Current sense shunt resistor circuit

Abstract

Current through a wire is sensed with a shunt resistor and a sense resistor in a current divider circuit. The values of the shunt resistor and sense resistor are related to provide a specified gain ratio to increase a dynamic range of current measurement. The sense resistor is a trimmable resistor, the configuration of which can be discerned from a look-up table based on a level of precision needed for current measurement. The two resistors can also be related by thermal coefficients to improve linearity of current measurements.

1 . A current sense circuit, comprising: 
 a shunt resistor in a current path for measuring current in the current path;    a sense resistor coupled to the shunt resistor for dividing current supplied to the shunt resistor; and    a resistance value for the sense resistor being selectable such that a ratio of the shunt resistor value to the sense resistor value produces a gain suitable for establishing a range of current measurements to cover a range of sensed current.    
   
   
       2 . The circuit according to  claim 1 , further comprising a voltage buffer disposed between the shunt resistor and the sense resistor.  
   
   
       3 . The circuit according to  claim 2 , wherein the current direction through the sense resistant is constant.  
   
   
       4 . The circuit according to  claim 2 , further comprising a biasing voltage in the voltage buffer.  
   
   
       5 . The circuit according to  claim 1 , where at least one of the shunt and sense resistors is selected to obtain a ratio of thermal coefficients for the resistors that is approximately 1.0.  
   
   
       6 . The circuit according to  claim 4 , further comprising a reference current value subtracted from a sensed current through the sense resistor to offset the bias voltage.  
   
   
       7 . The circuit according to  claim 6 , further comprising a reference current storage element coupled to the sense resistor for storing the reference current value.  
   
   
       8 . A current sense circuit, comprising: 
 a shunt resistor in a wire for measuring current through the wire;    a sense resistor coupled to the shunt resistor to provide a current divider path for the sensed current;    a switch disposed between the shunt resistor and the sense resistor for decoupling the sense resistor from the shunt resistor;    a voltage buffer coupled to the sense resistor and operable to maintain a single current direction through the sense resistor; and    the bias voltage being further operable to provide a reference current value in conjunction with the sense resistor when the switch is operated to decouple the sense resistor from the shunt resistor.    
   
   
       9 . The circuit according to  claim 8 , further comprising a reference current storage element for storing the reference current obtained when the switch is operated to decouple the sense resistor from the shunt resistor.  
   
   
       10 . The circuit according to  claim 9 , further comprising a summing element coupled to the storage element and operable to subtract the reference current value stored in the storage element from a sensed current value through the sense resistor when the switch is operated to couple the sense resistor to the shunt resistor.  
   
   
       11 . The circuit according to  claim 8 , further comprising a specified relationship between the values of the shunt resistor and the sense resistor to obtain a specified gain and dynamic range for measuring a current through the shunt resistor.  
   
   
       12 . A current sense circuit, comprising: 
 a shunt resistor connected to a wire for measuring current through the wire;    a sense resistor coupled to the shunt resistor to provide a current divider circuit;    a relationship between the shunt resistor and the sense resistor values to provide a specified gain and dynamic range for measuring current through the shunt resistor; and    a trimming mechanism coupled to the sense resistor to modify the value of the sense resistor to obtain the specified gain in relationship with the shunt resistor value.    
   
   
       13 . The circuit according to  claim 12 , wherein the trimming mechanism is a network of resistors, individual connections of which are interruptible to modify the overall resistance of the sense resistor.  
   
   
       14 . A method for sensing current in a wire, comprising: 
 providing a shunt resistor in the wire to generate a voltage related to current through the wire;    providing a sense resistor coupled to the shunt resistor to obtain a current divider circuit;    measuring current flow through the sense resistor; and    determining current flowing through the shunt resistor based on a specified relationship between the shunt resistor and the sense resistor.    
   
   
       15 . The method according to  claim 14 , further comprising establishing the relationship between the shunt resistor and the sense resistor based on a desired gain ratio between the shunt resistor value and the sense resistor value.  
   
   
       16 . The method according to  claim 14 , further comprising applying a biasing voltage between the shunt resistor and the sense resistor such that current flows through the sense resistor in one direction.  
   
   
       17 . A method for measuring current in a wire, comprising: 
 providing a shunt resistor in the wire to develop a voltage for measuring current through the wire;    providing a sense resistor coupled to the shunt resistor to obtain a current divider circuit;    providing a switch between the shunt resistor and the sense resistor to couple or decouple the sense resistor and the shunt resistor; and    obtaining a reference current through the sense resistor when the sense resistor is decoupled from the shunt resistor.    
   
   
       18 . The method according to  claim 17 , further comprising storing the reference current.  
   
   
       19 . The method according to  claim 17 , further comprising: 
 operating the switch to couple the sense resistor and the shunt resistor; and    determining a value for current flowing through the shunt resistor based on current flowing in the sense resistor and the reference current.    
   
   
       20 . A method for measuring current in a wire, comprising: 
 providing a resistance in the wire to develop a voltage when current flows through the wire;    dividing current through the wire between the resistance and a current sensor having a gain relationship with the shunt resistor; and    modifying the current sensor to change the gain relationship to establish a desired gain relationship between the current sensor and the resistance, whereby the current sensor is capable of sensing current through the resistance over an entire dynamic range of operation.    
   
   
       21 . The method according to  claim 20 , further comprising changing a resistance in the current sensor to obtain a desired gain relationship between the current sensor and the resistance.  
   
   
       22 . A method for setting a gain relationship between a shunt resistor in a wire for measuring current through the wire and a sense resistor coupled to the shunt resistor to form a current divider circuit, the method comprising: 
 passing a predetermined current through the shunt resistor and the sense resistor;    obtaining a current measure for the current flowing through the sense resistor;    determining an error percentage based on an expected value for the current measured in the sense resistor and the actually measured current through the sense resistor; and    adjusting the value of the sense resistor to reduce a difference between the expected value of the current and the actually measured current through the sense resistor.    
   
   
       23 . The method according to  claim 22 , further comprising determining an adjustment to the sense resistor based on the difference between the expected current value and the actually measured current value.  
   
   
       24 . The method according to  claim 23 , wherein determining an adjustment further comprises obtaining adjustment parameters for the sense resistor from a look-up table.  
   
   
       25 . The method according to  claim 22 , further comprising measuring current through the sense resistor a second time and comparing the measured current with an expected current value to determine an error percentage.  
   
   
       26 . The method according to  claim 25 , further comprising ensuring a difference between the second current measurement and the expected value of the second current measurement fall within specified error ranges related to the adjustment to the sense resistor.  
   
   
       27 . A method for forming a look-up table of values related to configuration of an adjustable sense resistor for use in conjunction with a current shunt resistor for measuring current in a wire, the method comprising: 
 providing values for resistors in resistor path configurations for a resistor network forming the sense resistor;    simulating selected interruptions in the resistor network to produce a percent change in overall resistance for the sense resistor;    storing the selected interruption configuration and the associated percent change; and    providing a list of the interruption configurations and the associated percent changes.