Fault indicating circuit for a current consuming load connected to an electronic switching device
BACKGROUND OF THE INVENTION 
     The invention relates to a circuit for reporting faults in a current consuming load connected to an electronic switching device wherein the fault is reported by means of an indicating device and with a control signal causing the load to be switched on being feedable to the input of the electronic switching device via a control line. 
     In such a circuit, as disclosed in DE-OS 22 03 426, faults are reported over a separate line leading to a display device common to a plurality of loads. 
     If a plurality of loads, particularly in a motor vehicle, are to be monitored individually, a large number of lines must be brought to the central operating location, namely one control line and one report line for each load. 
     SUMMARY OF THE INVENTION 
     It is the object of the invention to reduce the number of lines leading to the central operating location. 
     This is accomplished by according to the invention by a circuit for reporting faults in a current consuming load connected to an electronic switching device to whose input a control signal for causing the load to be switched on can be fed via a control line, wherein the input resistance of the electronic switching device is made variable by means of an impedance controlled by a fault signal and an indicator device for reporting a fault is actuated in dependence on the current flowing in the control line if the current in the control line deviates from the value at which it flows when there is no fault. Advantageous embodiments of the invention are defined in the dependent claims. 
     The invention results in the advantage that faults are indicated over the control line so that no separate report lines are required. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be explained with the aid of advantageous circuit examples which are illustrated in the drawings wherein: 
     FIG. 1 is a simplified illustration of the circuit according to the invention with an indictator element in the form of an indicator lamp; 
     FIG. 2 shows a modification of the circuit of FIG. 1 using a light emitting diode as the indicator element; 
     FIG. 3 shows modification of the circuit of FIG. 1 with an indicator device which additionally permits monitoring the brake light switch of a motor vehicle; and 
     FIG. 4 is a simplified illustration of a modification of the basic circuit according to the invention shown in FIG. 1. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     As shown in FIG. 1, an electronic switching device is marked with the reference numeral 1 and includes an input stage formed by a switching transistor 2 through which the current consuming load 3 connected to the positive pole of the voltage source is connected with ground potential which forms the negative pole. If the switch 4 disposed in a central operating location is closed, the base of transistor 2 receives, via a dropping resistor 5 and a control line 14, a control voltage which causes the load 3 to be switched on. The magnitude of this control voltage is fixed by Zener diode 6 at the value U z  and is less than the voltage U of the voltage source by the voltage drop across the dropping resistor 5. This voltage U z  is also present at the input of the window discriminator 7 which, however, at this value, does not permit current to pass through the indicator element, here shown as an indicator lamp 8. 
     Dropping resistor 5, window discriminator 7 and the indicator lamp 8 are combined in a display device 9 which is disposed in the area of the central operating location. 
     Fault detectors 10 and 11 are associated with the electronic switching device 1, with fault detector 10 transmitting a signal to OR gate 12 if the load is short-circuited and fault detector 11 transmitting a signal to OR gate 12 if the load current circuit is interrupted. The output signal produced at the OR gate 12 when there is a fault causes switch 13 to be closed. Switch 13 is preferably designed as a controllable semiconductor but could also be the closing contact of a relay actuated by the output signal of the OR gate 12. Switch 13 thus forms a controllable impedance which changes its value from &#34;zero&#34; (closed) to &#34;infinite&#34; (open). However it would also be possible, within the scope of the invention, to effect a change, for example if a switching transistor were used, between a first resistance value greater than zero and a second value which is greater than the first value but finite. 
     In any case, the difference in resistance has the result that the input resistance of the electronic switching device 1 takes on a lower value when there is a fault so that because of the then increased voltage drop across the dropping resistor 5 the input voltage to the window discriminator 7 is also reduced noticeably. The window discriminator 7 is designed in such a manner that, if there is an input voltage which is noticeably less than the value U z , it emits an output current to actuate the indicator lamp 8 which thus indicates the existence of a fault. Additionally, the window discriminator 7 produces a fault indication if its input voltage is greater than U z , for example if the control line 14 is interrupted due to a malfunction. 
     A short-circuit of the control line 14 to ground would in any case be indicated by the indicator lamp 8 lighting up because then the input voltage to the window discriminator 7 would be less than U z . 
     The input resistance measured between control line 14 and ground can of course also be varied in dependence on a fault in that switch 13&#39; or a switching transistor controlled by the output signal of OR gate 12, respectively, is connected in series into the control circuit as shown in FIG. 4. Then, however, the switch 13&#39; would have to be opened in the case of a fault so that a voltage greater than U z  would be present at the input of the window discriminator 7 whereupon the latter would cause the indicator lamp 8 to light up. 
     By qualitatively evaluating the voltage present across the control line 14 behind the resistor 5 by means of the window discriminator 7, it is possible to selectively determine various types of faults. 
     A particularly simple indicator device 15 is shown in FIG. 2 in which faults are indicated by a light-emitting diode 16 through which current flows directly from the control line. The indicator device 15 can be used in conjunction with the electronic switching device 1 shown in FIG. 1; in that case, the Zener diode 6 is not required. 
     With switch 13 open, i.e. if no fault is present, a small current flows through control line 14 which does not cause the light-emitting diode 16 to light up. The current is particularly small if transistor 2 is a VMOS transistor. If OR gate 12 causes switch 13 to close as a result of a fault, or if the control line 14 is short-circuited to ground, a greater current flows which causes the light-emitting diode 16 to light up. However, the circuit according to FIG. 2 does not indicate an interruption in control line 14. 
     An expanded circuit for additionally monitoring the operability of the brake light switch 4&#39; of a motor vehicle is shown in FIG. 3. 
     Between the window discriminator 7 and the indicator lamp 8, there is disposed an OR gate 17 which does not influence the above-mentioned fault indication via the first input 18. The second input 19 of the OR gate is connected with the output of an inverter 20 whose input is connected, on the one hand, in a high-resistance manner (resistor 21) with the positive pole of the voltage source and, on the other hand, via a control switch 22 actuatable by the pressure of the brake fluid and via an AND gate 23, which is provided for potential separation, to the connecting line between switch 4&#39; and resistor 5. 
     If switch 4&#39; is open, this connecting line is at ground potential, if switch 22 is likewise open. An H signal, which has been converted into an L signal at 19, is present at the input of the inverter 20. The indicator lamp 8 does not have current. 
     If the brake pedal is depressed, the brake switch 4&#39; is closed. An H signal is present at AND gate 23. If, after the brake pressure has been built up in the master brake cylinder, the control switch 22 responds, an H signal is present at the input of inverter 20 and an L signal at the input 19 of the OR gate 17 so that the indicator 8 continues to remain without current. 
     If, after actuation of the brake pedal, switch 4&#39; is not closed, for example, as a result of an incorrectly adjusted rod assembly, the inverter 20 receives an L signal once the control switch 22 has responded, causing the fault indicating lamp 8 to respond. 
     A fault is also indicated if switch 4&#39; is closed with a delay only after the brake pressure has been built up and after control switch 22 has closed. Since in this case, the indicator lamp 8 would light up only briefly, it is appropriate to store the fault signal for a certain period of time with the aid of a memory cell 24.