the double triode type, which comprises a'common cathode 2,-.and 'a'"-pair of' anodes'3 and 4 with their associated controlelectrodesorgrids'5 and 6. While a comm on cathode isindicatedya double triode having 1 separate cathode connections may be used .:with -an external common connection. The cathode 2 is connected to ground through a resistor l' in 'parallel-with'a shunt capacitor 8. The anodes 3 andd' are connected, through resistors-t and i respectively, to l a .1 source of operating 4 potential 1 indicated by B+.

'Cross-connections between anodes and con- "trol electrodes of the two halves or sections of device I are'provided by resistor ll,'for anode 3 and-control electrode 6, and :by resistor l2, for anode land"control-electrode 5. These resistors have shunt'capacitors l 3 and [4 respectively connected' in parallel "therewith. The purpose of these capacitors is to per-mit a faster rise of volt- 'ageat the controlielectrodes whenever a pulse is applied. controlelectrode ti is normally connected to ground through resistor l5, and similarly, control electrode 5 has a ground connectionthrougha'resistor it. The circuit connecti'ing :control electrode 6 to ground also includes 2 a switch ll which may be opened for'recycling the; multivibrator; asgwillbei more: fully explained .later.

My. invention. is a more articularly concerned 1 n with thenetworh forxsupplying aninputpulse to :the-multivibrator. iThisi-network comprises the previously ,mentioned: resistors: 9 and I 0,: and, in .additioma pair: of resistorsr l 9 and; 2 0.;connected in series obetween 1 anodes 1 3 and 4. A? coupling 10 :capacitor 2 1,. provides :a connection from the junctionof resistors. l9 and toaan input terminal 22, which is adapted to receive trigger pulses for the operation of the multivibrator circuit. Theout- :put; signals. or". pulses from: the: multivibrator are ld-available;at'aterminal 23 which is directly; connected to the-anode]. '.;The circuit comprises, in .addition,. a terminal 124, designated feed-back :output, which is connected to the-,anode 3,; and a terminal 25, designated feed-backinpuhwhich 20 is connected to the controlelectrode 6.

- In operation, veither. section of: device I con- "ducts, at any: one time but bOt'h 'SGCtlOIIS cannot .COIldllCl) simultaneously. In other --wor,d s, currentflows to.cathode 2 from. either anode ;3' or 25 anode 4 butdoes not. flow. from both simultaneously. Moreover, whichever; sectionw-is conduct- ..ing at: any a one time :remains; conducting until a voltage is applied to the circuitwhich hasthe required magnitude and polarity 5 to upset I this stable .state. The otherz-section' thereupon conducts and the circuit remains; stable in this .new state, until another voltage is applied to make the circuit revert 'toits original stable state.

The. reasons; for .these operating characteristics .becomeapparent .upon-consideration-of the crossconnections between the anodes and control. elecatrodes in both sections of-the devices. Assuming that the right-handqsectionasof .device [5118.0011- ducting, the anode 4 is at-acomparatively low 4 potential, whereas the cathodei. is at.-a certain positive potential above, ground. The potential atcathode 2 is higher thanthe potential at electrode'5 resulting from the voltage dividing action of resistors 12 and I6 operating onthe. potential at anode 4. Thus the left-hand section of the device is cut off or non-conducting. 'Since no current flows from anode'3, it is at a relatively high potential, approximating that of the source 3+, and, accordingly, the voltage applied to control electrode 6 of the right-hand section through" the voltage-dividing action of resistors ll a-nd I-5, 4 exceeds the voltageat theca'thode 2.

As long-as no extraneous voltages are-applied to the circuit; the condition 'describedabove 'will continue indefinitely. However, 1 if the" circuit is disturbed, for example by applying a negative pulse to control electrode 6, the conditions reverse. This pulse need be of suflicient magnitude to cause the resulting amplified positive pulse at the anode 4 to drive control electrode 5 into the conducting region. The resulting negative pulse at anode 3 is in phase with the original disturbing pulse and accelerates the triggering action, causing the right hand side of device I to become non-conducting and the left hand side to become conducting.

Anode 4 is then at a relatively high potential approximating B+ and the voltage coupled therefrom to control electrode 5 maintains the lefthand section in a conducting state. Also since current now fiows from anode 3, it is at a relatively low potential and the voltage coupled to control electrode 6 maintains the right-hand section of the device in a non-conducting or cutoff condition.

This new condition of device I is also stable and will be maintained until another disturbing voltage is applied to the multivibrator circuit. Such a disturbing voltage, for instance, may be a negative pulse applied to electrode 5. A similar sequence of events then occurs in reverse order, interrupting current flow in the left-hand section of device I and restoring the right-hand section to its original conducting state.

In accordance with my invention, trigger pulses are supplied to the multivibrator circuit at input terminal 22. The function of capacitor 2| is to isolate the multivibrator from any constant unidirectional potentials which may be applied to input terminal 22 in combination with the trigger pulses so that, effectively, only trigger pulses appear at the junction of resistors I9 and 26.

From this junction point the trigger pulses are applied to both anodes 3 and 4 to develop, at the control electrodes 5 and 6, voltages whose magnitudes are dependent upon the polarity of the pulses and the state of conduction of device I. Another function of capacitor 2| is to provide some differentiating action to sharpen the trigger signals. The time constant of capacitor 2'I together with resistors I9, 20, 9 and I0 is such as sharpen the trigger signals to the pulse shape required for operation.

In electronic counter chains or systems, it is desirable that the multivibrator respond to pulses of one polarity only. This particular multivibrator circuit is designed to respond to negative pulses only, and the input coupling circuit which I have described is particularly suitable for this purpose, as it provides excellent differentiation between positive and negative trigger pulses.

When a positive pulse is applied to the input terminal 22, a voltage is developed at the anode of either section of device I and is applied to the control electrode of the opposite section through the appropriate anode-to-control electrode coupling circuit. However, a positive trigger pulse can be effective only at the control electrode of a section of device I which, previous to the pulse, had been in a cut-off or non-conducting state. Accordingly, the positive pulse must be developed across a conducting anode before it can be applied to the control electrode of the non-conducting section. This causes a loading through this particular branch of the trigger pulse input circuit, which loading decreases the magnitude of the pulse developed at the control electrode in question so that the control electrode of the cut-ofi section is not driven sufficiently positive to cause a reversal of the conditions of stability in device 2.

Thus, for instance, assuming that a positive trigger pulse is supplied to terminal 22 at a time when the right-hand section of device I is conducting, anode 4 presents a comparatively low impedance to ground so that a very large proportion of the input pulse is developed across resistor 20, whereas only a small proportion is developed at anode 4. As a result, the voltage produced at electrode 5 by the positive trigger pulses is not large enough to permit current to flow to anode 3. At the same time, anode 3 presents a fairly high impedance so that a relatively high.

voltage is developed at electrode 6, but the righthand section of device I is already in a conducting state, so that the positive pulse has no effect. Thus a positive trigger pulse applied to input terminal 22 has no efiect on the multivibrator circuit and cannot upset its equilibrium or reverse the states of conduction of the two sections of device I. I

On the other hand negative trigger pulses applied to terminal 22 are not greatly attenuated in passing the anode of the non-conducting section of device I, on the way to the control electrode of the conducting section. Accordingly, a comparatively large negative trigger pulse is developed at the control electrode in question. This pulse is normally sufiicient to drive the control electrode below the cut-off voltage and to upset the state of equilibrium and reverse the conditions of the two sections of device I. Thus, for example, if a negative trigger pulse is applied to input terminal 22 at a time when the righthand section of device is conducting, a comparatively large negative trigger voltage is developed at anode 3. This negative voltage is then coupled to control electrode 6 and causes the righthand section of device I to become non-conducting, while causing the left-hand section to become conducting. If a negative trigger pulse is applied to input terminal 22 at a time when the left-hand section is conducting, a large negative voltage is then developed at anode 4 which is coupled to control electrode 5. The left-hand section of device I then becomes non-conducting, while the right-hand section becomes conducting. Thus in either case, no matter which section of device I is conducting, a negative input pulse, applied to input terminal 22, reverses the conditions of conduction as between the two sections.

When the multivibrator circuit which has been described is used in an electric counting chain or system, the output voltage is taken at terminal 23 and consists of alternate positive and negative increments of voltage for every negative trigger pulse or increment of voltage applied to the input terminal 22. Thus, assuming that operation begins with the right-hand section of device I in a conducting state, a negative pulse applied to input terminal 22 causes the right-hand section to become non-conducting and generates a positive increment of voltage at output terminal-23 A succeeding negative trigger pulse applied to input terminal 22 causes the right-hand section of device I to revert to its conducting state and produces a negative increment of voltage at output terminal 23. This action continues indefinitely and, for every two negative trigger pulses applied to input terminal 22, only one negative increment of voltage is produced at output terminal 23. V

The negative increments of voltage developed at terminal 23 may subsequently be utilized as 5. negative trigger pulses. Thus a second multivibrator circuit identical to that which has been described may have its input terminal connected to the output terminal 23. Since this-second multivibrator circuit operates on negative trige-r pulses only, it will reverse its condition of equilibrium once only, 'for every two consecutive reversals occurring in the first multivibrator circuit. Any number oi such multivibrator circuits may be connected in cascade, the output terminal of'each. one, in theichain being connected to the input terminal of the succeeding one. The number ofoccurrences of negative pulses is thus halved at each multivibrator circuit and a voltage dividing or counting chain is thereby constituted. The purpose of the switch i? is to permit recycling of the multivibrator by placing it definitely in a particular one of its two stable states. In the circuit of Fig. 1, opening switch it places a .high positive potential on control electrode 5, so" that the right-hand section of device I ismade conductive. When a series or" multivibrators oi this type is used as a counting chain, such a recycling circuit becomes necessary to insure that all of the multivibrators, start the count from one particular state. The feed-back input terminal 25 is provided for the purpose of receiving a pulse from an external source to reverse the conducting state of the multivibrator for certain special applications. Ihus in a counter chain, it may be desirable to reduce or to increase the number of reversals of a multivibrator element in the chain, so as to obtain a decimal counting system. This can be done by means of the feet back of pulses in proper sequence in the chain. Feedback output terminal 24 can be used to provide such a feedback pulse in a counter chain.

The trigger input coupling circuit which I have described has numerous advantages over circuits heretofor used for the same purpose. The impedance presented by the input circuit to the trigger pulse is considerably higher than that ohtainable with conventional circuits. In the place, the shunt path to the supply voltage is effectively removed because it occurs through resistors in series which may be either resistors 59 and 9, or resistors 20 and i0. Moreover, the amount of trigger pulse energy dissipated in the conducting anode is considerably less than in cir-' cuits used heretofore because there is a resistor Micro- Resistor Ohms Capacitor Inicro- Tubes Type farad While a specific embodiment hasv been shown and described, it will, of course, be understood that various modifications may be made without departing from the invention. The appended claims are therefore intended to coverany such modifications, within the true spirit and scope of the invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. A multivibrator circuit comprising a pair of cross-coupled electron discharge devices having two conditions of stability in which either one of said devices is conducting while the other is nonconducting, each of said devices having an anode, a source of anode operating potential, a pair of individual anode load impedances each connected between one of said anodes and said source, and a separate pair of impedances connected in series between said anodes, the junction point of said pair of impedances providing an input terminal for the-application of triggering signals to cause said multivibrator to alternate between said two conditions of stability.

2. A multivibrator circuit adapted to alternate between two stable states in response to signals of a predetermined polarity, comprising a pair of electron discharge devices each having an anode, a cathode and a control electrode, a source of unidirectional potential, individual load impedances connecting said anodes to one side of said source, a common self-bias impedance shunted by a capacitance and connecting both said cathodes to the other side of said source, crosscoupling means comprising an impedance capable of carrying unidirectional current connecting the anode of each device to the control electrode oi the other device, a pair of impedanoes capa ble of carrying direct current connecting said control electrodes respectively to said other side of said source, and a separate pair of impedances capable of carrying unidirectional current connected in series between said anodes, the junction point between said last named pair of impedances providing a terminal for the application of triggering signals to cause said devices to alternate between said stable states in which one or the other of said devices is conducting, and an output terminal connected to one of said anodes.

3. A' multivibrator circuit adapted to alternate between two stable states in response to signals of negative polarity, comprising a pair of electron discharge devices each having an anode, a cathode, and a control electrode, a source of unidirectional potential having a negative terminal and a positive terminal, a common self-bias impedance shunted by a capacitance and load connecting both said cathodes to said negative terminal, individual resistances connecting each of said anodes to said positive terminal, cross-coupling means comprising a pair of resistances respectively connecting the anode of each device to the control electrode of the other device, individual resistances connecting each of said control electrodes to said negative terminal, a separate pair of input resistances serially connected between said anodes, the magnitudes of said resistances being selected to provide symmetry in the circuits of said devices and to cause one 01' the other of said devices to conduct and to remain conducting indefinitely in the absence of ex ternal disturbances, thereby providing said two stable states, the junction point between said input resistances providing a terminal for the application of negative triggering signals to cause said devices to alternate between said stable states, and an output terminal connected to one of said anodes.

4. A multivibrator circuit adapted to alternate between two stable states in response to applied pulse signals of negative polarity, comprising a pair of electron discharge devices each having an anode, a cathode, and a control grid, a source of unidirectional potential having a negative terminal and a positive terminal, means connecting both said cathodes to said negative terminal, individual load impedances connecting each of said anodes to said positive terminal, cross-coupling means comprising a resistance shunted by a capacitance for respectively comiecting the anode of eachdevice to the control electrode of the other device, individual grid impedances connecting each of said grids to said negative terminal, a pair of input coupling impedances serially connected between said anodes, a connection point between said coupling impedances adapted to have negative trigger pulses impressed thereon 8 with respect to said negative terminal, said circuit being symmetrical and having two stable conditions of equilibrium in which one or the other of said devices is conducting in the absence of an applied trigger pulse, the value of said grid impedances being substantially lower than the value of said coupling impedances.

BURTON R. LESTER.

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

UNITED STATES PATENTS Number Name Date 2,404,047 Flory et a1. July 16, 1946 2,462,275 Morton et a1 Feb. 22, 1949 OTHER REFERENCES Electronics, February 1948, Predetermined Counter for Process Control, by Blume, pages 88-93.

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