Over-current protection circuit
TECHNICAL FIELD 
     The present invention relates generally to power supply protection. More specifically, this invention relates to a circuit that emulates the function of a fuse for protecting an electronic system and its power supply from damages caused when a more than expected amount of current is drawn from the power supply. 
     BACKGROUND OF THE INVENTION 
     A conventional method of protecting electronic systems from an over-current condition (that is, a condition when more than a predetermined amount of electric current is drawn from the power supply) is by using a separate power supply for each subsystem, together with a circuit breaker, or a replaceable fuse, along with each power supply, so that, when an over-current condition occurs, the fuse will melt, or the circuit breaker will trip, thereby disconnecting the power supply from the subsystem. 
     When the number of subsystems becomes large, the use of separate power supplies, together with the corresponding number of fuse boxes required, will take up a significant amount of physical space. In addition, when the number of subsystems increases, the task associated with storing spare fuses and replacing melt fuses may become cumbersome. 
     Accordingly, there is a need for an electronic circuit which can perform the function of a fuse for protecting electronic systems from an over-current condition. However, it is also desirable to have a protection circuit which requires no fuse replacement, or no resetting of a circuit breaker, after the occurrence of an over-current condition. 
     SUMMARY OF THE INVENTION 
     The present invention is a circuit for protecting an electronic system and its power supply from an over-current condition. The circuit comprises channel means responsive to a first control signal for passing an electric current from said power supply to said electronic system, first control means for supplying said first control signal to said channel means, sensor means coupled to said channel means for sensing the electric current passing through said channel means; and second control means coupled to said sensor means for disabling said first control means when said electric current passing through said channel means exceeds a predetermined value. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is, a schematic circuit diagram of a preferred embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION 
     Referring to FIG. 1, there is shown an illustrative embodiment of a power supply protection circuit 1 according to present invention. The circuit 1 comprises channel means in form of a first transistor Q1 for passing, through a current path between its emitter and collector, an electric current from a power supply 100 to a load 200. 
     In accordance with transistor characteristics, the current path through transistor Q1 is controlled by a first control signal applied to its base. 
     This first control signal is supplied from the collector of a second transistor Q2. The base of this second transistor Q2 is biased at a predetermined voltage level by resistors R3 and R4, while its emitter is biased at another predetermined voltage level by resistors R6 and R7. The first control signal is supplied by the second transistor Q2 though its emitter-collector junction. The first control signal supplied by the transistor Q2 is controlled by a second control signal applied to base of the transistor Q2. 
     The base of the second transistor Q2 is connected to the collector of a third transistor Q3. In accordance with transistor characteristics, the voltage between the collector and the emitter of the transistor Q3 is dependent upon the voltage difference between its base and emitter. This voltage is in turn equals to the voltage drop across resistors R8 and R9. 
     Resistor R9 lies in the aforementioned current path between the power supply 100 and the load 200 through transistor Q1. It senses the current through the current path by providing a voltage drop, V R9 , in accordance with Ohm&#39;s law, as a function of the current drawn by the load 200 from the power supply 100. When the current passing through the current path of Q1 is higher than a predetermined value, the voltage drop across resistor R9 will disable transistor Q2 from supplying said first control signal, as will be described hereinafter, thereby cutting off the current path. 
     Once the current path is cutoff, it will be kept in a cutoff state by circuit 300. Circuit 300 causes a current to flow into the base of transistor Q3 when the voltage at the emitter of Q3 is below a predetermined threshold value. In normal operation, no such current would flow into transistor Q3. 
     The voltage level at the base of transistor Q2 and the voltage level at the emitter of transistor Q2 are biased such that a current, I 12 , will automatically flow between the base of transistor Q1 and the collector of transistor Q2 when power is applied from power supply 100 to the circuit 1. 
     Transistor Q1 is selected to have a current amplification factor such that Q1 operates in saturation during normal operation. In a preferred embodiment of this invention, such amplification factor is provided by using a Darlington transistor as transistor Q1. 
     The current, I load , passing through transistor Q1, will cause a voltage drop of V R9  (=I load  * R9) across resistor R9. When the voltage difference between the emitter and the base of transistor Q3 less than V Q3be (on), the forward voltage of the base-emitter junction, transistor Q3 will switch off and no current will flow across resistor R8. In that case, the voltage difference between the emitter and the base of transistor Q5 is equal to the voltage drop across resistor R9. 
     As soon as the current I load  drawn by the load 200 from the power supply 100 reaches the point where V R9  is greater than V Q3be (on), transistor Q3 will switch on. When the emitter collector junction conducts, the collector voltage of transistor Q3 will be pulled up. Since the collector of transistor Q3 is connected to the base terminal of transistor Q2, when transistor Q3 switches on, the potential of the base of transistor Q2 will rise, causing an increase in the collector current of transistor Q2. This in turn will cause transistor Q1 to saturate more. It can be seen that transistor Q2 and Q3 together form a positive feedback path which controls the current flowing into transistor Q1. 
     When the current I load  increases to the point where transistor Q 1  goes from its saturation state to its linear state, the voltage difference between the emitter and the collector of transistor Q1 will increase, causing the voltage at the emitter of transistor Q3 to drop. 
     As the voltage at the emitter of transistor Q3 drops, its collector voltage will follow. This means that the base voltage of transistor Q2 will also drop. As the emitter of Q2 is biased by resistors R6 and R7, when the emitter voltage of transistor Q3 drops to a certain level, transistor Q2 will be cut off, which in turn will switch off transistor Q1. 
     When no current flows through transistor Q1, the voltage drop across resistor R9 will disappear. However, transistor Q3 will still switch on because the bias circuit 300 provides a current I b2  to the base of transistor Q3 when the emitter voltage of Q3 drops below a certain threshold value. This threshold value is approximately equal to the input voltage of the power supply 100, minus the voltage drop across diodes CR1 and CR2, and the base-emitter voltage of transistor Q3. Once the current I load  is high enough to cause the voltage at the emitter of Q3 to go below this voltage value, transistor Q3 will conduct independently of the voltage across resistor R9 and cut off transistors Q1 and Q2 in a regenerative manner. 
     Once circuit 1 is shutoff, it can be restarted in two conditions: 
     1. By bringing up the input voltage, V in  of the power supply from zero volt while the current drawn by load 200 is less than the cutoff value--In this case, because diode CR1 will not conduct if V in  is below its breakdown voltage, transistor Q1 is able to come up to saturation. 
     2. By removing the output load 200 completely--In this case, because the output load is removed, the output voltage will be allowed to rise due to the current path from the power supply 100 through diode CR1, resistor R2, diode CR2, resistors R8 and R9. 
     In the preferred embodiment of the present invention, a filter circuit 400 is optionally provided as shown to filter out noise signals. Also, as an option, an indicator circuit is also connected as shown to provide an indication when the power is cutoff by the protection circuit 1. 
     A set of exemplary values for the illustrative implementation of the power protection circuit 1 shown in FIG. 1 is given in table 1. 
     
                       TABLE 1                                                     
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           Vin = 18 volt                                                  
           R1 =  5.11K                                                    
           R2 =  1K                                                       
           R3 =  9.09K                                                    
           R4 =  90.9K                                                    
           R5 =  5.11K                                                    
           R6 =  2.49K                                                    
           R7 =  9.09K                                                    
           R8 =  100                                                      
           R9 =  0.5                                                      
           Q1 =  2N6041                                                   
           Q2 =  2N2222                                                   
           Q3 =  2N2222                                                   
           Q4 =  2N2907                                                   
           CR1 = IN746                                                    
           CR2 = IN4003                                                   
           CR3 = IN4003                                                   
           CR4 = IN4003                                                   
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     While the invention has been particular shown and described with reference to the above preferred embodiment, it will be understood by those skilled in the art that the foregoing and other changes in form and detail may be made therein without departing from the spirit and scope of the invention. For example, although the above embodiment is described with reference to a particular semiconductor technology (e.g. bipolar) and particular types of transistor devices (e.g. npn transistors), it will be obvious to those skilled in the art that other semiconductor technologies (e.g. metal oxide semiconductor, MOS) and other types of transistor devices may be used.