Abstract:
An apparatus and method for thermally compensating a voltage signal for a circuit protection device. The circuit protection device is coupled to a powered circuit and having current flowing through it. A bus carries the power therethrough. A sense resistor is electrically coupled to a bus for sensing the current flowing through the bus. A temperature sensitive circuit is coupled to the sense resistor for compensating ambient temperatures. The invention is useful for providing an accurate sense resistor that supplies a temperature independent current in a power line and the related voltage output of the sense resistor at varying ambient temperatures.

Description:
FIELD OF INVENTION 
     The present invention relates to an apparatus and method for sensing and translating a current signal into a temperature compensated voltage signal used by an Arc Fault Circuit Interrupter (AFCI) detection circuit 
     BACKGROUND 
     In general, several different types of sensing apparatus have been used to measure current in an electrical circuit. Such sensing apparatus may utilize sense resistors, current transformers, and Hall Effect sensors. These sensors are used to monitor and measure current as it is passes through the load of a circuit either for purposes of overcurrent protection or to control another circuit based on the measured current in the load. 
     Sensors are also used to detect wave forms and the shape of current which can indicate the presence of an arc fault. Arcing faults may be defined as the existence of a current path between two ends of a broken conductor located within an ionized gas, between two conductors supplying a load, or between a conductor and a ground. Arcing faults are characterized by low and erratic current flow. Arcing faults may be undetected by standard circuit breakers, because the current flow may be below the breaker&#39;s tripping threshold. Upon occurrence of an arcing fault, branch or load impedance may cause the current levels to be reduced to a level below the trip curve setting of the circuit breaker, causing the arcing fault condition to be undetected by a circuit breaker. In addition, an arcing fault which does not contact a grounded conductor or other grounded point will not trip a ground fault protected circuit. 
     During the current measurement, it is important to maintain galvanic isolation in order to assure that current does not flow directly between the load and the measuring circuit. Sense resistors, typically, are accurate, inexpensive, and provide a higher frequency response than the other two types of sensors mentioned above. However, a disadvantage of the sense resistors is that they provide no galvanic isolation. Another disadvantage is that the sensor output signal tends to vary with temperature variations. This is due to the composition of the sensing material used. Metals have an inherent property of changing resistance when ambient temperatures fluctuate. By way of example, the proportionality of a signal being converted at 25 degrees Celsius will be different from the same signal being converted at 66 degrees Celsius or −35 degrees Celsius. Similarly, the sensitivity of the circuit changes if there is a deviation from a nominal operational temperature of 25 degrees Celsius. Known sense resistor configurations do not compensate the current for this variance in ambient temperature. Therefore, a need exists to compensate for temperature induced signal drift. 
     On the other hand, both current transformers and Hall Effect sensors provide galvanic isolation. Current transformers also provide proportional signals with suitable accuracy at a wide operational temperature range. However, both current transformers and Hall Effect sensors are more expensive solutions. Further, the frequency content for current transformer and Hall Effect sensors will be lost or will be filtered out because they have a low pass filter type transfer function. A disadvantage of a low pass filter type transfer function is that the critical information of the frequency content provided at higher frequencies will be missed. A further disadvantage of current transformers is that they occupy a relatively large area of a printed circuit board. 
     SUMMARY OF INVENTION 
     It is therefore an object of the present invention to provide an improved sense resistor apparatus for supplying a valid current signal to an arc fault circuit interrupter detection circuit at varying ambient temperatures. 
     It is also an object of the present invention to provide an improved sense resistor that is inexpensive, accurate, responds at high frequencies, and imparts galvanic isolation at varying ambient temperatures. 
     In accordance with one aspect of this invention, a temperature compensated current sensor for a circuit protection apparatus comprises: a circuit protection device for coupling to a powered circuit having current flowing therein; a bus for carrying the power therethrough; a sensing resistor electrically coupled to the bus for sensing current flow through the bus; temperature sensitive compensation circuit coupled to the sense resistor for compensating ambient temperature; and an output for reading the current. 
     In accordance with another aspect of this invention, a sense resistor apparatus for providing a temperature independent current signal at varying ambient temperatures, comprises: a sense resistor for sensing a current passed through the sense resistor and generating a voltage signal; and at least one thermistor for thermally compensating the voltage signal generated through the sense resistor. 
     In accordance with another aspect of this invention an apparatus for thermally compensating a voltage signal for an AFCI circuit, comprises: a sense resistor for sensing a current passed through the sense resistor and generating the voltage signal; at least one thermistor for thermally compensating the voltage signal generated through the sense resistor; and an operational amplifier for conditioning a thermally compensated voltage signal before the thermally compensated voltage signal enters a detection circuit of an arc fault circuit interrupter device. 
     In accordance with another aspect of this invention, a method for translating a current signal into a temperature compensated voltage signal for an AFCI circuit, comprise: generating a voltage signal by passing the current signal through a sense resistor; applying the voltage signal through at least one thermistor to generate a thermally proportional voltage signal; amplifying the thermally proportional voltage signal by energizing an operational amplification circuit; and determining whether a detection circuit of an arc fault circuit interrupter device detects the thermally proportional voltage signal. 
     In accordance with another aspect of this invention, a method for thermally compensating a voltage signal, comprises generating the voltage signal by passing a current signal through a sense resistor; and applying the voltage signal through at least one thermistor to generate a thermally proportional voltage signal. 
     In accordance with another aspect of this invention, a method for thermally compensating a current sensor for a circuit protection apparatus comprises coupling a circuit protection device to a powered circuit having current flowing therein; coupling a bus for carrying the power therethrough; electrically coupling a sensing resistor to the bus for sensing current flow through the bus; coupling a temperature sensitive compensation circuit to the sense resistor for compensating ambient temperature; and reading an output of the current. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1   a ) is a side view of the sense resistor incorporated into a powered circuit. 
         FIG. 1   b ) is a perspective view of the sense resistor incorporated into a powered circuit. 
         FIG. 2  is a schematic circuit diagram of the sense resistor with a Positive Temperature Coefficient (PTC) thermistor configuration 
         FIG. 3  is a schematic circuit diagram of the sense resistor with a Negative Temperature Coefficient (NTC) thermistor configuration 
         FIG. 4  is a schematic circuit diagram of the sense resistor with a Positive Temperature Coefficient (PTC) thermistor configuration &amp; Negative Temperature Coefficient (NTC) thermistor configuration 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1   a ) refers to the components incorporated in a powered circuit  10  comprising a printed circuit board  100  comprising push to test switches  110 , power clip  120 , solenoid  130 , part of neutral trace  140 , differential sensor  150 , line terminal  160 , sense resistor  170 , neutral pigtail  180 , neutral lug  185 , and contact  190 . Power clip  120  connects printed circuit board  100  to a voltage source. 
     Current is sensed when a proportional voltage signal is generated by the passage of current through the sense resistor  170 . By switching on a load, a current is first sent through the partial of neutral trace  140  to the differential sensor  150 . The differential sensor  150  is used to detect hot to neutral situational arcs to safeguard ground wire devices. Solenoid  130  comprises a plunger  135  that extends upon reaction and detection of a hot to neutral situational arc by differential sensor  150 . From the differential sensor  150 , the current flows to the sense resistor  170  and then out through neutral pigtail  180  which is crimped to the neutral boss on a load center to complete the circuit and path of current with load center. The neutral pigtail  180  may also be used to attach a circuit breaker, such as an Application Specific Integrated Circuit (ASIC) detection circuit of an arc fault interrupter device. The sense resistor  170  is part of the current path of the ASIC detection circuit of an arc fault circuit interrupter device. 
     Referring to  FIGS. 1   a  and  1   b  in conjunction with  FIG. 2 , first  220  and second  221  Positive Temperature Coefficient (PTC) thermistors are connected to pins  14  and  15 , respectively of ASIC detection circuit  240 . A PTC thermistor  220  is a thermally sensitive semiconductor resistor that is well known in the art whose primary function is to exhibit a change in electrical resistance with a change in body temperature. One ordinarily skilled in the art may use a PTC thermistor  220  constructed from ceramic materials and linear in nature. A nominal PTC thermistor  220  measures 10 k Ohm of resistance at 25 degrees Celsius. Change in the resistance of a PTC thermistor  220  can be brought about either by a change in the ambient temperature or internally by self heating resulting from current flowing through the device. At room temperature, the resistance of PTC thermistor  220  is almost zero. Sense resistor  170  is connected between the first  220  and second  221  PTC thermistors. Line terminal  160  serves to connect a stab assembly used in the load center to the differential sensor  150 . 
     In accordance with the present invention, the resistance value of the sense resistor is selected such that the voltage generated will be sufficiently large enough to be used by the ASIC detection circuit  240  and small enough to not apply detrimental electric voltage. By way of example, the resistance value may be approximately 250 micro Ohms, although it is understood that other suitable resistance values may be selected. 
     Referring to  FIG. 2 , the preferred embodiment, the current flows from load neutral  200  through the sense resistor  170  (point  1  to point  2 ) and then to the neutral pigtail  180  (also considered earth). Voltage is produced as the current flows through the sense resistor  170 . This voltage potential is attenuated by the first  220  and the second  221  PTC thermistor. In the case of an overcurrent situation, resistance rises within the first  220  and the second  221  PTC thermistor. This additional resistance in the circuit has the effect of reducing the overall current. Once the overcurrent situation has been removed, the first  220  and the second  221  PTC thermistor will cool, in doing so their internal temperature drops resulting in the resistance returning to a low state. The first  220  and the second  221  PTC thermistor can be formulated to have switching temperatures as low as 0 degrees Celsius to well over 200 Celsius. The ambient temperature will force the first  220  and the second  221  PTC thermistor&#39;s resistive properties to change. If the ambient temperatures are above 25 degrees Celsius, then the resistive properties of the first  220  and the second  221  PTC thermistor will increase automatically as the first  220  and the second  221  PTC thermistor are passive electrical devices. Alternatively, if the ambient temperature is below 25 degrees C., then the resistive properties of the first  220  and the second  221  PTC thermistor will decrease to a low state. See the chart in  FIG. 2B . 
     Therefore, the first  220  and the second  221  PTC thermistor is directly proportional to temperature and has a different resistance value based on different ambient temperatures. The first  220  and the second  221  PTC thermistor dynamically compensates the voltage signal proportion to the changes in resistance of the metal in the first  220  and the second  221  PTC thermistor due to the exposure to ambient temperatures above and below and below 25 degrees Celsius. As the ambient temperature increases, then the resistive properties of the first  220  and the second  221  PTC thermistor will also increase. Also, if the ambient temperature increases, the voltage signal generated from the sense resistor  170  will increase. Prior to the voltage signal entering the ASIC detection circuit  240  of the AFCI device, the first  220  and the second  221  PTC thermistor will compensate for this change in voltage signal the following way:
 
 V   OUTPUT   =V   INPUT ×( R   FIXED   /R   PTC )
 
     V OUTPUT  is the thermally compensated voltage output. R PTC  is the resistance of the first  220  and the second  221  PTC thermistor at the specific ambient temperature. R FIXED  is the resistance of an off the shelf resistor that has a constant resistance value at a 25 degree Celsius ambient temperature nominally valued at 300 k Ohms. V INPUT  is the voltage generated as current passes through the sense resistor  170  at ambient temperatures. The thermally compensated voltage signal is equal to the ratio of the resistance of a first  220  and a second  221  PTC thermistor at 25 degrees Celsius divided by the resistance of a first  220  and a second  221  PTC thermistor at the ambient temperature as applied to the voltage signal generated by the sense resistor  170  at ambient temperatures. Therefore, the change in temperature for the voltage output is linearly inverse to that of the sense resistor  170  in order to mitigate any effects due to temperature change. 
     Once the voltage signal is attenuated, the output is then passed through an operational amplifier  230  for final conditioning. The operational amplifier  230 , located inside the ASIC circuit  240 , connects in a negative feedback configuration whereby there is high impedance and therefore negligible current. This negative feedback configuration allows the amplification of the voltage signal. The operational amplifier  230  is powered by 1016 VDC which is a 10 V DC source feeding the ASIC circuitry  240  and the ASIC circuitry  240  regulates the voltage signal to all the internal components. The output of the operational amplifier  230  is sent back to lead  13 . The amplification occurs because the voltage signal coming from the sense resistor  170  is a low voltage value and requires a higher voltage value for detection by the ASIC circuit  240 . The ASIC circuit  240  is responsible for detecting the arc. The ASIC circuit  240  is assigned to pick up the particular singularities or features of the current waveform sensed by the sense resistor  170 . The only way to observe an arc fault is if there is a current flowing through the powered AC line. 
       FIG. 3  shows another embodiment using one NTC thermistor  250  in contrast to the first  220  and the second  221  PTC thermistor. The NTC thermistor  250  would replace a negative feedback resistor with the condition that the operational amplifier  230  does not saturate. The Negative Temperature Coefficient (NTC) thermistor  250  is a thermistor whose zero-power resistance decreases with an increase in temperature. Therefore, a NTC thermistor  250  is inversely proportional to temperature increase and will compensate the voltage signal as follows:
   V   OUTPUT   =V   INPUT ×( R   NTC   /R   FIXED ) 
where the value of the thermally compensated voltage signal is equal to the ratio of the resistance of the NTC thermistor  250  at the ambient temperature divided by the resistance of the NTC thermistor  250  at a normal 25 degrees Celsius ambient temperature applied to the voltage generated through the sense resistor  170 . V OUTPUT  is the thermally compensated voltage output. R NTC  is the resistance of the NTC thermistor at the specific ambient temperature. R FIXED  is the resistance of an off the shelf resistor that has a constant resistance value at a 25 degree Celsius ambient temperature. V INPUT  is the voltage generated as current passes through the sense resistor  170  at ambient temperatures.
 
       FIG. 4  shows another embodiment using the configuration two PTC thermistors  220  and one NTC thermistor  250  simultaneously. In this embodiment, the sense resistor  170  will be able to measure the more dramatic changes in the electrical system due to varying ambient temperatures. There are situations where the first  220  and the second  221  PTC thermistor do not provide adequate compensation for the AC current in the power line and the voltage output of the sense resistor  170  at varying ambient temperatures. In this embodiment, the configuration of the first  220  and the second  221  PTC thermistor and the NTC thermistor  250  in combination with the sense resistor  170  allows for additional compensability. 
     While the present invention has been disclosed with reference to certain embodiments, numerous modifications, alterations, and changes to the described embodiment are possible without departing from the sphere and scope of the present invention. Accordingly, it is intended that the present invention not be limited to the described embodiments and equivalents thereof.