Abstract:
The application of electric power from a power source to a load is controlled by detecting the magnitude of electric current flowing to the load and producing a first signal level that is indicative of that magnitude. The first signal level is compared to a reference signal level which comparison produces an output signal. Either the first signal level or the reference signal level is altered in response to variation of voltage applied to the load. This results in the output signal indicating when electric power consumed by the load exceeds a threshold level. The flow of electric current from the power source to the load then is controlled in response to the output signal.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   Not Applicable 
   STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
   Not Applicable 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to circuits which control supplying electricity to a load, and more particularly to circuits which limit the amount of power consumed by a load. 
   2. Description of the Related Art 
   Many types of electrical apparatus are powered by direct current. The direct current may be derived from a battery or from an alternating to direct current power supply. For example, the power supply converts 120 volt or 250 volt alternating current from a utility company into a desired lower DC voltage level compatible with the electronic circuitry. 
   The voltage potential of these current sources can vary in magnitude for a number of reasons. For instance, the output voltage level of an AC to DC power supply can fluctuate with variation of the AC voltage into the power supply. The output voltage also can vary due to changes in the amount of current being drawn by loads attached to the power supply. The output voltage from a battery decreases as the charge on the battery depletes and can increase while the battery is recharging. 
   It often is beneficial to limit the amount of electric current that is allowed to enter or flow through electrical equipment. Limiting the amount of electric current to below a defined level can prevent damage to the equipment. Such current limiting also may prevent a failure of one component from causing damage to other components. 
   Current limiting circuits are well known and commonly detect voltage across a sensing resistor through which all or a defined portion of the electric current to a load flows. The level of the sensed voltage is in proportion to the magnitude of the electric current. That sense voltage level is compared to a reference voltage level that corresponds to the desired current limit. The result of that comparison can be used to control the flow of current to the load. For example, if the reference voltage level is exceeded, indicating an excessively high current flowing to the load, a disconnect device can be activated to terminate that current flow and prevent damage to the load. 
   Under certain circumstances it is desirable to limit the input power to a circuit to less than a prescribed wattage level prescribed. For example, equipment can be designed with greater latitude for clearances between conductors and other parameters of the circuit layout, when the equipment draws less then 200 volt-amperes. Conventional design standards are more stringent for equipment that may consume more electrical power. Therefore, it is desirable to ensure that this power level is not exceeded so that a less expensive and complicated circuit layout can be utilized in the equipment. 
   However, merely regulating the voltage or current applied to the apparatus does not ensure that the circuitry will not draw in excess of the desired power limit. For example, if the apparatus is nominally powered at 15 volts and consumes 150 watts of power, its input current is 10 amps. A power supply that limits the current level to 10 amps could allow the apparatus to consume more than a 150 watts when the input voltage rises above 15 volts. In this example if the supply voltage is greater than 20 volts, a design guideline of 200 volt-amperes is exceeded. As noted previously, such supply voltage variation is not uncommon, especially among battery powered equipment, which can vary significantly depending upon the charge level of the battery. For example, it is not unusual for a battery voltage to be relatively high when a battery charger is active and then decrease to about half that voltage level when battery is the discharged. 
   Therefore, it is desirable to provide a reliable, inexpensive circuit which limits the power consumption of an electrical load to less than a predefined level. 
   SUMMARY OF THE INVENTION 
   An apparatus is provided to perform a method which controls the application of electric power from a power source to a load. That apparatus comprises a current sensing circuit which detects the magnitude of electric current flowing to the load and produces a first signal level indicating that magnitude. A comparator is connected to the current sensing circuit and has a first input to which the first signal level is applied, a second input to which a reference signal level is applied. The comparator produces an output signal in response to comparing the first signal level and the reference signal level. 
   A circuit branch is connected to the comparator and alters either the first signal level or the reference signal level in response to variation of voltage applied to the load. A first embodiment of the apparatus provides a circuit branch that alters the first signal level and a second embodiment incorporates a different circuit branch which alters the reference signal level. Such alteration results in the output signal indicating when electric power consumed by the load exceeds a threshold level. A device that is connected to the comparator output, controls the flow of electric current from the power source to the load in response to the output signal. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic diagram of a first embodiment of a circuit for limiting the input power to a load; and 
       FIG. 2  is a schematic diagram of a second embodiment of a circuit for limiting the input power to a load. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   With initial reference to  FIG. 1 , a first current limiting circuit  10  for a power supply has a pair of input terminals  12  and  14  across which a DC voltage is applied. The first input terminal  12  receives the positive voltage line, while the second input terminal  14  is connected to the circuit ground. A current sensing resistor  16  is connected between the first input terminal  12  and a disconnect device  18 , which may comprise a semiconductor switch, an electromechanical relay, contactor or similar switching device. The disconnect device controls the flow of current to a first output terminal  20 . A second output terminal  22  is connected to circuit ground via the second input terminal. The load  24  being powered is connected across the two output terminals  20  and  22 . 
   An operational amplifier  26  has an inverting input coupled to the first input terminal  12  via an input resistor  28 . The non-inverting input terminal of the operational amplifier  26  is connected directly to a supply line  30  between the current sensing resistor  16  and the disconnect device  18 . Thus the inputs of the operational amplifier  26  are coupled to opposites sides of the current sensing resistor  16 . The signal at the output of the operational amplifier  26  corresponds to the voltage across the current sensing resistor  16  and thus, is proportional to the current flowing through that resistor to the load  24 . The current sensing resistor  16 , input resistor  28  and the operational amplifier  26  comprise a current sensing circuit  25 . 
   The output of the operational amplifier  26  is connected to the base, or control, electrode of a transistor  32  which also is part of the current sensing circuit  25 . The conduction path of the transistor  32  is connected between the inverting input of the operational amplifier  26  and an intermediate node  34 . Specifically, the transistor  32  is a PNP type with an emitter connected to the inverting input of the operational amplifier  26  and a collector connected to the intermediate node  34 . The intermediate node  34  is coupled to the supply line by a power limit resistor  36  and to the circuit ground by a bias resistor  38 . 
   The intermediate node  34  also is connected directly to the non-inverting input of a comparator  40  which has an inverting input connected to a source of a constant reference voltage V REF . The output of the comparator  40  is connected to the control input of the disconnect device  18 , thereby controlling the operation of that latter device. 
   During the operation of the first current limiting circuit  10 , the electric current flowing through the current sensing resistor  16  produces a voltage across that device that is proportional to the magnitude of that electric current. That voltage difference produces a voltage level at the output of the operational amplifier  26  indicative of the voltage across the current sensing resistor  16  and thus the current magnitude. The output of the operational amplifier  26  controls the conduction level of the transistor  32 , thereby causing a voltage to be produced across the bias resistor  38  that corresponds to the magnitude of current flowing to the load  24 . The comparator  40  compares the voltage level at the intermediate node  34 , i.e. the voltage across the bias resistor  38 , to the reference voltage V REF  that defines a limit for the current. When the voltage at the intermediate node  34  exceeds this reference voltage level, which occurs when the current limit has been exceeded, the output of the comparator  40  changes states, which causes the disconnected device  18  to terminate the flow of electric current to the load  24 . 
   For example, assume that the nominal supply voltage at the first input terminal is 15 volts and the first current limiting circuit  10  typically must allow 10 amps of current to flow to the load which normally consumes 150 watts. However, when the supply voltage exceeds 20 volts while the load continues to draw 10 amps, the power consumption now is greater than the 200 volt-ampere guideline. To prevent this from occurring, the power limit resistor  36 , in circuit branch  35 , modulates the current sense voltage at the intermediate node  34  in response to variation of the supply voltage at the first input terminal  12 . In the above example if the supply voltage rises from 15 to 25 volts, more current flows through the power limit resistor  36  which raises the voltage across the bias resistor  38  and thus, the voltage level at the intermediate node  34 . The rise in the voltage at the intermediate node  34 , connected to the non-inverting input of the comparator  40 , causes the output of the comparator to change states at a lower current level than when the supply voltage was at 15 volts. By the proper choice of resistor values in the first current limiting circuit  10 , the voltage developed across the bias resistor  38  can vary in a way that causes the comparator  40  to switch at a constant input current/input voltage product, thus resulting in a constant power limit. 
   The first current limiting circuit  10  senses the current flowing to the load  24  and limits the magnitude of that current flow. However, the threshold at which the current limiting occurs is varied in response to changes in the voltage of the electricity applied to the load. That process ensures that the power consumption of the load is limited to a predefined amount. 
   The first current limiting circuit  10  in  FIG. 1  modulates the voltage that corresponds to the current level, in response to changes in the level of the supply voltage. Alternatively, the modulation in response to the supply voltage variation can alter the level of the reference voltage applied to the comparator to produce a similar power level limiting. This alternative second current limiting circuit  50  is illustrated in  FIG. 2 . This limiting circuit  50  contains the components of the first input circuit with the exception of the power limit resistor  36 . Elements of this alternative circuit that are in common with the first current limiting circuit  10  in  FIG. 1  have been assigned identical reference numerals. 
   The second current limiting circuit  50  has an additional circuit branch  51  that has a Zener diode  52  connected in series with a first resistor  54  between the first and second input terminals  12  and  14 . The breakdown voltage of Zener diode  52  is the nominal operating voltage of the load. The sensing node  55  between the Zener diode  52  and the first resistor  54  is coupled by a second resistor  56  to the inverting input of a second operational amplifier  58 . The non-inverting input of the second operational amplifier  58  is connected directly to circuit ground via the second input terminal  14 . Third and fourth resistors  60  and  62  are connected in series between the non-inverting input of the second operational amplifier  58  and the inverting input of the comparator  40 . The output of the second operational amplifier  58  is connected to a junction between the third and fourth resistors  60  and  62 . 
   A voltage divider  65  is formed by a fifth resistor  66  and a sixth resistor  68  connected in series between a source of positive voltage (V+) and circuit ground. A reference node  64  between the fifth and sixth resistors  66  and  68  is connected to the inverting input of the comparator  40  to apply a reference voltage level to that input. Although the voltage divider  65  produces a constant reference voltage level, the actual voltage at the inverting input of the comparator  40  is modulated by the variation of the input voltage. 
   The second current limiting circuit  50  is configured based on the nominal current and voltage values during normal operation of the load  24 . Using parameters similar to the previous example, assume that the load  24  normally operates at a supply voltage of 15 volts and draws 10 amps of current for a nominal power consumption of 150 watts. When 10 amps flows through the current sensing resistor  16 , the current sensing circuit  25  will produce a given voltage (e.g. 1 volt) at the intermediate node  34 . The voltage divider  65  comprising the fifth and sixth resistors  66  and  68  is configured to produce that given voltage level (1 volt) at the inverting input of the comparator  40 . The breakdown voltage of the Zener diode  52  is 15 volts so that under these operating conditions it is non-conducting and the branch circuit  51  does not affect the voltage at reference node  64 . Therefore, both inputs to the comparator  40  are at the same voltage level and its output does not trigger the disconnect device  18 . 
   Until the Zener diode  52  breaks down and begins conducting, the second current limiting circuit  50  functions as a conventional current limiter. However, when the supply voltage at the first input terminal  12  exceeds the breakdown voltage (e.g. 15 volts), the Zener diode  52  conducts current. This causes the second operational amplifier  58  in the circuit branch  51  to produce a negative output voltage that is proportional to the amount that the supply voltage exceeds the Zener breakdown voltage. This negative output voltage pulls down the voltage produced by the at the voltage divider  65  at the reference node  64 . The alteration of the voltage level at the inverting input of the comparator  14  effects the threshold of the comparator so that it will change states when a lower voltage level occurring at the intermediate node  34 . Thus, the comparator will change states at a proportionally lower supply current level as the input voltage exceeds the voltage limit set by the Zener diode  52 . This change in threshold maintains a constant input current/input voltage product, thus maintaining a fixed power limit for the load  24 . 
   The foregoing description was primarily directed to a preferred embodiment of the invention. Although some attention was given to various alternatives within the scope of the invention, it is anticipated that one skilled in the art will likely realize additional alternatives that are now apparent from disclosure of embodiments of the invention. Accordingly, the scope of the invention should be determined from the following claims and not limited by the above disclosure.