Abstract:
A method of indicating multi-bit values using a single optocoupler indicates a first multi-bit value in response to a first range of optocoupler output voltages, and indicates a second multi-bit value in response to a second range of optocoupler output voltages. The first range is different from the second range.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    This application relates to optocouplers, and more particularly to a method of indicating multi-bit values using a single optocoupler. 
         [0002]    An optocoupler is a device that uses a short optical transmission path to transfer a signal between elements of a circuit, while keeping the circuit elements electrically isolated. One optocoupler configuration includes a photo diode that emits light that causes a photo transistor to turn ON and permit a flow of current, yielding an output voltage. Thus, the photo diode is able to control the photo transistor while remaining electrically isolated from the photo transistor. 
         [0003]    Optocouplers have wide tolerance ranges, such that an input current to an optocoupler may yield a wide range of output voltages. As a result, optocouplers used for data transmission are only used to pass single bit values (for example, a logic 0 is OFF, and a logic 1 is ON). Transmitting multi-bit data has required an optocoupler for each bit of data. 
       SUMMARY OF THE INVENTION 
       [0004]    A method of indicating multi-bit values using a single optocoupler indicates a first multi-bit value in response to a first range of optocoupler output voltages, and indicates a second multi-bit value in response to a second range of optocoupler output voltages. The first range is different from the second range. 
         [0005]    A system for indicating multi-bit values using a single optocoupler includes an optocoupler, a controller, and an analog to digital converter. The controller is operable to inject specific input currents into the optocoupler to yield an output voltage within one of a plurality of predefined ranges. The analog to digital converter is coupled to the optocoupler and is operable to indicate a multi-bit value in response to an output voltage of the optocoupler falling within one of the plurality of predefined ranges. Each of the plurality of predefined ranges is assigned a multi-bit value. 
         [0006]    These and other features of the present invention can be best understood from the following specification and drawings, the following of which is a brief description. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]      FIG. 1  schematically illustrates an example optocoupler. 
           [0008]      FIG. 2  schematically illustrates a table of example input currents and output voltage ranges corresponding to an optocoupler having a current transfer ratio of 50%-200%. 
           [0009]      FIG. 3  schematically illustrates a graph of example input currents and output voltage ranges corresponding to an optocoupler having a current transfer ratio of 50%-200%. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0010]      FIG. 1  schematically illustrates an example optocoupler  10  that includes a photo diode  12  and a photo transistor  14  that are electrically isolated from each other. A controller  16  controls a diode current I D  that flows through the photo diode  12  and causes the photo diode  12  to emit light. The emitted light causes photo transistor  14  to turn ON to allow transistor current I T  to flow, yielding output voltage V out . Resistor  18  (“R 1 ”) is coupled to the controller  16  and the photo diode  12 . Resistor  19  (“R 2 ”) is coupled to the photo transistor  14  and is coupled to an analog to digital converter  20 . The analog to digital converter  20  is operable to convert the analog output voltage V out  into a digital signal readable by a microprocessor (not shown). 
         [0011]    The magnitude of the transistor current I T  is governed by equation #1 below, and the magnitude of output voltage V out  is governed by equation #2 below. 
         [0000]        I   T   =I   D   *CTR    equation #1
 
         [0012]    where I T  is the transistor current; 
         [0013]    I D  is the diode current; and 
         [0014]    CTR is an optocoupler current transfer ratio (“CTR”) representing a ratio of the output current(“I T ”) to the input current (“I D ”). 
         [0000]        V   out   =I   T   *R   2    equation #2
 
         [0015]      FIG. 2  schematically illustrates a table  21  of example input currents  22   a - d , example output voltage ranges  24   a - d , and example binary value assignments  26  for an optocoupler having a CTR of 50%-200%, assuming R 2  is 1 kΩ. As shown in the table  21 , by injecting a specific diode current I D  into the optocoupler  10 , a specific voltage range can be achieved. By assigning a different multi-bit value  26  to each range  24   a - d , a single optocoupler  10  can be used to express a plurality of multi-bit values  26   a - d , even if the optocoupler has a widely varying CTR (e.g. 50%-200%). 
         [0016]    In the Example of  FIG. 2 , range  24   a  corresponds to multi-bit value  26   a  (“00”), range  24   b  corresponds to multi-bit value  26   b  (“01”), range  24   c  corresponds to multi-bit value  26   c  (“10”), and range  24   d  corresponds to multi-bit value  26   d  (“11”). The multi-bit values  26  can be used to indicate a state of a system, such as a lighting system. In one example multi-bit value  26   a  (“00”) corresponds to no fault, multi-bit value  26   b  (“01”) corresponds to an over-temperature fault, multi-bit value  26   c  (“10”) corresponds to an over-current fault, and multi-bit value  26   d  (“11”) corresponds to a hardware fault (e.g. damaged MOSFET). Of course, the multi-bit values  26  could be used to indicate other states, or even other pieces of information that are not states. 
         [0017]      FIG. 3  schematically illustrates a graph of example input currents and output voltage ranges corresponding to an optocoupler having a CTR of 50%-200%. In the example of the table  21  of  FIG. 2  and the graph of  FIG. 3 , each of the voltage ranges  24   a - d  are non-overlapping, and are spaced apart by a minimum voltage range spacing. In the example of the table  21 , the minimum voltage range spacing is 0.05 V. Of course the values of  FIG. 2  and graph of  FIG. 3  are only exemplary, and other current transfer ratios, current values, resistor values, and voltage separation ranges could be used. If the voltage ranges did overlap, statistical analysis techniques according to known methods could be used to determine which voltage range a given output voltage fell within. 
         [0018]    Referring to the values from table  21  of  FIG. 2 , the first current  22   a  of 0.00625 mA yields a first voltage range  24   a  of 0.003125-0.0125 V (50-200% of 0.00625 mA). The second current  22   b  of 0.125 mA amps yields a second voltage range  24   b  of 0.0625-0.25 V (50-200% of 0.125). The third current  22   c  of 0.6 mA yields a third voltage range  24   c  of 0.3-1.2 V (50-200% of 16). The fourth current  22   d  of 2.5 mA amps yields a fourth voltage range  24   d  of 1.25-5 V (50-200% of 64). 
         [0019]    Although only four voltage ranges  24   a - d  and four multi-bit values  26   a - d  have been described, it is possible that additional ranges and corresponding multi-bit values could be achieved. The maximum number of available non-overlapping ranges  24  would depends on the maximum tolerance of the CTR of a given digital optocoupler  10 , and on how discretely non-overlapping ranges can be divided across the entire current range of a given optocoupler. 
         [0020]    Although a preferred embodiment of this invention has been disclosed, a worker of ordinary skill in this art would recognize that certain modifications would come within the scope of this invention. For that reason, the following claims should be studied to determine the true scope and content of this invention.