United States Patent to 11, 3,786,278 Whitehouse Et Al
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United States Patent to 11, 3,786,278 Whitehouse et al. (45) Jan. 15, 1974 3,660,683 5/972 Wangard............. 3071252K (54) ELECTRONIC SWITCHING APPARATUS 3,217, 185 - 1 1/1965 Jansons........................... 307/223B (75). Inventors: Joseph Colin Whitehouse, Blaby; 3,621,287 11/1971 Peterson......................... 3071223 x Colin Arthur Wild, Leicester, both of England Primary Examiner-John Zazworsky (73) Assignee: The Rank Organisation Limited, Attorney-Joseph F. Brisebois et al. London, England (22 Filed: Jan. 31, 1972 57 ABSTRACT (21) Appl. No.: 222,178 A switching circuit for enabling a selected one of a 30) Foreign Application Priority Data number of channels. The circuit includes a number of Feb. 11, 1971 Great Britain...................4437/71 devices, one for each channel, each having a voltage? current characteristic with a negative-impedance re (52) U.S. Cl...............o 3071252 F, 307/242 gion separating two stable states. The devices are fed (51). Int. Cl. ........................................... H03k 17/72 in parallel from a source of constant current or con (58) Field of Search............... ... 307/252 F, 221, 225, stant voltage of a magnitude such that only one may 3071223, 242 assume one of the stable states. The devices may be for example programmable unijunction transistors or tunnel diodes. , (56). References Cited UNITED STATES PATENTS 3,629,614 12/1971 Matthews, Jr................. 3071223 x 5 Claims, 5 Drawing Figures PATENTEDJAN 151974 3,786,278 SHEET 1 OF 2 402.47% -6/6627V7 PATENTEDJAN 151974 3,786,278 SHEET 2 OF 2 3,786,278 ... 1 2 ELECTRONIC SWITCHING APPARATUS of the PUT 10 is as shown in FIG.1b; this characteristic has a peak voltage Vp and a valley voltage Vv sepa This invention relates to electronic switching appara rated by a negative impedance region and the value of . tus...: the peak voltage Vp is governed by the cathode-gate An object of the invention is to provide a switching voltage Vso. FIG. 1b also shows the load line for a resis apparatus for selecting one or more channels from a tance RA in series with the anode-cathode voltage number of similar channels and which is small, effi source Vs., and from this it is seen that three possible cient, versatile and reliable and has a low power dissi states exist for the conditions shown in FIG. 1a, namely pation. those labelled 11, 12 and 13. The state 12 however is According to one aspect of the present invention 10 unstable owing to its being situated on the negative im there is provided an electronic switching apparatus in pedance region, and in practice the PUT 10 may as cluding a plurality of electronic devices for switching sume either of the states 11 and 13 which are hereinaf selected ones of a corresponding plurality of channels, ter referred to as the 'low' and "high' states respec each of said devices being connected in series with a tively. ; : load in such a way that it can have only two stable 15. Turning now to the present invention as embodied in states, the devices being arranged for connection to a FIGS. 2 to 4, the principle of operation may be de constant-voltage or constant-current so that only a pre scribed with reference to FIG. 2. A number of PUTs determined number of the devices can exist in one sta designated PUT, PUT, . PUT are connected in ble state, means associated with each device for alter parallel across a constant current source (not shown). ing its condition to temporarily make only said one sta Associated with each PUT is a series load resistor RA, ble state available whereby another of the devices is RA, . RA and a voltage source such as Vsci driven to its other stable state, and a monitor associated with each device arranged to monitor which of its sta arranged in series with a current-limiting resistor such ble states it occupies. ". as Roi between its cathode and gate. The current Ic Preferably the devices have an output characteristic 25 delivered by the source is such that only a predeter which includes a negative impedance region. mined number (in this embodiment one) of the PUTs The invention also provides an electronic switching can be in the high state at any given time. Assume that apparatus including a plurality of programmable uni PUT is in the high state and that all the sources Vs junction transistors (PUTs) for switching selected ones . Vso are delivering the same voltage. If then the of a plurality of channels, a load in series with each 30 source, say, Visc is acted upon to reduce its voltage, the PUT to define high and low stable states of voltage and characteristic of the PUT is altered in the manner that current of the PUT, the series combinations of PUTs its peak voltage is reduced; the reduction in the voltage and loads being connected together in parallel for con of the source Vsc is made sufficient to bring the peak nection to a constant current source whose current is voltage of PUT, below the load line thus forcing PUT, sufficient to maintain only a predetermined number of to assume the high state. This in turn draws an in PUTs in the high state, means for applying a controlled creased current through PUT, and reduces the current voltage across the cathode and gate of each PUT, and available to PUT which therefore moves to the low means for monitoring each PUT to determine which of state. At this point the voltage of the source. Vso may its stable states it occupies at any given time. be returned to its original value, equal to that of the Preferably, the means for applying a controlled volt 40 other sources. By monitoring the anode-cathode volt age comprises a first Zener diode arranged to normally ages of the PUTs a multi-channel, two level output is determine the cathode-gate voltage of each PUT, and obtained and (in this embodiment, with only one high a second Zener diode of lower breakdown voltage. ar level) the circuit functions as an EXCLUSIVE-OR de vice. The outputs may be used to drive any suitable ranged to be selectively shunted across the first Zener 45 diode. - form of two level switch, for example reed relays may Preferably also, the first and second Zener diodes are be driven via amplifiers. arranged to control a transistor regulating the cathode The PUTs can be easily matched by suitable selection gate voltage of the relevant PUT. of of Vs. and R. The second Zener diode may be shunted across the FIG. 3 illustrates a practical embodiment of the basic first by a push-button or by a switching transistor. 50 circuit of FIG. 2. Each voltage source comprises a se An embodiment of the invention will now be de ries regulating transistor such as T, whose base bias is . scribed by way of example, with reference to the ac normally fixed by a resistor R2 and a Zener diode Z, companying drawings, in which: connected in series across power supply lines 20, 21. FIG. 1a illustrates a circuit incorporating a program which are fed by a power supply separate from the cont. mable unijunction transistor (PUT); 55 stant current source for the PUTs. To select, for exam FIG. 1b is a graph showing the characteristics of the ple, the channel of PUT, a pushbutton P, is depressed circuit of FIG. a to connect a further Zener diode Z across the diode FIG. 2 illustrates the basic form of an embodiment of Z. The reverse breakdown voltage of the further diode the invention; Z is less than that of the diode Z, and thus the voltage FIG. 3 is a more detailed illustration corresponding 60 at the gate of PUT, determined by T, is reduced and to FIG. 2; and the channel controlled by PUT, is selected as described FIG. 4 shows a modification of the circuit of FIG. 3. above. Each channel is identical, except that of PUT, has a capacitor C connected across the diode. Z. On Referring now to FIG. 1, as is known when a voltage switching on the power supply and constant current Vs is applied in series with a resistance Ra between the 65 source, the cathode-gate voltage of PUT, is held below gate and cathode of a programmable unijunction tran that of the other PUTs until the capacitor C is charged, sistor (PUT) 10 then the voltage/current characteristic thus ensuring that PUT, is selected initially. 3,786,278 3 4 As shown in FIG. 4, the push buttons P. etc. may be a plurality of programmable unijunction transistors replaced by series switching transistors, such as T1, (PUTs) connected to switch selected ones of a plu whose bases receive control pulses from a remote rality of channels, source, and a monitor may be connected across the a load in series with each PUT which defines high and anode and cathode of each PUT. As described above, low stable states of voltage and current of that these monitors may consist essentially of two-level reed PUT, the series combinations of PUTs and loads relays RR and RR driven through suitable amplifiers being connected together in parallel, A1 and A2. means supplying to said series combinations a con The invention comprehends the use of devices other stant current sufficient to simultaneously maintain than programmable unijunction transistors; any device O only one of said PUTs in the high state, m : having two distinct stable states may be used. It is pre means connected to apply a controlled voltage across ferred to use devices having a negative impedance re the cathode and gate of each PUT, and gion in their output characteristics, and other possible means connected to monitor each PUT to determine devices of this type are active amplifiers with positive which of its stable states it occupies at any given feedback, and tunnel diodes.