Abstract:
A fault monitor for the load portion of an electrical circuit may be connected to an existing circuit by a single connection, and may detect flaws within the load portion of the circuit through the detection of changes in voltage through a resistor, caused by changes in current flow due to changing resistance within the load portion of the circuit.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to testing of electrical circuits. More specifically, the invention provides a fault monitor for testing the load portion of an electrical circuit for faults.  
       DESCRIPTION OF THE RELATED ART  
       [0002]     Motor vehicles presently contain a variety of electronic devices, for example, light bulbs, which must be properly functioning both to ensure safety and to keep the vehicle in compliance with the statutorily prescribed safety equipment guidelines.  
         [0003]     Presently existing fault detection devices utilize switching devices with current sensing capabilities, or devices for measuring voltage drop across a low impedance resistor as part of the switching device. Such devices are typically expensive, especially if designed for monitoring high current devices. Furthermore, such devices generally need to be included in the original circuit design, or require substantial reworking of the original circuit design to be incorporated into the circuit.  
         [0004]     Accordingly, there is a need for a fault monitoring device that may be integrated into an existing electrical circuit without substantial modification of the existing circuit. There is a further need for a fault monitoring device having a lower cost than presently available devices.  
       SUMMARY OF THE INVENTION  
       [0005]     The present invention provides a fault monitor for the load portion of an electrical circuit that may be added to the circuit with only minimal modification. The fault monitor may be added by splicing a single connection into the existing circuit.  
         [0006]     A fault monitor of the present invention includes a power supply connected in series with a resistor, a connection for connecting to the circuit to be monitored with the load of that circuit in parallel with the resistor, a current sensor connected in series with the resistor, and switching means for opening and closing the connection between the power supply and the resistor and load.  
         [0007]     The invention may further include a means for determining whether current is flowing through the load before and after the test, because the test should be performed without current from other sources flowing through the load. The means for determining whether current is flowing through the load include a switching means connected in series with the resistor, and in parallel with the current sensor. By closing the switching means, any current flowing through the load will also be drawn through the parallel circuit including the current sensor. If the reading of the current sensor is ground, a test may be performed. If the current sensor reads any amount of current, the test must be delayed. A preferred test switching means include a transistor, such as an NPN digital transistor, connected between the resistor and a ground, with a switched pull-down resistor supplying the transistor&#39;s input. When a positive potential is supplied through the switched pull-down resistor, current may flow through the transistor to the ground, so that any current flowing through the load will also be drawn through the parallel resistor, test switching transistor, and also the current sensor. If the current sensor reads ground, there is no current flowing through the load, and a test may be performed. Otherwise, the test must be delayed.  
         [0008]     Once it is determined that no current is flowing through the load, power may be simultaneously applied through the parallel connected resistor and load. A preferred switching means for supplying power to the resistor and the load include a transistor, for example, a digital NPN transistor. A switched 12-volt supply supplies the input of the digital transistor, with a positive potential at the input permitting current flow through the transistor. In some preferred embodiments, the current flowing through the digital NPN transistor provides a negative input to a second PNP transistor, thereby permitting the flow of power through the PNP transistor to the load and resistor. The load, which may be a plurality of bulbs wired in parallel, will provide a resistance equivalent to that of a single resistor according to the well-known principals of Ohm&#39;s law. The equivalent resistance will change if one or more of the bulbs within the load have burned out, thereby changing the amount of current that may pass through the circuit for a given supply voltage. Changes in this current will result in a change in the voltage applied to a voltage sensor.  
         [0009]     After testing for faults within the load, the circuit should again be tested to see if current is flowing to the load from the load&#39;s main power source, to ensure that this power was not turned on during the test.  
         [0010]     Accordingly, it is an object of the present invention to provide a fault monitor for an electrical circuit that may be installed on a preexisting electrical circuit by merely making a single additional connection to the circuit.  
         [0011]     It is another object of the invention to provide a fault monitor capable of detecting the number of component failures within a load consisting of multiple electrical devices in parallel.  
         [0012]     It is a further aspect of the present invention to provide a fault monitor having a means for determining whether the load to be tested for faults is on or off before and/or after a test is performed.  
         [0013]     It is another object of the invention to provide a fault monitor having a means for protecting the components within the fault monitor from damage due to excessive voltage or current.  
         [0014]     It is further object of the invention to provide a fault monitor having lower costs than presently available fault monitors.  
         [0015]     These and other objects of the invention will become more apparent through the following description and drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]      FIG. 1  is a schematic view of a fault monitor according to the present invention, illustrated in conjunction with a circuit for which fault monitoring is desired. 
     
    
       [0017]     Like reference characters denote like elements throughout the drawing.  
       DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0018]     The present invention provides a fault meter for testing for faults within an electrical circuit. Referring to  FIG. 1 , a typical electrical circuit  10  with which the present invention will be used is illustrated. The circuit  10  includes a power supply  12  connected through a switching device  14  to a load  16 , and finally through a ground  18 . The load  16  illustrated in  FIG. 1  includes a plurality of bulbs  20 ,  22 ,  24  connected in parallel. The circuit  10  may represent, for example, a high mount stop light on a motor vehicle.  
         [0019]     The fault monitor  26  may be connected to the circuit  10  by splicing into the wire  28  at point  30 , or in other preferred embodiments may be connected or disconnected to the circuit  10  using the mating connectors  32 ,  34 .  
         [0020]     The fault monitor  26  includes a power supply  36  connected to switching mechanism  38 . A preferred switching mechanism includes a first transistor  40 , which may in the illustrated example be a PNP transistor  40  having an entrance  42 , input  44 , and emitter  46 . The power supply  36  may also be connected to a second transistor  48 , which in the illustrated example is a digital NPN transistor having an entrance  50 , an input  52 , and an emitter  54 . A pair of resistors  56 ,  58  are connected in series between the power supply  36  and the entrance  50  of the transistor  48 . The input  44  of the transistor  40  is connected between the resistors  56 ,  58 , thereby providing a voltage divide for the input  44 . The resistors  56 ,  58  may, in some embodiments, provide resistance of about 1.0 KΩ and about 4.7 K Ω, respectively. The input  52  is connected to a power source, for example, the switched 12-volt power supply  59 . The emitter  54  is connected to a ground  61 . When a positive potential is supplied from the power supply  59  to the input  52  of the transistor  48 , current is permitted to flow from the power supply  36  to the ground  61 . This current will provide a negative potential at the voltage divide between the resistors  58 ,  56 , thereby providing a negative potential to the input  44  of the transistor  40 , permitting current to flow therethrough from the power supply  36 . While the present invention is not limited to the illustrated switching mechanism  38 , the illustrated switching mechanism  38  provides for fine tuned switching through the digital transistor  48 , and the potential to handle higher current through the transistor  40 .  
         [0021]     The fault monitor  26  also includes a resistor  60 , and may also include a resistor  62 , in series with the emitter  46  of the transistor  40 . The resistors  60 ,  62  may in some embodiments provide resistance of about 330 KΩ and about 20 Ω, respectively. The resistor  62  limits the current passing through the load  16  to a level below that required to light the bulbs  20 ,  22 ,  24 , thereby ensuring that the test process does not light the bulbs  20 ,  22 ,  24  at a time when they should not be lit. A capacitor  66  connected in series with the ground  68  and in parallel with both the resistor  60  and the load  16  provides for a more uniform flow of current through the resistor  60  and load  16 . The capacitor  66  may in some embodiments provide a capacitance of about 0.1 μF. The diode  65  resists current flow in through the resistor  62  towards the power supply  36 .  
         [0022]     A sensor  70 , which may be an analogue filament resistance sensor for sensing voltage, is connected in series with the resistor  60 . The sensor  70  may be provided with a parallel connected capacitor  72  connected in series with the ground  74  for providing an even flow of current to the sensor  70 . The capacitor  72  may in some embodiments provide a capacitance of about 0.1 μF. The sensor  70  may also be provided with a parallel connected Zener diode  76 , connected in series with the ground  78 . The Zener diode  76  will resist current flow unless the voltage exceeds a predetermined maximum, in which case current will flow through the Zener diode  76  to the ground  78 , thereby protecting the sensor  70  from damage. In some preferred embodiments, the Zener diode may have a breakdown voltage of about 5.1 volts.  
         [0023]     The fault monitor  26  may also be provided with a means for determining whether current is flowing from the power supply  12  to the load  16 . The test must be conducted with the switch  14  open, and no current flowing through the bulbs  20 ,  22 ,  24 . The means  80  for detecting current flow from the power supply  12  includes a test switching means which, in the illustrated example, is the digital NPN transistor  82  having an entrance  84  connected in series with the resistor  60 , an input  86 , and an emitter  88  connected to the ground  90 . The input  86  is connected to a power supply  92  which in the illustrated example is a pull-down resistor switch. A resistor  94  may be provided between the resistor  60  and the transistor  82  to provide a voltage divide for the sensor  70 , thereby protecting the sensor  70  from damage from excess voltage. The resistor  94  may, in some embodiments, provide a resistance of about 100 KΩ.  
         [0024]     To test the load  16  for faults, the pull-down resistor switch  92  will apply a positive potential to the input  86  of the transistor  82 , so that if the switch  14  is closed, current from the power source  12  will flow through the transistor  82 , and to the sensor  70 . If the sensor  70  indicates that no current is flowing, then a test may be performed. If current is flowing through the load  16 , then the test must be delayed. The resistors  60 ,  94  minimize the current that is drawn away from the load  16 , so that if the bulbs should be lit, they will remain lit during the test.  
         [0025]     To perform the test, the switched 12-volt power supply  59  will apply a positive potential to the input  52  of the transistor  48 , thereby permitting current to flow from the power supply  36  through the transistor  48 . A negative potential will thereby be applied to the input  44  of the transistor  40 , thereby permitting current from the power supply  36  to flow therethrough. This current will therefore be supplied to the resistor  60  and sensor  70 , and to the bulbs  20 ,  22 ,  24 . As can be seen from  FIG. 1 , the voltage applied to both the resistor  60  and to each of the bulbs  20 ,  22 ,  24  will remain constant. However, if one or more of the bulbs  20 ,  22 ,  24  has burned out, the current will vary according to the well-known principals of Ohm&#39;s law. According to well-known principals, the bulbs  20 ,  22 ,  24  will provide a resistance equal to that of a hypothetical single resistor, designated herein as R eq . The resistance of R eq  can be determined by the equation:  
         1     R   eq       =       1     R   20       +     1     R   22       +     1     R   24             
 
 where R 20 , R 22 , and R 24  each denote the resistance of the bulbs  20 ,  22 ,  24 , respectively. Therefore,  
         R   eq     =       1       1     R   20       +     1     R   22       +     1     R   24           .         
 
         [0026]     The current flowing through the circuit will flow according to the equation V=IR, where V is the voltage, I is the current, and R is the resistance. Therefore, the current flowing through the load  16  may be determined by the equation  
       I   =       V   R     .         
 
 Because the current flow through the resistor  60  to the sensor  70  is negligible, R eq  plus the resistance of the resistor  62 , hereinafter R 62 , will determine the total current flowing through the circuit. Therefore, the current flowing through the circuit is determined by the equation  
       I   =       V       R   eq     +     R   62         .         
 
 With the current thereby determined and the resistance of the resistor  60  (hereinafter R 60 ) being constant, the voltage applied to the sensor  70  (hereinafter V 70 ) becomes V 70 =IR 60 . 
 
         [0027]     Suppose that bulb  20  is burned out. Now,  
         R   eq     =       1       1     R   22       +     1     R   24           .         
 
 The absence of the contribution of the bulb  20  to R eq  means that R eq  will increase, thereby causing less current to flow through the system. Likewise, if a second bulb  22 ,  24  burns out, R eq  will become equal to R 24 , causing even less current to flow through the system. Obviously, if all bulbs  20 ,  22 ,  24  burn out, then no current will flow through the system. Therefore, if no bulbs  20 ,  22 ,  24  have burned out, the voltage sensed by the sensor  70  will be at a maximum, and will decrease for increasing numbers of burned out bulbs. 
 
         [0028]     After completion of the test, the switched 12-volt power supply  59  is turned off, and the pull-down resisted switch  92  is again activated, to make sure that the switch  14  has not been closed during the test. If the sensor  70  reads something other ground, the switch  14  has been closed, and the test is invalid.  
         [0029]     From the above description, it will become apparent to those skilled in the art that the above described test procedure may, if desired, be controlled by an appropriate microprocessor that has been programmed to carry out the test at various intervals, and provide indicia of bulb failure, for example, a dashboard light.  
         [0030]     While a specific embodiment of the invention has been described in detail, it will be appreciated by those skilled in the art that various modifications and alternatives to those details could be developed in light of the overall teachings of the disclosure. Accordingly, the particular arrangements disclosed are meant to be illustrative only and not limiting as to the scope of the invention which is to be given the full breadth of the appended claims and any and all equivalents thereof.