Abstract:
The present disclosure is directed to a current limiting circuit. The current limiting circuit may include a load, a first switch that controls current supplied to the load, and a first resistive network. The current limiting circuit may further include a voltage divider connected across the first resistive network and including a thermistor. The current limiting circuit may further include a first bipolar junction transistor that controls switching of the first switch. The output terminal of the voltage divider may be connected to a base junction of the first bipolar junction transistor.

Description:
TECHNICAL FIELD 
       [0001]    The present disclosure relates generally to electrical systems, and more particularly to electrical systems including a temperature compensated current limiting mechanism. 
       BACKGROUND 
       [0002]    Traditional locomotives are known to use several on-board electrical systems to drive an output or load. Similarly, other electronic devices such as portable hand-held or wireless devices include load-driving circuitry. Typically, such electrical systems and devices include mechanisms to limit the amount of current that may be supplied to the load or output circuit. One such mechanism is a current limiter, which limits the current that may be supplied to the load or output circuit. 
         [0003]    A conventional current limiting circuit  200  is illustrated in  FIG. 2  that limits the current supplied to load  201 . Circuit  200  uses the base-to-emitter voltage, or V EB , of PNP transistor Q 1  to limit the current when load  201  attempts to draw current that exceeds a predetermined limit value. As the load  201  draws increasing current, the V EB  of transistor Q 1  increases from 0 mV to 650 mV based on the voltage drop across resistor R SENSE . A V EB  of 650 mV begins to “turn on” the PNP transistor Q 1 , which increases the voltage drop across resistor R GATE . This increase in the voltage across resistor R GATE  begins to “turn off” PMOS (or p-type MOSFET) Q 2  because the gate voltage of PMOS Q 2  begins to increase. This turning off of Q 2  continues until the current limit is reached. The above configuration actively limits the load current in this manner. The arithmetic expression of the current limit value (I LIM ) at room temperature may be given by: 
         [0000]        I   LIM   =V   BE   ÷R   SENSE ≈0.650V÷ R   SENSE   (1)
 
         [0004]    Equation (1) above shows that one may select a current limit value by means of selecting an appropriate R SENSE  resistor. In  FIG. 2 , circuit  200  limits the current deliverable to load  201  at approximately 217 mA (217 mA≈650 mV÷3Ω). 
         [0005]    Circuit  200 , however, suffers from variation over temperature, since the V EB  of Q 1  depends upon temperature for a given emitter-base current. An emitter-base current that produces a V EB  of 650 mV at 25° C. produces approximately 800 mV at −40° C. and 420 mV at 150° C. That is, the threshold V EB  value at which Q 1  “turns on” increases with a decrease in temperature. It will be apparent from equation (1) that such a change in the V EB  threshold value from 650 mV at room temperature will result in a large variation in the current limit for load  201 . Specifically, the load current will limit at too high for low temperatures, and too low at high temperatures. 
         [0006]    U.S. Pat. No. 5,587,649 discloses a scheme that recognizes the variation in the V EB  threshold value for transistor Q 1  in a current limiting circuit. The &#39;649 patent suggests replacing sense resistor R SENSE  with a combination of resistors including a thermistor having a negative temperature coefficient. 
         [0007]    While the &#39;649 patent may disclose a current limiting circuit that may take into account the variation in the V EB  threshold value for transistor Q 1 , the disclosed current limiting circuit does not attempt to maintain a low variation in the current limit over a wide temperature range. Instead, the disclosed current limiting circuit assumes that the maximum current demand of load  201  changes over temperature and the current limit tracks this change in current demand over temperature. Accordingly, the disclosed current limiting circuit may not be able to provide a low variation in the current limit over a wide temperature range. 
         [0008]    Further, the disclosed current limiting circuit may not be useful for systems in which a high load current is required. This is because commercially available thermistors have a large resistance value as a result of which the combination of resistors including the thermistor will have a large effective resistance. As a result, a low load current (of the order of milli-amps) will cause a large voltage drop across the resistor combination sufficient to “turn on” transistor Q 1 , thereby limiting the load current to the milli-amps range. 
         [0009]    The presently disclosed current limiting circuit and system including the same is directed to overcoming one or more of the problems set forth above and/or other problems in the art. 
       SUMMARY 
       [0010]    In accordance with one aspect, the present disclosure is directed to a current limiting circuit. The current limiting circuit may include a load, a first switch that controls current supplied to the load, and a first resistive network. The current limiting circuit may further include a voltage divider connected across the first resistive network and including a thermistor. The current limiting circuit may further include a first bipolar junction transistor that controls switching of the first switch. The output terminal of the voltage divider may be connected to a base terminal of the first bipolar junction transistor. 
         [0011]    According to another aspect, the present disclosure is directed to a locomotive. The locomotive may include a first electrical module and a second electrical module driven by the first electrical module. The second electrical module may include a load, a first switch that controls current supplied to the load, and a first resistive network. The second electrical module may further include a voltage divider connected across the first resistive network and including a thermistor. The second electrical module may further include a first bipolar junction transistor that controls switching of the first switch. The output terminal of the voltage divider may be connected to a base terminal of the first bipolar junction transistor. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]      FIG. 1  illustrates a pictorial view of an exemplary consist of two locomotives. 
           [0013]      FIG. 2  illustrates a conventional current limiting circuit. 
           [0014]      FIG. 3  illustrates an exemplary temperature compensated current limiting circuit that may be provided in an electrical system of the locomotive of  FIG. 1 . 
           [0015]      FIG. 4  illustrates a difference in variation in the current limit between the circuit of  FIGS. 2 and 3 . 
           [0016]      FIG. 5  illustrates an exemplary temperature compensated current limiting circuit that may be provided in an electrical system of the locomotive of  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION 
       [0017]      FIG. 1  illustrates a consist  100  comprising a plurality of locomotives  120 , the plurality including at least a first and a last locomotive  120 . Each locomotive  120  may include a locomotive engine  140 . In one embodiment, locomotive engine  140  may comprise a uniflow two-stroke diesel engine system. Those skilled in the art will also appreciate that each locomotive  120  may also, for example, include an operator cab (not shown), facilities used to house electronics, such as electronics lockers (not shown), protective housings for locomotive engine  140  (not shown), and a generator used in conjunction with locomotive engine  140  (not shown). 
         [0018]    While not shown in  FIG. 1 , consist  100  may comprise more than two locomotives  120 . Additionally, consist  100  may also comprise a variety of other railroad cars, such as freight cars or passenger cars, and may employ different arrangements of the cars and locomotives to suit the particular use of consist  100 . In an embodiment, the locomotives within consist  100  communicate with each other through, for example, wired or wireless connections between the locomotives. Particular examples of such connections may include, but are not limited to, a wired Ethernet network connection, a wireless network connection, a wireless radio connection, a wired serial or parallel data communication connection, or other such general communication pathway that operatively links control and communication systems on-board respective locomotives of a consist. 
         [0019]      FIG. 3  illustrates an exemplary temperature compensated current limiting circuit  300  limiting the current supplied to load  301 . Circuit  300  operates from a power supply of 12V. It will be apparent, however, that the power supply value can be any other value, which is large enough to properly bias the components to the right of PNP transistor Q 1 . For example, the power supply could be 2.5V, 3.5V, 5.5V, etc. 
         [0020]    Like circuit  200 , circuit  300  also includes PMOS Q 2 , capacitors C 1 , C 2 , and resistors R GATE  and R SENSE . While the embodiment discloses capacitors C 1  and C 2  as having a value of 10 μF, it will be understood that any suitable value may be used for capacitors C 1  and C 2 . Similarly, the value of R GATE  as 15 kohms is arbitrary and any other suitable value may be used. The value of R SENSE  is also arbitrary (here 3 ohms) and can be adjusted based on the desired current limit. 
         [0021]    Compared to circuit  200 , circuit  300  may include a voltage divider formed by resistor R 1 , negative temperature coefficient thermistor TH 1 , and resistor R 2 . One end of the voltage divider may be connected to one end of R SENSE  and the other end of the voltage divider may be connected to the other end of R SENSE . Exemplarily, R 1  and R 2  may have a value of 49.9K and thermistor TH 1  may have a resistance value of 100K at room temperature. It will be understood that these values are only exemplary, and that R 1 , R 2 , and TH 1  may take on other values. The base terminal of the PNP transistor Q 1  may be connected to the output of the voltage divider such that thermistor TH 1  is connected between the base and emitter terminals of PNP transistor Q 1 . Next, the temperature compensation aspect of circuit  300  will be explained. 
         [0022]    As discussed in the background section, the threshold V EB  value for transistor Q 1  increases with a decrease in temperature and decreases with an increase in temperature. As a result, for example, when the temperature increases, a lower load current will be sufficient to create a voltage drop across R SENSE  that is enough to “turn on” transistor Q 1 . In circuit  300 , since thermistor TH 1  has a negative temperature coefficient, TH 1 &#39;s resistance value decreases with an increase in temperature. Accordingly, the decrease in the V EB  threshold is compensated by a decrease in the output voltage of the voltage divider, where the output voltage of the voltage divider equals V EB . Similarly, if the temperature decreases, the resistance of TH 1  increases to compensate for an increase in the V EB  threshold for transistor Q 1 . 
         [0023]      FIG. 4  illustrates the variation in current limit between the conventional circuit  200  and the disclosed exemplary circuit  300  across a wide range of temperatures. Line  401  is the variation of the current limit for the conventional circuit  200  and line  402  is the variation of the current limit for the disclosed circuit  300 . As can be seen from  FIG. 4 , the variation in the current limit from the room temperature current limit decreased to about ±10% overall for circuit  300  compared to roughly +23% at −40° C. and −35% at 120° C. for circuit  200 . The graphical illustration of the current limit variation employed the Murata part number NCP15WF104F03RC for the negative temperature coefficient thermistor TH 1 . 
         [0024]    Various modifications can be made to circuit  300 . For example, in a functionally equivalent circuit  300 , the p-type transistor Q 1  could be replaced by an n-type transistor Q 1 . Similarly, the p-type MOSFET Q 2  may be replaced by an n-type MOSFET Q 2 . Moreover, Q 1  may be replaced by a comparator circuit. 
         [0025]      FIG. 5  illustrates another exemplary temperature compensated current limiting circuit  500  limiting the current supplied to load  201 . Like the circuit in  FIG. 3 , a voltage divider formed by a negative temperature coefficient resistor (TH 1 ) with resistances R 1  and R 2  drives the base of transistor Q 1 . An increase or decrease in resistance of TH 1  with a change in temperature compensates for the corresponding change in V EB  threshold for transistor Q 1 . Further, MOSFET Q 2  in circuit  300  has been replaced with a bipolar junction transistor Q 2 . 
         [0026]    Additionally, resistor R GATE  in circuit  300  has been replaced with resistor R BIAS  which is connected with the base and collector junctions of transistor Q 2 . It will be apparent to a skilled artisan that circuit  500  could also serve its purpose if resistor R GATE  was provided in circuit  500  like in circuit  300 . 
       INDUSTRIAL APPLICABILITY 
       [0027]    The disclosed current limiting circuit may provide a low variation in the current limit over a wide temperature range. By providing a negative temperature coefficient thermistor in the base drive of transistor Q 1 , the temperature variance of the threshold V EB  value can be compensated. Moreover, the disclosed current limiting circuit may have an advantage over conventional circuits in that the current limiting circuit  300  may be operable for both small and large load currents. This operation over a wide load current range is made possible by the provision of two separate paths to transistor Q 2 —a first low resistance path through R SENSE  and a second high resistance path through the voltage divider. Most of the load current will flow through R SENSE , whose value can be adjusted based on the desired load current operating range. 
         [0028]    It will also be understood that circuit  300  can be employed in an electrical module of locomotive  100  that is being driven by another electrical module of locomotive  100 . Moreover, it will be apparent that circuit  300  may be utilized in any electrical system where a current limiting mechanism is desired. For example, circuit  300  may be utilized in a battery pack. Circuit  300  may be utilized, for example, in a handheld device where a current limiting mechanism is desired. 
         [0029]    It will be apparent to those skilled in the art that various modifications and variations can be made to the disclosed current limiting circuit. Other embodiments will be apparent to those skilled in the art from consideration of the specification and practice of the disclosed circuit. It is intended that the specification and examples be considered as exemplary only, with a true scope being indicated by the following claims and their equivalents.