Abstract:
At a contact input circuit, a voltage at a switching device is sensed and the voltage is associated with a status of a switching device. The contact input circuit is operated according to the sensed voltage regardless of the value of the sensed voltage. The power usage of the contact input circuit is maintained to be within a predetermined range of power consumption values regardless of the value of the sensed voltage. Wetting voltages can be continuously monitored and the approaches described herein can monitor open contact, closed contact, and open field wire conditions.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    Utility application entitled “Loop Powered Isolated Universal Contact Input Circuit and Method for Operating the Same” naming as inventors Parag Acharya and Ravindra Desai, and having attorney docket number 268492 (130842) is being filed on the same date as the present application, the content of which is incorporated herein by reference in its entirety. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The subject matter disclosed herein relates to sensing information associated with switching devices and, more specifically, to sensing this information according to a wide range of operating conditions. 
         [0004]    2. Brief Description of the Related Art 
         [0005]    Different types of switching devices (e.g., electrical contacts, switches, and so forth) are used in various environments. For example, a power generation plant uses a large number of electrical contacts (e.g., switches and relays). The electrical contacts in a power generation plant can be used to control a wide variety of equipment such as motors, pumps, solenoids and lights. A control system needs to monitor the electrical contacts within the power plant to determine their status in order to ensure that certain functions associated with the process are being performed. In particular, the control system determines whether the electrical contacts are on or off, or whether there is a fault near the contacts such as open field wires or shorted field wires that affect the ability of the contacts to perform their intended function. 
         [0006]    One approach that a control system uses to monitor the status of the electrical contacts is to send an electrical voltage (e.g., a direct current voltage (DC) or an alternating current (AC) voltage) to the contacts in the field and determine whether this voltage can be detected. The voltage, which is provided to the electrical contacts for detection, is known as a wetting voltage. If the wetting voltage levels are high, galvanic isolation in the circuits is used as a safety measure while detecting the existence of voltage. Detecting the voltage is an indication that the electrical contact is on or off. A wetting current is associated with the wetting voltage. 
         [0007]    Various problems have existed with previous devices. For example, the contacts need to be isolated from the control system, or damage to the control system may occur. Also, the control system may need to handle a wide variety of different voltages, but previous devices only handle voltages within a narrow range. Previous devices have also been inflexible in the sense that they cannot be easily changed or modified over time to account for changes in the operating environment. All of these problems have resulted in general dissatisfaction with previous approaches. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0008]    A universal contact input circuit is provided that can operate across the entire wetting voltage range that is provided. In one aspect (and to enhance efficiency), the circuit automatically adjusts its impedance with wetting voltage in an attempt to keep the circuit power dissipation almost constant throughout the wetting voltage range. In still other aspects, the circuit can detect the contact status (e.g., open or closed), and are also capable of monitoring the wetting voltage. 
         [0009]    In many of these embodiments and at a contact input circuit, a voltage at a switching device is sensed and the voltage is associated with a status of a switching device. The contact input circuit is operated according to the sensed voltage regardless of the value of the sensed voltage. The power usage of the contact input circuit is maintained to be within a predetermined range of power consumption values regardless of the value of the sensed voltage. 
         [0010]    In some aspects, the wetting voltage of the switching device is monitored. In other aspects, a range of voltage values is determined by the monitoring. In still other aspects, the monitoring is performed continuously. 
         [0011]    In some examples, the operation converts the sensed voltage to a useable voltage regardless of the value and type of the sensed voltage. The type of sensed voltage may be a direct current (DC) voltage or an alternating current (AC) voltage. In other examples, the status of the switching device may be an open status or a closed status. 
         [0012]    In others of these embodiments, a contact input circuit includes a fixed attenuator sensing circuit and a control circuit. The fixed attenuator sensing circuit is configured to sense a voltage at a switching device and the voltage is associated with a status of a switching device. The control circuit is coupled to the fixed attenuator sensing circuit. The control circuit is configured to operate the contact input circuit according to the sensed voltage regardless of the value of the sensed voltage and maintain the power usage of the contact input circuit to be within a predetermined range of power consumption values regardless of the value of the sensed voltage. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]    For a more complete understanding of the disclosure, reference should be made to the following detailed description and accompanying drawings wherein: 
           [0014]      FIG. 1  comprises a block diagram of a contact input circuit according to various embodiments of the present invention; 
           [0015]      FIG. 2  comprises a circuit diagram of a contact input circuit according to various embodiments of the present invention; 
           [0016]      FIG. 3  comprises a plot of inverse power dissipation versus input voltage according to various embodiments of the present invention; 
           [0017]      FIG. 4  comprises a circuit diagram of a contact input circuit according to various embodiments of the present invention; 
           [0018]      FIG. 5  comprises a circuit diagram of a contact input circuit according to various embodiments of the present invention; 
           [0019]      FIG. 6A  and  FIG. 6B  comprise circuit diagrams of a contact input circuit according to various embodiments of the present invention; 
           [0020]      FIG. 7  comprises a plot of the inverse of power dissipation according to various embodiments of the present invention; 
           [0021]      FIG. 8  comprises a circuit diagram of a contact input circuit according to various embodiments of the present invention; 
           [0022]      FIG. 9  comprises one example of a lookup table according to various embodiments of the present invention; 
           [0023]      FIG. 10  comprises a block diagram of a contact input circuit according to various embodiments of the present invention; and 
           [0024]      FIG. 11  comprises a block diagram of a contact input circuit according to various embodiments of the present invention. 
       
    
    
       [0025]    Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity. It will further be appreciated that certain actions and/or steps may be described or depicted in a particular order of occurrence while those skilled in the art will understand that such specificity with respect to sequence is not actually required. It will also be understood that the terms and expressions used herein have the ordinary meaning as is accorded to such terms and expressions with respect to their corresponding respective areas of inquiry and study except where specific meanings have otherwise been set forth herein. 
       DETAILED DESCRIPTION OF THE INVENTION 
       [0026]    The approaches described herein provide contact input circuits that are power efficient and which can handle a wide range of wetting voltage ranges (e.g., from approximately 15 Vdc to approximately 350 Vdc). In some aspects, the contact input circuit employs a voltage attenuator to accommodate the wide voltage range. 
         [0027]    In other aspects, the present approaches provide a universal contact input circuits that can handle an entire wetting voltage range. To be efficient, the circuit automatically adjusts its impedance (with respect to the wetting voltage) to keep the circuit power dissipation almost constant throughout the wetting voltage range. Besides being able to handle a large wetting voltage range and detect the contact status (e.g., open or closed), the circuits described herein can monitor the wetting voltage. 
         [0028]    In yet other aspects, the contact input circuits described herein maintain the dissipated power (within a range or around a certain value) by either changing the circuit impedance continuously or intermittently as the wetting voltage changes. In some aspects, the wetting voltage is sensed or measured either continuously or in discrete steps. 
         [0029]    In some examples, a contact input circuit is used to detect the status of a remotely located relay contact or other types of switching devices. The wetting voltage applied to such a relay can be in the range of approximately 15 Vdc to approximately 220 Vdc; approximately 110 Vac/60 Hz; or approximately 230V/50 Hz. These ranges match customer choices of using wetting voltages of approximately 24 Vdc, 28V, 48 Vdc, 125 Vdc, 220 Vdc, 110 Vac/60 Hz or 230 Vac/50 Hz (to mention a few examples). 
         [0030]    The circuits provided herein are cost effective to construct, provide efficient power dissipation over a wide range of operating voltages, and reduce circuit part count. In these respects, the circuit topologies described herein are loop powered (i.e., they do not use an external power source but instead use the sensed voltage as a power source). The topologies of a universal contact input circuit described herein use a low count of simple passive components. The dissipation is controlled either continuously or intermittently to improve the overall efficiency of the circuit. Increased reliability of the circuit is achieved compared to previous approaches. Additionally, a universal contact input circuit allows the accommodation of any last minute changes of customer specifications (of the wetting voltage) and allows, for example, a customer to conveniently stock spare parts. 
         [0031]    Referring now to the drawings and in particular to  FIG. 1 , a block diagram of a contact input circuit  100  is illustrated. The contact input circuit  100  receives an input voltage as may exist across the contacts of a switching device (e.g., a contact, not shown in  FIG. 1 ). The input voltage is fed to a fixed attenuator  108  and then into a set of control switches  110 . The control switches  110  are configured to control a variable current sink comprised of a variable resistor or variable current regulator  102 . 
         [0032]    In some examples, the control switches  110  may be configured to output a signal representative of a measured input voltage to a control system across an isolation barrier through a first isolator  112 . Further, and also optionally, the resistor or variable current regulator  102  may be configured to determine the status of the switching device through a load and contact status sensing module  104 , which also can communicate with the control system across the isolation barrier through a second isolator  106 . So configured, a contact input circuit  100  can be provided to operate with a variety of input voltages and to vary an amount of wetting current across contacts of a switching device in accordance with, at least in part, the perceived input voltage. The first isolator  112  and the second isolator  106  communicate with a control system or processor (not shown). The control system may also include any combination of processing devices that execute programmed computer software and that are capable of analyzing information received from the contact input circuit  100 . 
         [0033]      FIG. 2  illustrates a circuit diagram of a contact input circuit  200 . The contact input circuit  200  includes a first switch/sink module  204  and a second switch/sink module  206  that are parallel to one another across the inputs to the contact input circuit  200 . Each switch/sink module  204 ,  206  receives voltage from an input, for example, from one or more input contacts across a switching device (illustrated here as simulated voltage input  202 ). The received voltage may be provided by a power supply coupled to the switching device or from a device coupled to the switching device. 
         [0034]    With respect to the first switch/sink module  204 , the received voltage enters a resistive voltage divider consisting of resistors  208  and  210 , with the signal between the resistors  208  and  210  being provided through resistor  212  to a drain of a transistor  214  (here shown as an N-channel FET, though other transistor types may be equally as suitable, including BJT transistors, CMOS transistors, and other FETs). The gate of transistor  214  is connected across one or more pull-up resistors  220  to the input voltage, as well as to a pull down resistors  216  (forming another resistor voltage divider circuit at the gate of the transistor  214 ). A second transistor  218  is provided such that its drain and source are in parallel with the pull down resistor  216  at the gate of the first transistor  214 . The gate of the second transistor  218  is connected to at least one zener diode  224  that is configured to block current from flowing to the gate of the second transistor  218  until the input voltage achieves a particular minimum voltage to trigger the zener. For example, the zener diode may be approximately 50V, or more precisely 56V by one approach, though almost any value is possible and can be selected by a designer according to the desired behavior specifics of the circuit  200 , including the desired granularity of the input voltage ranges. 
         [0035]    As input voltage increases from 0V, the first transistor  214  will begin to sink current commensurate with the attenuated voltage at its gate input, thus creating power dissipation across the various resistors and a wetting current across the contacts of the switching device. As the input voltage increases beyond the voltage of the zener diode  224  (e.g., above approximately 50V), the current will begin to flow though the zener diode  224  and through voltage divider resistors  226  and  228 , with the signal at the middle of voltage divider resistors  226  and  228  being fed to the gate of the second transistors  218  through a resistor  222 . Eventually, the second transistor  218  will turn on and shunting the pull down resistor  216 , thus creating a low input voltage to transistor  214  and stopping transistor  214  from sinking any current. Instead, a new current sink path is created through pull up resistors  220  and transistor  218 . This new current sink path is of higher resistance than that through the first transistor  214  and resistors  208  and  212 . Thus, as the first current sink path is removed by transistor  214  shutting off, the resistance of the entire current sink path increases, which reduces current therethrough, and reduces dissipated power. 
         [0036]    The second switch/sink module  206  can be provided that is nearly identical to the first switch/sink module  204  except for a few components. For example, the second module  206  will include the resistive voltage divider consisting of resistors  230  and  232 , with the signal between the resistors  230  and  232  being provided through resistor  234  to a drain of a transistor  236 . Like the first switch/sink module  204 , the gate of transistor  236  is connected across one or more pull-up resistors  242  to the input voltage, as well as to a pull down resistor  238 . A second transistor  240  is provided such that its drain and source are in parallel with the pull down resistor  238  at the gate of the first transistor  236 . Like the first switch/sink module  204 , the gate of the second transistor  240  is connected to at least one zener diode, and in this example, is connected to two zener diodes  246  and  248  in series. The zener diodes  246  and  248  in this example are simply the same value as zener diode  224 , thus creating a voltage block that is double the voltage block of the zener diode  224 . Other zener diode  246 ,  248  values are possible according to the desired behavior of the circuit  200 , though it is preferred to select a combined value of zener diodes  246 ,  248  that exceed that of the first module so that a staggered switching may occur, some of the benefits of which will be described with respect to  FIG. 3  below. 
         [0037]    Like the first switch/sink module  204 , as the input voltage exceeds the combined voltage of the zener diodes  246 ,  248 , current will eventually flow through the diodes  246 ,  248  and through the divider resistors  250 ,  252  and through gate input resistor  244  so that the second transistor  240  turns on and shuts pull down resistor  238  to turn off first transistor  236 . Again, as the resistance path through transistor  236  and resistors  230  and  234  was much less than the resistance path through transistor  240  and the pull up resistors  242 , the overall resistance of the current sink path increases, thus lowering the current therethrough and lowering the overall power dissipation. 
         [0038]    It may be beneficial to provide the pull up resistors  220 ,  242  as multiple resistors each in series as is shown (or in parallel, or with resistors beyond the two shown in  FIG. 2 ) so that the power dissipated is spread across the multiple pull up resistors  220 ,  242  to prevent device failure. Further, it may also be beneficial to make the resistor  234  coupled to the drain of the first transistor  236  in the second module  206  greater than the sink resistor  212  coupled to the drain of the first transistor  214  of the first switch/sink module  204 . This is because, when configured as described, the first transistor of the second module  206  will continue to sink current at higher input voltages than will the first transistor  214  of the first switch/sink module  204  due to the comparative voltages of the zener diodes  224  and  246 ,  248 . 
         [0039]    Further, though only two switch/sink modules  204 ,  206  are illustrated here, any number of switch/sink modules can be utilized, primarily dependant upon how tight of a power dissipation band  314  is desired (see  FIG. 3 ) or how much granularity is desired on the input voltage. 
         [0040]    Referring now to  FIG. 3 , an example plot  300  of the inverse of the power dissipation (on the y-axis) versus the input voltage (on the x-axis) is illustrated in accordance with an approach described with respect to  FIG. 2 . The curve  302  represents the power dissipation at each specified input voltage. With continuing reference to  FIG. 2 , during segment  308  of  FIG. 3 , transistors  214  and  236  will remain on. As the input voltage reaches and exceeds the voltage of the first zener diode  224  at voltage point  304 , transistor  214  will turn off and transistor  218  will turn on. This increases the overall resistance of the current sink path, which reduces the current therethrough, which lowers the power dissipation, as is shown by the jump in power dissipation at point  304 . During segment  310 , transistors  218  and  236  will continue to sink current until the input voltage rises above the value of the zener diodes  246 ,  248  of the second module  206  at point  306 . At this point  306 , transistor  236  will shut off and transistor  240  will turn on, again increasing the overall resistance and lowering the current and total power dissipation. So configured, the total power dissipation is kept roughly within a desired power dissipation band  314  as may be optimized for providing appropriate wetting current across the contacts of the switching device across a wide range of input voltages. 
         [0041]    Returning to  FIG. 2 , a load and contact status sensing circuit  254  portion is described. The input voltage is fed across a sensing resistor  256 , which creates a voltage that is fed in parallel to an optocoupler  258  across an isolation barrier. The light sensing transistor portion of the optocoupler  258  will sense light from the light emitting diode LED portion of the optocoupler  258 , which will allow current to flow through its base and resistor  260 . This creates a current on the output, which allows current from a control-side power source to flow through pull down resistor  264  to produce a low output signal representative of current flow on the contact inputs. This signal can then be fed to a processing device for processing thereof. 
         [0042]    Further, it is noteworthy that, as configured, the contact input circuit  200  is operated from power supplied across the input terminals to the contact input circuit  200 , which eliminates the need for additional power sources or other external components to power the circuit. This has the effect of reducing implementation cost of the contact input circuit  200 , as well as improving its compatibility with existing installations and/or new installations using varying control systems. 
         [0043]    Turning now to  FIG. 4 , another example of a contact input circuit  400  is illustrated. Like with  FIG. 2 , input voltage is simulated for illustration purpose by voltage source  402 . The voltage input is fed across a voltage divider  404  consisting of resistors  406 ,  408 ,  410 ,  412 , and  414 , which may correspond to the fixed attenuator  108  illustrated in  FIG. 1 . A set of optocouplers  418 ,  420 ,  422 ,  424  and corresponding base resistors  426 ,  428 ,  430 ,  432  comprise the control switches  110  of  FIG. 1 , with the resistor or variable current regulator  102  of  FIG. 1  being shown at  434 . A representative load across the switching device is shown by resistor  444 . A load and contact status sensing circuit  454  is provided to sense a load across a load resistor  403  as was described with respect to the load and contact status sensing circuit  254  of  FIG. 2 . 
         [0044]    The current regulator  434  includes a transistor  436  (shown here as a N channel FET, though other transistor types may be suitable) with its drain coupled to the input voltage and its source coupled to a load resistor  440 , which is in series with load resistor  444 , which returns to ground. The gate of the transistor  436  is coupled to a series of zener diodes  446 ,  448 ,  450 ,  452 ,  442  that establish the voltage at the gate. Pull up resistor  438  is coupled between the voltage input and the gate. 
         [0045]    The input of each optocoupler  418 ,  420 ,  422 ,  424  is placed across one of the voltage divider resistors  408 ,  410 ,  412 ,  414 , for example, optocoupler  418  is connected across resistor  408 , optocoupler  420  is connected across resistor  410 , optocoupler  422  is connected across resistor  412 , and optocoupler  424  is connected across resistor  414 . Each optocoupler output is placed in parallel with one of the zener diodes  446 ,  448 ,  450 ,  452 . For example, the output of optocoupler  418  is in parallel with zener diode  446 , the output of optocoupler  420  is in parallel with zener diode  448 , the output of optocoupler  422  is in parallel with zener diode  450 , and the output of optocoupler  424  is in parallel with zener diode  452 . 
         [0046]    The values of the resistors  406 ,  408 ,  410 ,  412 ,  414  are selected so that, in operation, as the input voltage increases, the optocouplers  418 ,  420 ,  422 ,  424  will be activated one by one across the allowable input voltage span (for example, evenly spaced between 0 and 500V). As each optocoupler is activated, the output will shunt its respective zener diode. Thus, as the input voltage increases, more optocouplers become active, thus shunting more zener diodes, thus lowering the drive voltage at the gate of the transistor  436 . As the gate drive voltage is lowered, the current through the transistor  436  drops, thus reducing the wetting current and reducing the power dissipated. The result is a stepped power dissipation curve similar to was shown in  FIG. 3  that keeps the power dissipation within an approximate band  314  over the entire input voltage range. 
         [0047]    Referring now to  FIG. 5 , another example of a contact input circuit  500  is described. The contact input circuit  500  shows a representative input voltage  502  fed across a voltage divider  504  consisting of resistors  506 ,  508 ,  510 ,  512 ,  514 ,  516 ,  518 , and  520 . The nodes between each resistor of the voltage divider  504  are each fed into the base of individual transistors through individual input resistors in the switching circuit  524 . For example, the node between resistor  506  and  508  is coupled to the base of transistor  526  through resistor  540 ; the node between resistor  508  and  510  is coupled to the base of transistor  528  through resistor  542 ; the node between resistor  510  and  512  is coupled to the base of transistor  530  through resistor  544 ; the node between resistor  512  and  514  is coupled to the base of transistor  532  through resistor  546 ; the node between resistor  514  and  516  is coupled to the base of transistor  534  through resistor  548 ; the node between resistor  516  and  518  is coupled to the base of transistor  536  through resistor  550 ; and the node between resistor  518  and  520  is coupled to the base of transistor  538  through resistor  552 . 
         [0048]    Each transistor is coupled to the gate of a current sink transistor  574  (here shown as an N channel FET, though other transistors may be suitable) through load resistors  554 ,  556 ,  558 ,  560 ,  562 ,  564 , and  566 . The gate of the current sink transistor  574  is also coupled to a voltage divider circuit comprised of a pull up resistor  570  and a pull down resistor  572 . Each transistor and load resistor combination is in parallel with the pull down resistor  572  coupled between the gate of the current sink transistor  574  and ground. The drain of the current sink transistor  574  is coupled to the input voltage with its source coupled to a load resistor  568  representative of a load across the contact inputs. 
         [0049]    The values of the resistors  506 ,  508 ,  510 ,  512 ,  514 ,  516 ,  518 , and  520  of the voltage divider  504  are selected so that, in operation, as the input voltage increases, each transistor  526 ,  528 ,  530 ,  532 ,  534 ,  536 ,  538  will turn on one-by-one across the allowable input voltage span (for example, evenly spaced between 0 and 500V). For example, the value of resistor  508  may be the highest while the value of resistor  518  may be the lowest (with resistor  520  provided as a minimum basis resistance to trigger the last transistor in the series and resistor  506  being the largest and acting as an attenuating resistor) so that as voltage increases, the voltage at the top of resistor  508  will be the first to activate a transistor (i.e., transistor  526 ) and the voltage at the top of resistor  520  will be the last to activate a transistor (i.e., transistor  538 ). As each transistor is activated, current begins to flow through each transistor  526 ,  528 ,  530 ,  532 ,  534 ,  536 ,  538  and its respective load resistor  554 ,  556 ,  558 ,  560 ,  562 ,  564 ,  566 . 
         [0050]    As the input voltage increases from 0V, it will eventually reach a level through the voltage divider resistors  570  and  572  above the threshold of the current sink transistor  574 , which will then allow current to flow therethrough in relation to the gate voltage. With no transistors of the switching circuit  524  on, the resistance to the gate of the current sink transistor  574  will be at its highest, and thus its voltage will be at the highest as well, which allows more current to flow. As the input voltage increases, eventually transistor  526  will turn on, allowing current to flow through resistor  554 . The resistance of resistor  554  in parallel with the pull down resistor  572  lowers the total resistance seen at the gate of the current sink transistor  574 , which resultantly lowers its current throughput, and lowers the respective power dissipation. As the voltage continues to rise, the other transistors will also turn on one-by-one and their respective load resistors will lower the gate resistance, thus lowering the gate voltage, which lowers the current and the power dissipation. The values of the load resistors  554 ,  556 ,  558 ,  560 ,  562 ,  564 ,  566  may be selected, by one approach, to be continuously decreasing (i.e., load resistor  554  may have a higher value than load resistor  566 ) so that the current output is tuned according to the input voltage to keep the power dissipation from the wetting current within an approximate band or range across the entire input voltage range. 
         [0051]    Referring now to  FIG. 6A  and  FIG. 6B , yet another example of a contact input circuit is described. The example contact input circuit  600  includes a representative input voltage  602  fed across a voltage divider  604  consisting of resistors  606 ,  608 ,  610 ,  612 ,  614 ,  616 ,  618 ,  620 ,  622 ,  624 , and  626 . The nodes between each resistor of the voltage divider  604  are each fed into the input of an Open Collector Schmidt inverter (herein after referred to as Schmidt inverter for brevity) of the switching circuit  628  through a resistor  630  and across a clamping diode  632 . For example, the node between resistor  606  and  608  is coupled to the input of Schmidt inverter  634 ; the node between resistor  608  and  610  is coupled to the input of Schmidt inverter  642 ; the node between resistor  610  and  612  is coupled to the input of Schmidt inverter  648 ; the node between resistor  612  and  614  is coupled to the input of Schmidt inverter  654 ; the node between resistor  614  and  616  is coupled to the input of Schmidt inverter  660 ; the node between resistor  616  and  618  is coupled to the input of Schmidt inverter  666 ; the node between resistor  618  and  620  is coupled to the input of Schmidt inverter  672 , and the node between resistor  620  and  622  is coupled to the input of Schmidt inverter  678 . The output of each Schmidt inverter  634 ,  642 ,  648 ,  654 ,  660 ,  666 ,  672 ,  678  is coupled to a pull up resistor  636  and the input of a second Schmidt inverter. For example, the output of Schmidt inverter  634  is coupled to the input of Schmidt inverter  638 ; the output of Schmidt inverter  642  is coupled to the input of Schmidt inverter  644 ; the output of Schmidt inverter  648  is coupled to the input of Schmidt inverter  650 ; the output of Schmidt inverter  654  is coupled to the input of Schmidt inverter  656 ; the output of Schmidt inverter  660  is coupled to the input of Schmidt inverter  662 ; the output of Schmidt inverter  666  is coupled to the input of Schmidt inverter  668 ; the output of Schmidt inverter  672  is coupled to the input of Schmidt inverter  674 ; and the output of Schmidt inverter  678  is coupled to the input of Schmidt inverter  680 . 
         [0052]    Each of the second Schmidt inverters is coupled to a resistor that is tied to the source of a current sink transistor  685 . For example Schmidt inverter  638  is coupled to resistor  640 ; Schmidt inverter  644  is coupled to resistor  646 ; Schmidt inverter  650  is coupled to resistor  652 ; Schmidt inverter  656  is coupled to resistor  658 ; Schmidt inverter  662  is coupled to resistor  664 ; Schmidt inverter  668  is coupled to resistor  670 ; Schmidt inverter  674  is coupled to resistor  676 ; and Schmidt inverter  680  is coupled to resistor  682 . These resistors are in parallel between the source of the current sink transistor and the Schmidt inverters to form a collective current sink load resistance. 
         [0053]    A current regulator circuit  684  is provided, including current sink transistor  685  (shown here as an N channel FET, though other transistor types may be suitable) with its drain coupled to the input voltage. The gate of the current sink transistor  685  is coupled to a pull up resistor  686  and to a zener diode  687  as well as diodes  688  and  689 . By this, the voltage at gate of the current sink transistor  685  will be set to the value of the zener diode  687  (here set to an example value of approximately 7.5V, though other values are possible) plus the diode drop voltage of the other optional diodes  688  and  689 . The source of the current sink transistor  685  is coupled to the collective current sink load resistance formed by the set of resistors  640 ,  646 ,  652 ,  658 ,  664 ,  670 ,  676 , and  682  in parallel. 
         [0054]    The values of the resistors of the voltage divider  604  are selected so that, in operation, as the input voltage increases, a voltage on the input of each first Schmidt inverter will rise above the threshold voltage of the first Schmidt inverter causing its output to go low, thus causing the output of the coupled second Schmidt inverter to go high. For example, as the voltage at the top of resistor  608  exceeds the threshold input voltage for Schmidt inverter  634 , the Schmidt inverter  634  output will go low, causing the second Schmidt inverter  638  to output a high signal. This process will continue itself with each respective Schmidt trigger set as the input voltage increases. 
         [0055]    Prior to the voltage at each resistor of the voltage divider  604  exceeding the respective Schmidt inverter voltage, the output of each second Schmidt inverter  638 ,  644 ,  650 ,  656 ,  662 ,  668 ,  674 ,  680  will remain tied to ground. Thus, each load resistor  640 ,  646 ,  652 ,  658 ,  646 ,  670 ,  676 ,  682  will be tied in parallel between the source of the current sink transistor  685  and ground, which decreases the collective source resistance. With a lowered source resistance, the current sink transistor  685  will sink more current (as compared to a higher source resistance) to raise the voltage its source. In order to sink the necessary current provided through current sink transistor  685 , the Schmidt inverters  638 ,  644 ,  650 ,  656 ,  662 ,  668 ,  674 ,  680  may be, by one example, open-collector Schmidt inverters. As the input voltage rises, more Schmidt inverters will go from lo to high, thus removing their respective load resistors from the collective parallel source resistance and effectively increasing the resistance seen by the source. As this resistance increases in steps (as the input voltage increases), the current sink transistor  685  will have to sink less current to keep its source voltage up, which reduces the wetting current and keeps the dissipated power within a band. 
         [0056]    Referring now to  FIG. 7 , an example plot  700  of the inverse of the power dissipation (on the y-axis) versus the input voltage (on the x-axis) is illustrated in accordance with the approach described with respect to  FIG. 6A  and  FIG. 6B . Similar to the example plot  300  of  FIG. 3 ,  FIG. 7  shows curve  702  represents the power dissipation at each specified input voltage. Each input voltage segment or range  704 ,  706 ,  708 ,  710 ,  712 , and  714  refers to an input voltage range between the activation of the various Schmidt inverter pairs. So configured, as the input voltage increases, more Schmidt inverter pairs are activated, thus lowering the sink current resultantly keeping power dissipation within an approximate ideal power dissipation range shown by band  716 . 
         [0057]    Referring now to  FIG. 8 , a circuit diagram of a contact input circuit  800  is described. A voltage source  802  representative of the input voltage across the input to the contact input circuit  800  is shown. The input voltage is fed to a drain of a transistor  820  (although an N channel FET is illustrated, other various transitory types may be appropriate). The voltage is fed through resistor  816  and across clamp diode  818  to the gate of the transistor  820 . As the input voltage exceeds about 10V, a voltage at the source of the transistor  820  will remain approximately 6-8V and serves as a power supply for the contact input circuit  800 . The power is fed through a resistor  822  into an optocoupler, which transmits the signal across an isolation barrier to an output served by a pull up resistor  826 . This output can then be fed to a processing device to provide the status of the contacts (i.e., closed, open, powered, and so forth). 
         [0058]    The input voltage is also fed to an inverting input of an op amp  808  through resistors  804  and  806 . The non-inverting input of the op amp  808  receives a reference voltage  810 . A feedback resistor  812  is provided between the output of the op amp  808  and the inverting input and establishes a gain (in comparison to the input resistors  804  and  806 ) for the op amp  808  to amplify the difference between the attenuated input voltage signal and the reference voltage  810 . The op amp  808  receives supply power from the source of the transistor  820 , as described above. Thus, because the op amp  808  inverts the difference between the input voltage signal and the reference voltage  810 , as the input voltage increases, the output voltage of the op amp  808  will reduce. The resistor  814  represents the resistive load of a current sensing module. As the voltage across the resistor  814  decreases, the current also decreases. Thus, as the input voltage increases, the output wetting current decreases. Thus, linear control over the power dissipation is provided as compared to the stepped control described above. 
         [0059]    Returning again to  FIG. 6A  and  FIG. 6B , other aspects of the contact input circuit  600  are described. The Schmidt inverters are configured to change state as the input voltage increases. This state information can be provided to a processing device such that the processing device can know the present input voltage range. With brief reference to  FIG. 7 , or example, the processing device can determine if the input voltage is in range  704 ,  706 ,  708 ,  710 ,  712 , or  714 . This is particularly useful when an exact input voltage is not required but where knowledge of an approximate range would be useful for the processing device. 
         [0060]    Returning again to  FIG. 6A  and  FIG. 6B , two separate approaches are illustrated to provide the range data to a processing device. The first approach  696  involves the use of a serial analog-to-digital converter (ADC)  697  that may receive an attenuated voltage, for example, across resistor  626 . The digitized voltage value can then transmitted serially through an isolator  698  across an isolation barrier (e.g., with an optocoupler) for use by a processing device. 
         [0061]    By another approach,  690 , the outputs of the Schmidt inverters  638 ,  644 ,  650 ,  656 ,  662 ,  668 ,  674 ,  680  are each fed into one input of a serializer  692  which can then be fed into an isolator  694  for transmission across an isolation barrier for use by the processing device. Referring to  FIG. 9 , an example lookup table is illustrated that may be utilized by a processing device to convert the received data in this second approach  690  to voltage range information according to at least one approach. 
         [0062]    Referring next to  FIG. 10 , another example of a contact input circuit  1000  is described. The contact input circuit  1000  includes a fixed attenuator  1002  that receives the input from the input contacts. The attenuator  1002  acts as a voltage sensing block and feeds an attenuated version of the input voltage to an analog to digital converter (ADC) or voltage controlled oscillator  1004 . The ADC/VCO  1004  sends a digitized signal through a first isolator  1006  (such as an optocoupler) across an isolation boundary to a processing device for processing thereof. Based on known transform functions or tables, the processing device will know the present input voltage and can decide an appropriate wetting current. 
         [0063]    After deciding the appropriate wetting current, the processing device then sends a digital signal representative of the selected wetting current back across the isolation boundary through a second isolator  1008  to a digital-to-analog converter (DAC)  1010 . The DAC then converts the digital signal to an analog output. The variability of the analog output then can be used to vary a wetting current provided on or by load  1012 , as has been described above. The first isolator  1006  and the second isolator  1008  communicate with a control system or processor (not shown). The control system may also include any combination of processing devices that execute programmed computer software and that are capable of analyzing information received from the contact input circuit  1000 . 
         [0064]    Accordingly, by this approach, the contact input circuit is capable of operating with a wide range of input voltages while providing a processing device a relatively precise real-time measurement of the input voltage. The processing device can then utilize this information to control a wetting current as well as make other decisions or take other actions with respect to the circuit  1000 . 
         [0065]    Referring now to  FIG. 11 , another example of a contact input circuit  1100  is described. The contact input circuit  1100  is similar to the contact input circuit of  FIG. 1  and like numbered elements operate in the same way. In this respect, the contact input circuit includes a fixed attenuator  1108 , a set of control switches  1110 , a variable resistor or variable current regulator  1102 , a load and contact status sensing module  104 , a first isolator  1112 , a second isolator  1106 . In contrast to the example of  FIG. 1 , the circuit of  FIG. 11  includes a switching device  1122  (e.g., a contact), a resistor  1124  that is connected electrically in parallel to the switching device  1122 , and an optional rectifier  1126 . A wetting voltage source  1120  is connected to the switching device  1122 /resistor  1124 . The optional rectifier  1126  converts and AC voltage to a DC voltage. The resistor  1124  is close to the switching device in the field and allows the detection of an open field wire condition. By “open field wire” condition, it is meant a break in either of the two wires that connect the customer terminal block and the switching device (e.g., contact) in the field (e.g., remote location to the contact input circuit  1100 ). 
         [0066]    It will be appreciated that the various examples described herein use various components (e.g., resistors and capacitors) that have certain values. Some of these values may be shown in the figures. If not shown, these values will be understood or are easily obtainable by those skilled in the art and, consequently, are not mentioned here. 
         [0067]    It will be appreciated by those skilled in the art that modifications to the foregoing embodiments may be made in various aspects. Other variations clearly would also work, and are within the scope and spirit of the invention. The present invention is set forth with particularity in the appended claims. It is deemed that the spirit and scope of that invention encompasses such modifications and alterations to the embodiments herein as would be apparent to one of ordinary skill in the art and familiar with the teachings of the present application.