Abstract:
A device for detecting a fault current in an electronic apparatus has a first and at least one second electric conductor, the first electric conductor carrying a first electric potential and the at least one second electric conductor carrying a second electric potential. The device is characterized by the fact that an electric sensing conductor having a fixed electric rest potential whose value lies between the first and the at least second electric potential is situated between the electric conductors.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to a device for detecting a fault current in an electronic apparatus.  
       BACKGROUND INFORMATION  
       [0002]     When used in motor vehicles, in particular, an ever-present supply potential, combined with temperature and moisture influences as well as ion-supplying substances such as de-icing salt, wiper fluid surfactants or plastic additives, may result in corrosion of contact materials, degradation of plastics or migration on printed circuit boards, producing conductive paths having unwanted cross or fault currents. Over time, these paths become increasingly less resistant, so that the fault currents rise and any surrounding plastic parts may overheat or ignite. However, since the fault currents are often below the response threshold of automotive. fuses, it is not possible to detect and avoid them, in particular in the case of continuous plus wires, by activating the automotive fuses or by interrupting the connection to the battery via an ignition key.  
         [0003]     An electric consumer, which is connectable to an a.c. voltage via a connector and two supply lines and has a sensing conductor and a detector circuit for detecting a fault current due to low-resistance shorting of the electric consumer produced by water or another conductive liquid, is known from U.S. Pat. No. 5,159,517. The connector also accommodates an interrupter circuit which interrupts the two supply lines if an overcurrent is detected. The detector circuit may be situated in either a housing of the electric consumer or in the connector, the sensing conductor in the latter case being run from the electric consumer to the connector, together with the two supply lines, as a third conductor.  
         [0004]     Another known method for detecting and avoiding fault currents in power networks involves the use of fault current circuit breakers which cut off the entire power supply in the event of a fault. However, it is not possible to use a procedure of this type in motor vehicles, since this may result in dangerous, undefined operating states. Furthermore, there is no need for this, since the vehicle voltage is usually below the level of the safety extra-low voltage.  
       SUMMARY OF THE INVENTION  
       [0005]     The device according to the present invention for detecting a fault current in an electronic apparatus, having a first and at least one second electric conductor, the first electric conductor carrying a first electric potential and the at least one second electric conductor carrying a second electric potential, has the advantage over the related art that it provides a very safe, simple and cost-effective means of detecting fault current paths or fault currents at an early stage and therefore in a timely manner. For this purpose, an electric sensing conductor having a fixed electric rest potential, the value of which lies between the first and the at least one second electric potential, is situated between the electric conductors.  
         [0006]     The rest potential of the sensing conductor is derived, in principle, from the mean value of the first and second electric potentials. In this manner, in the event of a fault, an evaluation circuit, a microprocessor and/or a threshold switch may very easily and unambiguously detect a shift in the electric rest potential of the sensing conductor in the direction of the first or the second electric potential.  
         [0007]     Since the device according to the present invention is able to detect faults at a very early stage, it is also advantageous that, to avoid undefined states of the electronic apparatus, the evaluation circuit, the microprocessor and/or the threshold switch emit(s) a warning signal upon detection of a fault, the device being designed in such a way that the electronic apparatus is not deactivated.  
         [0008]     The device according to the present invention is used in a particularly advantageous manner in connection with printed circuit boards, plastics having integrated or deposited conductive tracks, contact means and/or lead frames. In this regard, the first electric conductor, the at least one second electric conductor and/or the sensing conductor is/are designed as conductor tracks, contacts and/or lead frame tracks. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]      FIG. 1  shows a first exemplary embodiment of the device according to the present invention for a printed circuit board.  
         [0010]      FIG. 2  shows a second exemplary embodiment of the device according to the present invention for a lead frame; and  
         [0011]      FIG. 3  shows a third exemplary embodiment of the device according to the present invention for a contact means. 
     
    
     DETAILED DESCRIPTION  
       [0012]      FIG. 1  shows a first exemplary embodiment of device  10  according to the present invention. A first electric conductor  18  and a second electric conductor  20  are deposited in the form of a first conductor track  22  and a second conductor track  24  onto a printed circuit board (PCB)  12  of an electronic apparatus  14 , for example a motor vehicle control unit  16 . While first conductor track  22  carries a first electric potential V 1 , second conductor track  24  is connected to a second electric potential V 2 . Since the two conductor tracks  22  and  24  thus have different potentials, a low-resistance short between them must be avoided. However, a fault-caused, for example, by saline spray water, wiper fluid surfactants or dissolved plastic additives-resulting in ionic migration involving corrosion or migration of conductor tracks  22 ,  24  or in a degradation of the plastics causes unwanted cross or fault currents which may produce substantial heating or ignition of the plastic parts. In the case of motor vehicle control unit  16 , this even poses the danger of the vehicle catching fire, since it is not possible to interrupt the connection via a vehicle key, in particular in the case of a permanent plus connection between first conductor track  22  and the positive pole of a battery  26  illustrated in  FIG. 2 . Moreover, since cross or fault currents often lie below the response threshold of automotive fuses, such fuses are also unable to provide adequate protection.  
         [0013]     For this reason, according to the present invention, a sensing conductor  28 , which in the case of printed circuit board  12  is also designed as a conductor track  30 , is provided between susceptible conductor tracks  22 ,  24 . Sensing conductor  28  is connected to an evaluation circuit  32 , a microprocessor  34  and/or a threshold switch  36  and has a fixed electric rest potential V R , the value of which lies between first electric potential V 1  and second electric potential V 2 . This value may, in principle, be formed from the mean value (arithmetic, geometric or the like) of first and second electric potentials V 1  and V 2 . If, in the event of a fault, a conductive path occurs between first and second conductor tracks  22 ,  24 , rest potential V R  of sensing conductor  28  shifts in the direction of either first electric potential V 1  or second electric potential V 2 . This potential variation is detected by evaluation circuit  32 , microprocessor  34  and/or threshold switch  36 , and a warning signal W is emitted. Since the evaluation is highly resistive, warning signal W is emitted far in advance of an actual endangerment caused by critical, low-resistance states. To additionally avoid the danger of undefined states—in particular during the operation of a motor vehicle—device  10  is designed in such a way that it is unable to deactivate electronic apparatus  14  in the event of a fault. Intervention into faulty electronic apparatus  14  therefore takes place later on in a repair shop without endangering the vehicle occupants.  
         [0014]      FIG. 2  shows a second exemplary embodiment of device  10  according to the present invention to detect a fault current for a preferably plastic-coated lead frame  38 . First conductor  18  designed as lead frame track  40  and second conductor  20  designed as lead frame track  42  are connected to a supply potential V batt  and a ground potential GND of battery  26 , respectively, so that first electric potential V 1  of first electric conductor  18  corresponds to supply potential V batt , and second electric potential V 2  corresponds to ground potential GND. With reference to  FIG. 1 , lead frame tracks  40  and  42  in this case also form integral parts of electronic apparatus  14 . In the case of motor vehicle control unit  16 , lead frame tracks  40 ,  42  may be used, for example, to supply power to power components or high-current consumers, which are not illustrated.  
         [0015]     Sensing conductor  28 , which is also in the form of lead frame track  44 , is situated between lead frame tracks  40 ,  42  and connected to evaluation circuit  32 , microprocessor  34  and/or threshold switch  36 . As illustrated in  FIG. 1  above, sensing conductor  28  in this case also carries a fixed rest potential V R , the value of which—in particular the mean value—lies between first and second electric potentials V 1  and V 2 . Because both the detection and warning of fault currents are carried out as described for  FIG. 1 , they are not discussed in any further detail here.  
         [0016]      FIG. 3  shows a third exemplary embodiment of device  10  according to the present invention for a contact means  46 , first conductor  18 , second conductor  20  and a further conductor  48  being designed as injected or inserted contacts  50 . If contact means  46  is, for example, a connector  52 , contacts  50  are contact pins  54 . In the case of a socket (not illustrated), contacts  50  may also be designed as contact receptacles.  
         [0017]     First and second potential V 1  and V 2  may be applied to contacts  50  via a connection to battery  26 , or potentials V 1  and V 2  may be applied thereto only by inserting contacts  50  into a receptacle provided for them, but not illustrated. Moisture penetrating contact means  46 , a degradation of plastic or the like may produce a low-resistance short and thus a fault current between first electric conductor  18  and second electric conductor  20 . To enable this fault current to be detected at an early stage, contact means  46  has sensing conductor  28  between the two conductors  18  and  20  in the form of an integrally cast or inserted pin  56 . As shown in  FIGS. 1 and 2  above, this pin is connected to evaluation circuit  32 , microprocessor  34  and/or threshold switch  36  via a line (not illustrated) and provided with a fixed rest potential V R , the value of which—in particular, the mean value—lies between first and second electric potentials V 1  and V 2 . As a result of the shift in rest potential V R  in the direction of first or second electric potential V 1  or V 2 , produced by one of the aforementioned influences, it is thus possible to detect fault currents at an early stage using evaluation circuit  32 , microprocessor  34  and/or threshold switch  36 . According to this exemplary embodiment, a warning signal W is emitted upon detection of a fault, and device  10  is designed in such a way that it does not deactivate the electronic apparatus connected to contact means  46  in order to avoid undefined states which may pose a safety risk, for example in a motor vehicle.  
         [0018]     In conclusion, note that the illustrated exemplary embodiments are not limited to  FIGS. 1 through 3  or to the arrangement, shape or number of conductors or sensing conductors. It is therefore entirely conceivable for lead frame  38  shown in  FIG. 2  to have a sensing conductor  28  in the form of a conductor track, a flexible line or the like. The same applies to the two other exemplary embodiments illustrated here. It is also possible to use an appropriately designed sensing conductor  28  in a cable or a cable harness so that the protective means extends from electronic apparatus  14  via the cable harness to contact means  46 , and additional actuators and/or sensors connected to the cable harness may be included in the fault current detection function.