Abstract:
A simple voltage window sensing circuit including a current indicator, two Zener diodes and two digital transistors wherein the diode breakdown voltages are selected such that they define the upper and lower voltage limits of a voltage window, each diode causing a corresponding transistor to conduct when the breakdown voltage is exceeded, the indicator in series with a first transistor which is controlled by the diode which defines the lower limit and the transistors in parallel such that the first transistor only conducts when a voltage is within the window and hence the indicator only indicates when the voltage is within the window.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     Not applicable. 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not applicable. 
     MICROFICHE APPENDIX 
     Not applicable. 
     BACKGROUND OF THE INVENTION 
     The present invention relates to voltage sensing circuitry, and more particularly to threshold window circuits which determine whether a measured voltage lies within a given voltage range. 
     Window comparator circuits have long been used in various electronic industries to determine whether a measured parameter is within a window bound by upper or lower limits. One particularly useful application for window sensing is in voltage sensing circuits which determine if a voltage between two nodes is within a window. Typical circuits of this type generate a signal when a measured voltage is within the window. 
     One well-known voltage window sensing circuit includes, among other circuitry, two comparators, two voltage dividers (e.g., four resistors) and an output pull-up resistor. Unfortunately, while being sufficiently accurate, a large parts count renders these types of comparators relatively expensive and requires significant printed circuit board space. For these reasons, when costs and circuit board space need to be minimized, comparators of this well-known type are unsatisfactory. 
     As is evident from the foregoing, a need exists for a cost-effective, space-minimizing voltage-window sensing circuit capable of preserving the functionality of the common window comparator circuit at a relatively low cost and which requires minimal circuit board space. 
     BRIEF SUMMARY OF THE INVENTION 
     It has been recognized that a simple arrangement including two Zener diodes, two “digital” transistors (two bipolar junction transistors, each transistor having resistors situated across its base-emitter junction and in series with its base) and a current sensor can be constructed which has a small parts count, is extremely inexpensive, is extremely reliable, and requires relatively little circuit board space. To this end, one inventive embodiment comprises first and second Zener diodes which are chosen such that their first and second breakdown voltages, respectively, essentially define the lower and upper limits of a voltage window needed in a given application. 
     The Zener diode cathodes are linked to a first node and the first and second Zener diode anodes are linked to first and second bases of the first and second transistors, respectively. Negligible amounts of current flow into these bases until the respective Zener diode voltages are exceeded. The collector of the second transistor is linked to the first node and its emitter is linked to the second node. The indicator is linked in series with the first transistor between the first and second nodes. When either transistor conducts, current passes from the first node to the second node. The current sensor may be any sensing element, such as a light emitting diode, an opto-coupled or magnetically-coupled circuit, an LED, etc. 
     When so constructed the circuit defines three voltage ranges between the first and second nodes, a first range below the lower voltage window limit, a second range within the window and a third range above the upper voltage window limit. To this end, when voltage between the first and second nodes is below the lower window limit (i.e., in the first range), neither Zener diode conducts, corresponding transistors are off and hence no current is sensed by the current sensor. 
     When the voltage between the first and second nodes exceeds the lower window limit but not the upper window limit, the first Zener diode breaks down causing the first transistor to conduct. Current passing through the first transistor is sensed by the sensor which indicates a voltage within the voltage window. 
     When the voltage between the first and second nodes exceeds the upper window limit both the first and second Zener diodes break down and corresponding transistors conduct. Assuming the current sensor has some resistance, all current passes through the second transistor and not the first such that the current sensor will not sense a current and will not indicate a voltage within the window. 
     Thus, one object is to determine if a voltage between the two nodes is within a voltage window. Another object is to achieve the aforementioned object inexpensively. This is accomplished by configuring a circuit with a reduced parts count. 
     Yet another object is to achieve the aforementioned objects while requiring only minimal circuit board space. This object also is accomplished by configuring a circuit with a reduced parts count. 
     In a preferred embodiment of the invention a resistor can be linked in series with the first transistor between the first and second nodes to ensure that no current passes through the first transistor when the voltage between the first and second nodes is above the upper voltage window limit. 
     In yet another embodiment of the present invention, a second resistor can be linked between the second collector and the first node such that the node between the second collector and second resistor becomes a voltage dividing node and wherein the first resistor is then linked between this dividing node and the first collector. Preferably the second resistor resistance is much greater than the first resistor&#39;s resistance (e.g., 10 to 1000 times the first resistor resistance). In fact, in at least one embodiment, the second resistor may be altogether removed. 
     The foregoing and other objects, advantages, and aspects of the present invention will become apparent from the following description. In the description, reference is made to the accompanying drawings which form a part hereof, and in which there is shown, by way of illustration, a preferred embodiment of the present invention. Such embodiment does not necessarily represent the full scope of the invention, however, and reference must also be made to the claims herein for properly interpreting the scope of this invention. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
     FIG. 1 is a schematic circuit diagram in accordance with one embodiment of the present invention; 
     FIG. 2 is a second schematic circuit diagram in accordance with a second embodiment of the present invention; and 
     FIG. 3 is a third schematic circuit diagram in accordance with a third embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to FIG. 1, a voltage window sensing circuit  10  can be used to indicate a voltage across a first node  12  and a second node  13 . Circuit  10  includes a current indicator  16 , a first Zener diode  20  having a first cathode  22  and a first anode  24  and characterized by a first breakdown voltage  26 , a second Zener diode  40  having a second cathode  42  and a second anode  44  and characterized by a second breakdown voltage  46 , a first bipolar junction digital transistor  30  having a first base  32 , a first collector  34 , and a first emitter  36  and a second bipolar junction transistor  50  having a second base  52 , a second collector  54 , and a second emitter  56 . 
     Indicator  16  includes a light emitting diode  80  which is opti-coupled to a light sensitive switch  82 . Diode  80  and switch  82  electrically isolate circuit components to the left thereof in FIG. 1 from components to the right. While all illustrated embodiments include this type of indicator, other types are contemplated. When current flows through diode  80 , diode  80  lights up and switch  82  senses that light causing current to flow therethrough which indicates diode  80  current. 
     First cathode  22  is linked to first node  12 . First anode  24  is linked to first base  32  of first transistor  30  such that a negligible amount of current flows into first base  32  until the first breakdown voltage  26  is exceeded. Likewise, second cathode  42  is linked to first node  12 , and second anode  44  is linked to second base  52  of second transistor  50  such that a negligible amount of current flows into the second base  52  until the second breakdown voltage  46  is exceeded. By selecting the first diode  20  to have a lower breakdown voltage  26  than the second diode  40 , the value of the first breakdown voltage  26  sets the lower voltage limit  26 ′ of the window, and the value of the second breakdown voltage  46  sets the upper voltage limit  46 ′ of the window. Selecting the appropriate first Zener diode  20  and second Zener diode  40  depends, therefore, on the respective lower voltage limit  26 ′ and upper voltage limit  46 ′ required by a particular application. 
     By choosing a first breakdown voltage  26  which is distinct from a second breakdown voltage  46 , three regions of operation are defined. In Region I, the measured voltage between the first node  12  and second node  13  does not exceed the first breakdown voltage  26  set by the first Zener diode  20  or the second breakdown voltage  46  set by the second Zener diode  40 . Thus, both the first digital transistor  30  and second digital transistor  50  remain in their cutoff regions of operation, and consequently, only negligible current flows into the first collector  34  or second collector  54 . With little or no current flowing through transistor  30  indicator  16  indicates that no current is sensed. Thus, during Region I operation, indicator  16  fails to generate a signal indicating that the measured voltage is outside of the voltage window. 
     During Region II operation the measured voltage between first node  12  and second node  13  exceeds the first breakdown voltage  26  of the first Zener diode  20 , but not the second breakdown voltage  46  of the second Zener diode  40 . Hence, the first transistor  30  turns on. Since the first transistor  30  is turned on (i.e., effectively behaving as a closed circuit) and the second transistor  50  remains off (i.e., effectively behaving as an open circuit), current flows from first node  12  to second node  13  through the first collector  34  of the first transistor  30  and is received by the first emitter  36 . Current thus passes through current indicator  16  and indicator  16  generates a signal indicating that the measured voltage is within the voltage window, exceeding the lower voltage limit  26 ′ of the window but not the upper voltage limit  46 ′. 
     During Region III operation the measured voltage between first node  12  and second node  13  exceeds the first breakdown voltage  26  of first Zener diode  20  and the second breakdown voltage  46  of second Zener diode  40 . Thus, both the first transistor  30  and the second transistor  50  are turned on, thereby potentially facilitating current flow. However, due to internal resistance of current indicator  16 , essentially all available current flows from first node  12  to second node  13  through the second collector  54  where it is received by the second emitter  56 . Since current flows through the path of least resistance, no current flows through first transistor  30  or current indicator  16  and, consequently, indicator  16  fails to generate a signal. Absence of a signal indicates that the measured voltage is outside of the voltage window. 
     The above embodiment assumes that indicator  16  is characterized by some resistance such that, when both transistors  30  and  50  are on, the indicator resistance blocks current flow and all current passes through transistor  50 . This may not always be the case. For example, where indicator  16  causes no resistance (e.g., a Hall effect sensor), current may pass through each transistor  30  and  50  when both transistors are on, effectively causing current division. In this case the above described embodiment may still operate properly if indicator  16  is chosen only to indicate a current which is greater than the current which flows through transistor  30  when both transistors  30  and  50  are on. Thus, indicator  16  would have a current threshold which would have to be exceeded prior to indicating current. 
     Although the above-described embodiment linked the current indicator  16  between the first collector  34  and the first node  12 , the circuit functions identically if, in an alternative embodiment, the first collector  34  is linked to the first node  12  and the current indicator is linked between the first emitter  36  and the second node  13 . 
     Referring now to FIG. 2, a second embodiment of the invention is illustrated which links a first resistor  60  in series with first transistor  30 . Resistor  60  can be linked in series with the first transistor  30  by either linking the first resistor  60  between the first emitter  36  and the second node  13 , or between first collector  34  and first node  12 . In either case diode  80  may be placed in series or in parallel with resister  60 . In any of the embodiments including resistor  60 , resistor  60  provides additional resistance in series with transistor  30  to block current when each of transistors  30  and  50  are both potentially conducting (i.e., on). Operation of the configuration in FIG. 2 is essentially identical to operation of the FIG. 1 configuration and therefore is not again explained here in detail. 
     Referring still to FIG. 2, most preferably a second resistor  62  is linked between node  12  and the second transistor collector  54  such that a third or voltage dividing node  14  is found therebetween. In this case the series arrangement of resistor  60 /indicator  16  and transistor  30  is linked between nodes  14  and  13 . Operation with second resistor  62  is similar to the operation described above. In either case, with current indicator  16  linked in parallel with first resistor  60 , indicator  16  detects voltages across first resistor  60  when current passes therethrough. 
     The spirit of the present invention is not limited to any embodiment described above. Rather, the details and features of an exemplary embodiment were disclosed as required. Without departing from the scope of this invention, other modifications will therefore be apparent to those skilled in the art. Thus, it must be understood that the detailed description of the invention and drawings were intended as illustrative only, and not by way of limitation. For example, referring again to FIG. 1, while the invention is described as including digital resistors  30  and  50 , other switching configuration may be used such as transistor and resistor arrangements. For instance, transistors  30  and  50  in FIG. 1 may be replaced by first and second transistors with first and second base-to-emitter resistors and first and second series resistors in series with the bases. In this embodiment the first and second series resistors may be positioned between an adjacent Zener diode and a corresponding base or, in the alternative, may be positioned in series with the base and a Zener diode between the Zener diode and node  12 . 
     Moreover, a less accurate although still advantageous embodiment is illustrated in FIG.  3 . In FIG. 3 most of the components are similar to the components in FIG.  1  and therefore are not described again in detail. The FIG. 3 embodiment is unique in that base-to-emitter resistors are not linked to transistors  100 ,  102 . In addition, resistors  104  and  106  are in series with respective Zener diodes and transistor bases and between node  12  and respective Zener diodes. In the alternative, resistors  104  and  106  could be placed between Zener diodes and transistor bases. Operation of the FIG. 3 embodiment is essentially the same as operation of the FIG. 1 embodiment. Features of FIG.  3  and FIG. 2 could be combined to construct additional contemplated embodiments. 
     To apprise the public of the scope of this invention, the following claims are made: