Alarm circuit for car alternator
The invention relates to an alarm circuit for an alternator for charging a car battery. 
     The object of the invention is to provide an alarm circuit for indicating a defect such as non-rotation or de-energization in a car battery charging alternator, this alternator comprising at least two terminals, one positive, one negative, connected to the armature by one or more rectifier diodes. 
     According to the invention, the alarm circuit comprises a voltage divider formed by two resistors in series connected between a positive terminal and a negative terminal of the alternator, the center point of this divider being connected to the control terminal of an electronic commutation circuit provided in series with a warning device between the terminals of the battery, and rectifier means being provided in the supply circuit of the battery for avoiding said latter to discharge through the voltage divider. 
     The electronic commutation circuit is for example a transistor circuit of which the output path is closed when the voltage applied to the control electrode reaches a predetermined value, either when increasing or decreasing. In particular, a Darlington circuit may be used having a base electrode connected to the centre point of the voltage divider and a collector-emitter output path serially connected with the warning device. 
    
    
     The invention will be more readily understood on reading the following description with reference to the accompanying drawings, in which: 
     FIG. 1 is a diagram of the alarm circuit according to the invention. 
     FIG. 2 shows the connection of the armature of the alternator to the different output terminals. 
    
    
     Referring now to the drawings, the alternator is three-phase and its armature is connected to the terminals B+ and B- of the battery supply circuit via three rectifier diodes, one on each phase; a third negative terminal D- is also connected to the armature by three diodes. 
     The alternator therefore comprises two terminals B+ and B- for supplying the battery 1 which feeds the variable load 2; it also comprises an excitation terminal E which is supplied by a known regulator 3 connected to the terminals B+ and B-. 
     Apart from these terminals, there are also terminal D- and an auxiliary terminal Baux.+ which is connected directy to the terminal B+. 
     The alarm circuit according to the invention comprises a voltage divider formed by two resistors 4 and 5 in series connected between the terminals Baux.+ and D-, the centre point of this divider, i.e. the point of junction of the resistors 4 and 5 is connected to the base of a Darlington circuit 6 of which the emitter is connected to the negative terminal of the battery and the collector to the positive terminal of the battery via a warning lamp 7 and a switch 8. 
     The functioning of the circuit when the switch is on makes it possible to indicate, by the lamp glowing, the stopping of the alternator, the de-energization thereof and the disconnection of one of terminals E, B+ or B-. 
     If the alternator is stopped or not energized, but rotating, no voltage appears between the terminals B+ and D- and no current circulates in the divider since the diodes of the terminal B- isolate the battery from terminal D-, the base of the Darlington circuit is at the positive potential of the battery via the resistor 5 and the Darlingtion circuit 6 being saturated, the battery feeds the lamp 7 collector-emitter circuit of the Darlington circuit 6 and the warning lamp glows. 
     If the positive terminal B+ is disconnected, the output voltage between B+ aux. and D- is a function of the speed of the alternator and always greater than the regulation voltage, it follows that the base is at a high voltage which saturates the Darlington circuit 6, whereby the warning lamp glows. 
     In normal running with the alternator rotating, the terminal D- is at the same potential as terminal B-, the resistors 4 and 5 function as voltage divider and their value is such that the potential applied to the base of Darlington circuit 6 is lower than the limit of conductions of the transistor and no current circulates in the lamp 7 which is extinguished. 
     As above indicated, the terminal D- is separated by diodes from the terminal B- of the battery supply circuit. The battery does not discharge through the voltage divider 4, 5 which is exclusively supplied with current delivered by the alternator. 
     If the terminal D- does not exist, the resistor 4 may be connected to the terminal B-, on condition that an separating diode be inserted between the terminal B- and the negative terminal of the battery, avoiding the battery feeding the voltage divider. 
     A sharp commutation of the load 2 results in an overvoltage which saturates, for a certain period of time, the battery 1, which provokes the blocking of the regulator 3 and consequently the de-energization of the alternator which lights up the warning lamp. 
     This phenomenon is avoided by connecting means to the base of the Darlington circuit 6 for eliminating the transient voltage variations, for example a capacitor 9 connected between the base and the collector of the Darlington circuit. The presence of the capacitor 9 will not disturb the normal running of the Darlington circuit since the latter has a high gain and a high input impedance.