Patent Publication Number: US-2009224186-A1

Title: Dual opto-coupler optical isolation circuit

Description:
The application claims priority to U.S. Provisional Application No. 61/033,923 which was filed on Mar. 5, 2008. 
    
    
     BACKGROUND OF THE INVENTION 
     The present application is directed toward an isolator circuit for isolating a high voltage circuit from a low voltage control circuit. 
     Frequently in industrial applications, a high voltage or high current system must be monitored to ensure that the electrical power properties of the system meet select criteria, such as remaining within a voltage range, or remaining within a current range. Such systems frequently have power variations and fluctuations, such as transients, which can potentially damage sensor equipment and controllers. 
     One solution to problems caused by transients, which is recognized in industry, is gap isolation of the controller via opto-couplers, inductance couplers, capacitor couplers, or other gap isolation circuits. By way of example, an opto-coupler provides a circuit which converts an electrical signal to an optical signal, and reconverts the signal back to an electrical signal. The optical connection isolates the load from the controller for reasons such as safety, while still allowing the signal to be transmitted. Other gap isolators operate similarly with a different type of signal being transmitted across the gap. (IE an inductance coupler will convert the signal to inductance and then back into an analog electrical signal instead of using optical signal.) While such an arrangement addresses the potential problems caused by a high voltage load in direct connection with a controller, it can give rise to new problems due to a scaling factor present in all gap isolators. 
     The scaling factor of an opto-coupler is the factor by which the analog signal is modified, and results from the conversion from an electrical signal to an optical signal and the reconversion from an optical signal to an electrical signal. The scaling factor in opto-couplers, as well as in other gap isolators, can vary significantly between batches due to variations in the manufacturing process, even from the same manufacturing line. 
     As a result of the scaling factor, the output of the opto-coupler is scaled to a different magnitude than the input, while still retaining the signal characteristics of the input signal. Currently, in order to compensate for the signal scaling effect described above, each individual device must be calibrated to determine the magnitude of the scaling, and a compensating circuit must then be used to achieve the desired measurement accuracy. Individual calibration requires a significant time investment as well as raises costs associated with production. Similar problems arise when other forms of gap isolators such as capacitor isolators, inductance isolators, etc. are used. 
     SUMMARY OF THE INVENTION 
     Disclosed is an electrical control circuit. The control circuit has a sensor on a load and at least a first and a second gap isolator. The sensor detects the electrical properties of the load. The sensor signal is passed through the first gap isolator, which has a certain scaling factor. The signal is then sent through a second gap isolator which provides a scaling factor which is nearly the same scaling factor as in the first gap isolator and outputs an analog signal representing the sensor signal to a controller. 
     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 
         FIG. 1  illustrates a schematic diagram of an embodiment of the gap isolation circuit. 
         FIG. 2   a  illustrates a first opto-coupler schematic such as would be used the embodiment of  FIG. 1 . 
         FIG. 2   b  illustrates a second opto-coupler schematic such as would be used in the embodiment of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Utilizing a gap isolator for isolating a control circuit from a high voltage source results in signal scaling where the magnitude of the signal is altered and other signal characteristics are retained. When it is desirable to detect a power characteristic which would be altered by the gap isolator scaling, such as current magnitude, it is necessary to devise a way to compensate for the variability in scaling factors. One method which can be used to compensate for scaling variability is to utilize a first and a second gap isolator with similar scaling factors and combine them to cancel out the effect of the scaling factor. An example of the above described circuit using opto-couplers is illustrated in  FIG. 1 . 
     In the example of  FIG. 1  an optical isolation circuit has a current sensor  100 , a first opto-coupler  200 , a second opto-coupler  300 , and an operational amplifier (op-amp)  400 . The current sensor  100  can be, alternately, any sensor with which it would be desirable to isolate the load being sensed from the control circuit, such as a high voltage circuit and a low voltage user interface control circuit. 
     The current sensor  100  is connected to an input  202  of the first opto-coupler  200 . The first opto-coupler  200  has an output  204  which connects to an op-amp  400  at an input  402 . The op-amp  400  has an output  404  which is connected to an input (anode)  304  of a photo-diode (pictured in  FIG. 2 ) of the second opto-coupler  300 . The op-amp  400  additionally has an input  406  connected to the emitter of a photo-transistor of the second opto-coupler  300  (pictured in  FIG. 2 ). The second opto-coupler  300  additionally has a cathode  302  where an analog signal, which has been scaled in the first opto-coupler  200  and replicated in the second opto-coupler  300 , is sensed. 
     The sensor  100  has two connections  102 ,  104  to a load  500 . The sensor  100  detects electrical properties of the load  500 , such as current, frequency, voltage, etc., and outputs an analog signal at a sensor output  106 . The analog signal represents the load  500  characteristic(s) being measured by the sensor  100  and additionally reflects any transients or other power fluctuations which occur in the load  500 . 
     The analog output  106  is connected to an opto-coupler  200  which accepts the analog signal at an input (anode)  202 , converts the signal to an optical signal, transmits the optical signal across an air gap, and then reconverts the signal into an analog electrical signal which is output at the opto-coupler output (emitter)  204 . 
     The opto-coupler output  204  of the first opto-coupler  200  is connected to an op-amp  400  input  402 . The op-amp  400  then conditions the signal, to force the voltage of the transistor emitter (pictured in  FIG. 2 ) of the first opto-coupler  200  to be the same as the voltage of the transistor emitter of the second opto-coupler  300 . Methods and circuits for performing this conditioning using an op-amp are known in the art. 
     Once the conditioning has been performed, the signal is input into the second opto-coupler  300  at input (anode)  304 . The second opto-coupler  300  is connected such that the second opto-coupler  300  scales the signal the same as the first opto-coupler  200  scaling. By way of example if both of the opto-couplers  200 ,  300  had a scaling factor of X, the first opto-coupler  200  would multiply the signal magnitude by X, and the second opto-coupler  300  would multiply the signal magnitude by X, resulting in a signal at the cathode  302  of the second opto coupler which accurately represents the sensed input to opto-coupler  100 , while not retaining modifications to the original signal input  202  caused by a single opto-coupler isolator. The scaling factor can be any value, such as a function f(n). As a result of the first scaling factor and the second scaling factor being the same or nearly the same, the final system output of the dual opto-coupler system, as measured at  302 , is an analog signal which accurately represents the desired characteristic of the load being monitored. 
     In order to achieve proper replication in the above circuit it is necessary for both of the opto-couplers  200 ,  300  to have similar scaling values. The amount of variance between the scaling values which is allowable depends on the particular application, with a greater need for sensor precision requiring a lesser variance between the opto-coupler scaling values. One way to solve this problem is to utilize opto-couplers from the same batch. In this way, any impurities, or deviations in the manufacturing process which are present in one of the opto-couplers will additionally be present in the other. Another way would be to calibrate each opto-coupler and pair it with a second opto-coupler with a scaling value within the desired tolerance. Additionally, the opto-couplers can be created as independent components or as part of a circuit on one integrated circuit package which would help ensure similar scaling factors for each opto-coupler for the same reasons as using opto-couplers from a single batch. 
     Example opto-couplers  200 ,  300 , which can be used in the example of  FIG. 1 , are shown in  FIGS. 2   a  and  2   b.  The opto-coupler  200  illustrated in  FIG. 2   a  utilizes a photo-diode  210  and a photo-transistor  232 . The sensor signal enters the opto-coupler  200  at the photo-diode  210  through an input  202 . The photo diode  210  emits an optical signal across the light gap  250 , which is received by the photo-transistor  232 . The photo-signal switches the photo-transistor  232  on, which allows current flow from a collector  212  to an opto-coupler emitter  230 . In the example of  FIGS. 1 and 2   a  the opto-coupler emitter  230  is connected to the opto-coupler output  204 . The opto-coupler  200  additionally connects the photo-diode  210  to ground through an output  220 , and connects the photo-transistor  232  to a voltage source through collector  212 .  FIG. 2   b  illustrates a schematic of the second opto-coupler  300 . In order to replicate the scaling of the opto-coupler, as described above, the connections are made in a different manner, such that the cathode  302  of the photo-diode  310  will accurately represent the analog signal input from the sensor into the dual opto-coupler at input  202 . In the schematic illustrated in  FIG. 2   b,  the collector input  322  for the photo-transistor  312  is again connected to a voltage source. The opto-coupler emitter  330  is, however, connected to an input of the op-amp  400 . The output  404  (pictured in  FIG. 1 ) of the op amp  400  is connected to an input (anode)  304  of the photo-diode  310 . This configuration places the second opto-coupler  300  in a feedback path of the op amp  400 . 
     Placing the second opto-coupler in the feedback path allows the current flowing through the photo-transistor to be the same. Since the second opto-coupler  300  has the same or nearly the same scaling factor as the first opto-coupler, the op-amp forces the two inputs of the first and second opto-coupler to have the same voltage level. Therefore, the current that flows through the photo-transistor of the first opto-coupler is the same as the current that flows through the photo-transistor of the second opto-coupler, and the input current on the high voltage side is duplicated on the low voltage side. 
     It is known that any other circuit configuration whereby the first opto-coupler  200  and the second opto-coupler  300  have the same scaling factor will function as described above, and will meet the elements of the present application. It is additionally known that a similar circuit can be constructed using any other type of gap isolators, instead of opto-couplers, and still fall within the above disclosure. 
     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.