Universal field convertible 3-wire switch
BACKGROUND 
     The invention relates to 3-wire switches for connecting a load to a power supply. There are four types of configurations of 3-wire switches: (1) a P type DC switch where current flows from a DC source through the switch first and then to the load; (2) an N type DC switch where current flows from a DC source first through the load and then through the switch, such that the load is hot even when the switch is off: (3) a P type AC switch where the hot AC line is wired first through the switch and then through the load; and (4) an N type AC switch where the hot AC line is wired first through the load and then through the switch, such that the load is hot even when the switch is off. 
     An installer must stock three different switch modules to service these four configurations. One type of switch module services the P type DC configuration. A second different switch module services the N type DC configuration. A third switch module services both the P type AC and N type AC configurations. 
     SUMMARY 
     The present invention provides a single universal switch for servicing all four configurations. The switch is convertible in the field to any of the configurations without making internal changes to the switch. The switch is automatically converted from one configuration to another in accordance with the connection format of the three wires of the switch. The installer merely chooses which configuration is desired, and connects the power supply and load in the given connection pattern therefor. This installation connection wire programming is simple and efficient, and eliminates the need for stocking a plurality of dedicated switches as before. 
     In one aspect, the present invention evolved from efforts to provide versatile switching circuitry having the noted universal capability and also operable over a wide range of voltages. 
     Though not limited thereto, one such application is in proximity switch control circuitry wherein it is desired to use the output signal from a proximity switch to trigger or control a load across given power lines. The load control switch must tolerate a main current path which has large voltage swings. However, the voltage available from the proximity switch output signal is not large enough to match the triggering requirements of the load control switch. 
     In the preferred embodiment of the present invention, the available signal (which may be from a proximity switch) is used to throw another switch, such as an input transistor, to pass rectified line current to a conditioning circuit which then supplies predetermined drive requirement current for the load control switch, such as a load transistor, which then allows line current through the load. 
     The circuitry of the invention is particularly simple and effective and may be used to control one or more loads between the power lines. 
     In another aspect of the invention, the switching control circuitry enables a normally open or normally closed mode to be chosen individually for each load. The incoming signal may thus cause some loads to turn ON, while causing other loads to turn OFF. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic circuit diagram of a 2-wire switch. 
     FIG. 2 is a schematic circuit diagram of a 3-wire switch. 
     FIG. 3 is a schematic circuit diagram of a P type DC switch configuration. 
     FIG. 4 is a schematic circuit diagram of an N type DC switch configuration. 
     FIG. 5 is a schematic circuit diagram of a P type AC switch configuration. 
     FIG. 6 is a schematic circuit diagram of an N type DC switch configuration. 
     FIG. 7 is a schematic circuit diagram of a basic building block switch configuration useful for teaching the present invention. 
     FIG. 8 is a schematic circuit diagram of a universal switch constructed in accordance with the invention. 
    
    
     Detailed Description 
     FIG. 1 shows a 2-wire switch having two wires or terminals for connection to a power supply for controlling load current. FIG. 2 shows a 3-wire switch having first and second main terminals or wires 1 and 2 for connection to a power supply, and having a third load terminal or wire 3 for connection to a load. The present invention relates to a 3-wire switch. 
     FIGS. 3 through 6 illustrate the four types of configurations of the 3-wire switch of FIG. 2, and the three wires or terminals are labeled comparably to FIG. 2 to facilitate clarity. 
     FIG. 3 shows a P type DC configuration switch, with main terminal 1 connected to the positive side of a DC power supply such as a battery, and main terminal 2 connected to the negative side of the battery, which terminal may also be grounded. A switch is placed in series between the load and terminal 1 such that current flows through the switch fifst and then the load back to terminal 2 or to ground. FIG. 4 shows an N type DC switch configuration, with current flowing from terminal 1 first through the load and then through the switch back to terminal 2. In this configuration, the load is still hot even with the switch OFF. FIG. 5 shows a P type AC switch with the hot terminal 1 connected through the switch first and then through the load back to the neutral or grounded terminal 2. FIG. 6 shows an N type AC switch configuration with the hot wire 1 connected first through the load and then through the switch back to ground or neutral terminal 2. 
     In a typical embodiment of FIG. 3, the ON/OFF switch 10 is a transistor which may be triggered into conduction by a control signal or the like from a variety of sources depending upon the application. One typical application is in proximity sensing where the control signal generated from a proximity sensor is used to trigger transistor switch 10 into conduction. Current flows through transistor 10 first and then through the load. 
     If it is desired to convert the switch of FIG. 3 to an N type DC configuration as shown in FIG. 4, the installer cannot merely reverse the connection of terminals 1 and 2 across the power supply in order to have the current flow first through the load. If the connection of terminals 1 and 2 in FIG. 3 is reversed, current will not flow in the reverse direction through transistor 10. Thus a separate dedicated switch must be used to achieve the switch configuration of FIG. 4. 
     The P type AC switch configuration of FIG. 5 may be converted to the N type AC switch configuration of FIG. 6 merely by reversing the connection of terminals 1 and 2. This is because the ON/OFF switch 12, such as a triac, is bidirectional to conduct the AC current. Thus, a single switch module may service both the P type AC and the N type AC configurations of FIGS. 5 and 6. 
     It is thus seen that in order to service all four of the switching configurations of FIGS. 3 through 6, a total of three switch modules are necessary. A first switch module services the P type DC switch configuration of FIG. 3. A second different switch module services the N type DC switch configuration of FIG. 4. A third switch module services both the P type AC and the N type AC switch configurations of FIGS. 5 and 6. 
     FIG. 7 shows a basic building block switching structure useful for teaching the present invention. The same 3-wire numeration is used as before for clarity. First and second main terminals 1 and 2 are provided for connection to either an AC or DC power source, and terminal 2 may be either grounded or neutral. A third load terminal is provided for connection to a load 20. A diode rectifier bridge 22 is connected at its AC inputs 24 and 26 between the first and third terminals 1 and 3. A switching transistor 28 is connected between the DC outputs 30 and 32 of bridge 22. Transistor 28 may be a FET or bipolar of either polarity. In one embodiment, transistor 28 is a PNP transistor with its emitter connected to the positive DC output 30 of the bridge and its collector connected to the negative DC output 32 of the bridge. Load 20 may be connected to terminal 2 or grounded if terminal 2 is grounded. 
     In operation the switch module of FIG. 7 services the P type DC switch configuration by connecting terminal 1 to the positive side of a DC power source and connecting terminal 2 to the negative side of a DC power source. Current flows from terminal 1 to the positive DC output 30 of the bridge, then through transistor 28 when triggered by gate or base drive on trigger line 34, then to the negative DC output 32 of the bridge, then through load 20 back to ground or to terminal 2. 
     The switch module of FIG. 7 services the N type DC switch configuration by connecting terminal 2 to the positive side of the DC power source and connecting terminal 1 to the negative side of the DC power source. Current then flows from terminal 2 through load 20 to the positive DC output 30 of the bridge then through transistor 28 when triggered ON, then to negative DC output 32 then to terminal 1. 
     The switch module in FIG. 7 services the P type AC configuration by connecting terminal 1 to the hot side of an AC power source and connecting terminal 2 to the neutral side of the AC power source. In the first half cycle with terminal 1 swinging positive relative to terminal 2, current flows from terminal 1 to the positive DC output 30 of the bridge then through transistor 28 when triggered ON, then to negative DC output terminal 32 then through load 20 back to terminal 2 and/or to ground. In the other half cycle, with terminal 1 swinging negative relative to grounded or neutral terminal 2, current flows from terminal 2 through load 20 to the positive DC bridge output 30, which is at ground or neutral, then through transistor 28 to negative DC bridge output 32, then back to terminal 1. 
     The switch module of FIG. 7 services the N type AC configuration by connecting terminal 1 to the neutral side of the AC power source and connecting terminal 2 to the hot side of the AC power source, with operation comparable to that above described. 
     All four configurations are thus furnished by a single universal switch module as wire programmed at installation, and affording field convertibility between the configurations without internal changes to the switch module. The installer merely programs the configuration according to installation wired connection of terminals 1, 2 and 3. 
     In order to afford short circuit and overload protection, it is necessary that nonlatching switch means such as transistor 28 be used within the bridge. A nonlatching switch stops conducting when the trigger or control signal is terminated. A latching type switching device, such as an SCR or a triac, is not amenable to short circuit and overload protection because these types of switches remain conducting in DC modes even after termination of the trigger signal, as long as a sufficient holding current flows through the main electrodes. In AC applications, even after termination of the trigger signal, the latching switch will remain conducting until the end of the half cycle, i.e. until the next zero crossing of the current through the main electrodes. Turn-off must be effected much faster than this, especially for a 60 hertz cycle, to protect the switching device from destruction due to overload or short circuit current. 
     The triggering of transistor 28 presents practical problems, particularly in a hardwired trigger connection embodiment. For example, if a triggering voltage source is referenced to one of the lines 1 or 2, the switch module of FIG. 7 will not work in practical applications of all the configurations of FIGS. 3 through 6. This is because the reference level changes when the installation connection of lines 1 and 2 is reversed to convert between P and N type configurations. This changing reference level for the trigger circuitry is accommodated in prior devices by using dedicated switch modules appropriately referenced. 
     FIG. 8 shows a universal 3-wire switch module 40 in its preferred construction in accordance with the invention, including the triggering circuitry 36 of FIG. 7. In FIG. 8, switch circuit 40 has first and second main terminals 1 and 2, and a third load terminal 3 for connection to load 20 as before. Like reference numerals from FIG. 7 are used where appropriate in FIG. 8 to facilitate clarity. Trigger circuit 36 thus affords the triggering base drive on line 34 to the load transistor switch 28 in bridge 22. 
     Circuit 40 includes an input diode bridge rectifier 42 connected between the power lines at input terminals 44 and 46 of the bridge. Bridge 42 thus has positive and negative DC output terminals 48, and 50, respectively. &#34;Rectified line current&#34; shall mean the DC current at terminal 48 or 50, derived from power lines 1 and 2, regardless of whether the power lines 1 and 2 are AC or DC biased. Input switch means, such as input transistor 52, is connected to the input bridge rectifier 42 and passes rectified line current therethrough from the bridge under control of signal 54. 
     In one particularly desirable aspect of the invention, the preferred construction accommodates a control signal 54 which is not matched to the load control requirements. For example, the voltage levels available in control signal 54 may not be great enough to meet the triggering requirements for load control transistor switch 28 which must withstand large voltage swings across the power lines 1 and 2. Power supply or voltage setting means 56 is provided between the DC outputs 48 and 50 of the auxiliary or input bridge 42. This voltage setting or power supply means includes a zener diode 58 and a resistor 60 connected in series between points 48 and 50, and a storage capacitor 62 connected in parallel with zener diode 58. In one particular application, the zener diode 58 is selected to yield a 7.5 voltage supply between points 65 and 50. In the noted particular application, this voltage is provided on terminals 66 and 67 and is the power supply for a proximity sensor which outputs a 7.5 volt control signal 54 on line 64 in response to the given sensed condition, and outputs a zero volt signal in the absence of the given condition. This control signal 54 on line 64 is referenced to a point within bridge 42, preferably negative DC point 50, to be described more fully hereinafter. This referencing within the bridge provides automatic intelligent reference level transfer such that the control signal&#39;s reference follows either the more positive or the more negative polarity of terminals 1 or 2, regardless of the above-noted P or N type configuration. 
     NPN input transistor 52 has its collector connected through resistor 60 to the positive DC output terminal 48 of the bridge, and has its base connected to a signal line 64. Input transistor 52 is thus in a normally blocking OFF state in the absence of signal 54, and is driven into conduction in response to the presence of signal 54, which conduction passes rectified line current from terminal 48 through transistor 52. 
     The input switch means may alternatively or additionally include PNP transistor 68 having its base connected through resistor 69 to signal line 64 and also connected through resistor 70 to the negative DC output terminal 50 of bridge 42. The emitter of transistor 68 is connected through resistor 60 to the positive DC output terminal 48 of the bridge. PNP transistor 68 is thus in a normally conductive ON state in the absence of signal 54 due to the positive biasing of its emitter-base junction from positive DC terminal 48 through the emitter-base junction of transistor 68 which is return referenced through resistor 70 to negative DC terminal 50. In response to the presence of signal 54, transistor 68 is biased to its blocking OFF state and thus blocks the flow of rectified line current therethrough from DC terminal 48. 
     Circuit 40 further includes conditioning means 72 conditioning the rectified line current from input switch means 52 to predetermined drive requirements. In preferred form, conditioning means 72 comprises a current source supplying a constant current drive on line 34 for controlling load transistor switch 28. The preferred type of current source is a current mirror driven by the current on line 74 through resistor 76 from transistor 52, and outputting a predetermined proportional level drive current on line 34. 
     Current mirrors are known in the art, and will be only briefly described. Current mirror 72 comprises one or more left-hand transistors 78 base driven by the current on line 74 from input transistor 52 and passing rectified line current from input transistor 52 through transistor 78 back to a return point, negative DC terminal 50 of bridge rectifier 42. The current mirror further includes one or more right-hand transistors 80 base driven in common with the left-hand transistor 78 from input transistor 52, and passing drive current on line 34 therethrough to return point 50 of bridge rectifier 42. 
     Transistor 78 is an NPN transistor having its collector and base connected to line 74 from the emitter of transistor 52. Transistor 80 is an NPN transistor having its base connected to line 74, and its collector connected to line 34. The emitters of transistors 78 and 80 are connected through respective resistors 82 and 84 to a common point 86 which is connected to return point 50 of bridge 42. 
     In operation, a current I 1  flows through lefthand transistor 78, and a current I 2  flows through the right-hand transistor 80. Between common base point 88 and common emitter point 86, there are two parallel circuit branches. The left-hand circuit branch is through the base-emitter junction of transistor 78 and through resistor 82. The right-hand circuit branch is through the base-emitter junction of transistor 80 and through resistor 84. The voltage drop through each branch between points 88 and 86 is identical since the branches are in parallel. The voltage drop across the left branch is the base to emitter drop of transistor 78 plus the IR drop across resistor 82, as shown in equation 1 for the voltage across points 88 and 86. 
     
         V.sub.88-86 =V.sub.BE78 +I.sub.1 R.sub.82                  (1) 
    
     Likewise, the voltage drop through the right-hand branch is the base to emitter drop of transistor 80 plus the drop across resistor 84, as shown in equation 2. 
     
         V.sub.88-86 =V.sub.BE80 +I.sub.2 R.sub.84                  (2) 
    
     Inserting equation 1 into equation 2 to set the voltage drops equal results in equation 3. 
     
         V.sub.BE78 +I.sub.1 R.sub.82 =V.sub.BE80 +I.sub.2 R.sub.84 (3) 
    
     The base to emitter voltage drops are negligible compared to the IR drops across the resistors, particularly for larger size resistors. Eliminating the base to emitter voltage drops in equation 3 results in equation 4. 
     
         I.sub.1 R.sub.82 =I.sub.2 R.sub.84                         (4) 
    
     Solving equation 4 for I 2  yields equation 5. 
     
         I.sub.2 =I.sub.1 (R.sub.82 /R.sub.84                       (5) 
    
     Conditioning circuit 72 thus pulls an amount of current I 2  from line 34 which is a mirror of the amount of current I 1  on line 74 multiplied by a given constant, namely the ratio of the resistance values of resistors 82 and 84. 
     As above noted, switch 40 includes an output diode bridge rectifier 22, connected in series with load 20 between power lines 1 and 2. Transistor 28 is driven by conditioning means 72 to an ON state passing line current through load 20. In preferred form, load transistor 28 is a PNP transistor having its base connected to line 34, and its emitter and collector connected to the respective positive and negative DC output terminals 30 and 32 of bridge 22. 
     In operation, the constant current pulled through transistor 80 from line 34 supplies the base drive for transistor 28 to turn the latter ON and allow line current, whether AC or DC, to pass through load 20. Furthermore, switch module 40 may be connected in any of the configurations in FIGS. 3 through 6. In the absence of signal 54, transistor 52 is OFF, whereby transistors 78 and 80 are OFF, and hence there is no driving base current for transistor 28, whereby the latter is in an OFF state blocking line current flow through load 20. Load 20 thus is operated in a normally open (NO) mode. 
     Another load 90 may alternatively or additionally be operated in a normally closed (NC) mode. In response to the absence of gate signal 54, input transistor 68 is ON, whereby rectified line current is supplied from positive DC terminal 48 of bridge 42 to line 92 through resistor 94. A second conditioning circuit 96 is provided, comparable to conditioning circuit 72. A predetermined proportional level constant drive current is thus afforded on output line 98 which supplies the base drive for a second load transistor 102. Transistor 102 is connected in a second output diode bridge rectifier 104 which is connected at its AC inputs between first terminal 1 and another third terminal 3a. In the absence of signal 54, load transistor 102 is thus base driven into a conductive ON state to pass line current, whether AC or DC, through load 90. Load 90 is thus operated in a normally closed (NC) mode with switch module 40 connected in any of the configurations of FIGS. 3 through 6. 
     It is recognized that various modifications are possible within the scope of the appended claims.