Abstract:
In a light-emitting-diode lamp, there is provided an input impedance-changing circuit for establishing a low input impedance circuit when the light-emitting-diode lamp is missingline turned off. This input impedance-changing circuiting comprises Shunt circuit section, and a detector circuit section. The shunt circuit section includes a low impedance element and a controllable switching device connected in series. The detector circuit section detects turning off of the light-emitting-diode lamp and, in response to such detection, close the switching device to thereby cause electric current to flow through the shunt circuit section in order to simulate lower input impedance of the light-emitting-diode lamp. When the light-emitting-diode lamp replaces a conventional traffic signal incandescent lamp, the input impedance-changing circuitry prevents the conflict monitor of the already installed traffic-light lamp system to detect a high lamp impedance and accordingly a faulty lamp.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention is concerned with a fault-indicating impedance-changing circuit installed into a load, in particular but not exclusively a light-emitting-diode (LED) lamp. 
     2. Brief Description of the Prior Art 
     Incandescent lamps are conventionally used in traffic lights. As well known to those of ordinary skill in the art, a traffic light also includes a conflict monitor (a) to detect turning on of the lamp when it should be turned off and (b) to sense the impedance of the filament of the incandescent lamp to detect failure of the lamp. This conflict monitor is usually designed to operate with incandescent lamps having a rated nominal power of the order of 150 watts and an impedance lower than 1500 Ω. 
     When the power switch through which the incandescent lamp is turned on and turned off is open, a small current is supplied to the filament of the incandescent lamp through a shunt impedance element connected in parallel to the power switch. Upon selecting the impedance of the shunt impedance element, the two following factors are taken into consideration: 
     the impedance of the incandescent lamp is lower than 1500 Ω; and 
     the voltage measured across the incandescent lamp must not exceed a given voltage threshold when the lamp is in good condition, since detection of a voltage amplitude across the lamp higher than this given voltage threshold indicates a failure of the lamp. 
     A newly developed technology enables production of LED lamps that meet with the traffic signalling standards regarding light intensity. These LED lamps consume an electric power as low as 20 watts. 
     However, replacement of an incandescent lamp by a LED lamp raises the problem that light emitting diodes must be supplied with direct current and the input impedance of the required ac-to-dc power supply, included in the LED lamp, is high (when the LED lamp is turned off) if compared to the impedance of the filament of an incandescent lamp. 
     The easiest solution to the above discussed problem is to permanently connect an impedance element lower than 1500 Ω in parallel to the LED lamp. However, this solution is itself the source of the following problems: 
     a resistive impedance (resistor) will increase the level of electric power consumed by the LED lamp; 
     a reactive impedance (capacitor and/or inductor) will reduce the power factor of the lamp; and 
     in case of a failure of the light emitting diodes and/or the ac-to-dc power supply, the impedance of the LED lamp does not change sufficiently to allow the conflict monitor to detect a fault. 
     Upon detection of a fault, namely turning on of a lamp when it should be turned off or failure of a lamp, the conflict monitor activates a safety system to cause all the red and yellow lamps of the traffic light to flash for thereby warning the automobilists crossing the corresponding junction. 
     As road safety must not be neglected, LED lamps must be designed to enable the conflict monitor to detect a fault and activate the safety system in view of warning the automobilists. 
     OBJECTS OF THE INVENTION 
     An object of the present invention is therefore to overcome the above discussed problems by providing a fault-indicating impedance-changing circuit usable in a LED lamp. 
     SUMMARY OF THE INVENTION 
     More specifically, in accordance with the present invention, in an electric load, there is provided an input impedance-changing circuitry for establishing a low input impedance circuit of the electric load when this electric load is turned off. The input impedance-changing circuitry comprises (a) a shunt circuit section including a low impedance element and a controllable switching device connected in series, and (b) a detector circuit section for detecting turning off of the electric load and for closing, in response to detection of turning off of the electrical load, the controllable switching device. Closure of the switching device establishes a shunt circuit including the low impedance element and thereby establishes the low input impedance circuit of the electric load. 
     Also according to the present invention, in a light-emitting-diode lamp, there is provided an input impedance-changing circuitry for establishing of low input impedance circuit. the light-emitting-diode lamp when this light-emitting-diode lamp is turned off, comprising a shunt circuit section including a low impedance element and a controllable switching device connected in series, and a detector circuit section for detecting turning off of the light-emitting-diode lamp and for closing, in response to detection of turning off of the light-emitting diode lamp, the controllable switching device. This establishes a shunt circuit including the low impedance circuit element and thereby establishes the low input impedance circuit of the light-emitting-diode lamp. 
     Further in accordance with the present invention, there is provided a traffic-light lamp system comprising: 
     a light-emitting-diode lamp comprising an input impedance-changing circuitry for establishing a low input impedance circuit of the light-emitting-diode lamp when that light-emitting-diode lamp is turned off, this input impedance-changing circuitry comprising: 
     a shunt circuit section including a first low impedance element and a first controllable switching device connected in series; and 
     a detector circuit section for detecting turning off of the light-emitting-diode lamp and for closing, in response to detection of turning off of the light-emitting-diode lamp, the first controllable switching device in order to establish a shunt circuit including the first low impedance element and thereby establish the low input impedance circuit of the light-emitting-diode lamp; and 
     a second controllable switching device interposed between a source of electric power and the light-emitting-diode lamp for selectively turning on and turning off the light-emitting-diode lamp. 
     Establishing a low input impedance circuit in a light-emitting-diode lamp when turned off prevents a conventional conflict monitor of a traffic-light lamp system to detect failure of the light-emitting-diode lamp through detection of a high impedance of that lamp. 
     According to a first preferred embodiment: 
     the second controllable switching device comprises a power switch for supplying, when this power switch is closed, the light-emitting-diode lamp with electric power from the above mentioned source and thereby turning on the light-emitting-diode lamp, and a second impedance element connected in parallel with the power switch; 
     the source of electric power is an ac source, the light-emitting-diode lamp further comprises (a) a set of light emitting diodes, (b) a rectifier circuit section supplied with ac voltage and current from the ac source through the first controllable switching device and having an output for delivering rectified voltage and current, and (c) a power converter supplied with rectified voltage and current from the rectifier circuit section for producing dc voltage and current supplied to the set of light emitting diodes, and the shunt circuit section is connected between the output of the rectifier circuit section and the ground and is therefore supplied with rectified voltage and current from the rectifier circuit section; and 
     the shunt circuit section comprises a resistor forming the first low impedance element, a capacitor and the first controllable switching device connected in series. 
     In accordance with a second preferred embodiment of the present invention: 
     the detector circuit section comprises a comparator having a first input supplied with a predetermined voltage threshold, a second input supplied with a voltage across the capacitor, and an output for delivering a first signal when the amplitude of the voltage across the capacitor is lower than the predetermined voltage threshold, this first signal being indicative of turning off of the light-emitting-diode lamp and being supplied to the first controllable switching device to close that first switching device; 
     the set of light emitting diodes comprises a plurality of subsets of serially interconnected light emitting diodes, these subsets of serially interconnected light emitting diodes being connected in parallel; 
     the detector circuit section detects a dc current flowing through each subset of serially interconnected light emitting diodes and produces a second signal when no dc current is flowing through a predetermined number of subsets; and 
     the detector circuit section detects the amplitude of the dc current supplied to the set of light emitting diodes when the power switch is closed, produces a third signal when the amplitude of the dc current supplied to the set of light emitting diodes, when the power switch is closed, is higher than a predetermined current threshold and, in response to the second and third signals, prevents the first signal to reach the first controllable switching device to close that first switching device. 
     In accordance with a third preferred embodiment of the subject invention: 
     the detector circuit section comprises a comparator having a first input supplied with a predetermined voltage threshold, a second input supplied with a voltage across the capacitor, and an output for delivering a first signal when the amplitude of the voltage across the capacitor is lower than the predetermined voltage threshold, this first signal being indicative of turning off of the light-emitting-diode lamp and being supplied to the first controllable switching device to close that first switching device; 
     the detector circuit section (a) detects the amplitude of the ac voltage supplied to the rectifier circuit section when the power switch is closed, and produces a second signal when the amplitude of the ac voltage supplied to the rectifier circuit, when the power switch is closed, is higher than a predetermined voltage threshold, (b) detects the amplitude of the dc current supplied to the set of light emitting diodes when the power switch is closed, and produces a third signal when the amplitude of the dc current supplied to the set of light emitting diodes, when the power switch is closed, is higher than a predetermined current threshold; and 
     in response to the second and third signals, prevents the first signal to reach the controllable switching member to close that switching member. 
    
    
     The objects, advantages and other features of the present invention will become more apparent upon reading of the following non restrictive description of a preferred embodiment thereof, given by way of example only with reference to the accompanying drawings. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the appended drawings: 
     FIG. 1 is a schematic diagram of the electronic circuit of a LED (light-emitting-diode) lamp incorporating a fault-indicating impedance-changing circuit embodying the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Although the preferred embodiment of the present invention will be described hereinafter with reference to an application of a fault-indicating impedance-changing circuit according to the invention to a LED lamp, it should be kept in mind that this example is not intended to limit the range of applications of the present invention. 
     Referring to FIG. 1 of the appended drawings, the LED lamp is generally identified by the reference  1 . Lamp  1  comprises a set  2  of light emitting diodes such as  3 . The set  2  is formed of a plurality of subsets such as  4  of serially interconnected light emitting diodes  3 . The subsets  4  of serially interconnected light emitting diodes  3  are connected in parallel to each other to form the set  2 . 
     A current-to-voltage converter, namely a resistor  5  has a first terminal  6  connected to the cathode  7  of the last light emitting diode  3  of each subset  4 , and a second terminal  8  connected to a first terminal  9  of a current-to-voltage converter  10 . The current-to-voltage converter  10  has a second terminal  11  connected to the ground. As illustrated in FIG. 1, the current-to-voltage converter  10  is formed of two serially interconnected resistors  12  and  13 . 
     Those of ordinary skill in the art will appreciate that a voltage signal having an amplitude representative of the magnitude of the dc current flowing through each subset  4  of light emitting diodes  3  is produced across the corresponding resistor  5  and the serially interconnected resistors  12  and  13 , and is available on the terminal  6  of resistor  5 . In the same manner, those of ordinary skill in the art will appreciate that the two serially interconnected resistors  12  and  13  produce a voltage signal having an amplitude proportional to the magnitude of the current flowing through all the subsets  4  of light emitting diodes  3 . Of course, the serial resistors  12  and  13  can be replaced by a single resistor having a corresponding resistance value. 
     The set  2  of light emitting diodes  3  is supplied by an ac (alternating current) source  14 . Alternating voltage and current from the ac source  14  is supplied to a full-wave rectifier bridge  15  through a conflict monitor  16 , a load switching device  17 , and an overcurrent protection  18 . The alternating voltage and current from the ac source  14  is rectified by the full-wave rectifier bridge  15  and supplied to the anode  19  of the first diode  3  of each subset  4  through an ac-to-dc power converter  20 . As explained in the following description, the load switching device  17  comprises a controllable power switch  21  to selectively connect the ac source  14  to the lamp  1  in order to selectively switch the lamp  1  on and off. 
     As indicated in the foregoing description, the current flowing in all the subsets  4  of light emitting diodes  3  flow through the serial resistors  12  and  13  of the current-to-voltage converter  10 . Accordingly, the serial resistors  12  and  13  convert the total current flowing through the set  2  of light emitting diodes  3  into a corresponding current-representative voltage signal delivered on an output  22  of converter  10 . 
     The lamp  1  further comprises a power factor controller  23 . In the illustrated example, the controller  23  is the power factor controller manufactured and commercialized by the company Motorola and identified by the reference MC34262. To allow the power factor controller  23  to perform a current feedback control of the supply of the set  2  of light emitting diodes  3 , a linearizing circuit  24  is required. 
     The voltage/current characteristic of a light emitting diode is sensitive to temperature and the current through a light emitting diode changes very rapidly and non linearly with the voltage across this light emitting diode. For example, for a given type of light emitting diode widely used in the fabrication of traffic lights, a constant voltage of 1.8 volt will produce in the light emitting diode a current of about 7.5 mA at a temperature of −25° C., a current of about 20.5 mA at a temperature of +25° C., and a current of about 30 mA at a temperature of +60° C. The amplitude of the current through the light emitting diode at a temperature of +60° C. is therefore, for a constant voltage, about 1.6 time higher than the amplitude of the current at a temperature of +25° C. Voltage feedback control would therefore be very detrimental to the durability of light emitting diodes. 
     It is obvious from the foregoing description that voltage feedback control of the supply of a light emitting diode is not desirable, and that current feedback control is required to ensure durability of the light emitting diodes. 
     The controller  23  is not capable of conducting a direct current feedback control of non linear loads such as light emitting diodes. To enable the controller  23  to current feedback control the set  2  of light emitting diodes  3 , the linearizing circuit  24  is interposed between the output  22  of the voltage-to-current converter  10  and an input  25  of the power factor controller  23 . The function of the linearizing circuit  24  is to transform the non linear relation between the LED supply dc voltage at the output  26  of the power converter  20  and the dc current supplied to the set  2  of light emitting diodes  3  into a linear relation. 
     The linearizing circuit  24  is, in fact, a filter circuit formed of passive elements. More specifically, the linearizing circuit  24  comprises a resistor  27  having a first terminal  28  connected to the output  22  of the current-to-voltage converter  10 , and a second terminal  29  connected to the input  25  of the controller  23 . The linearizing circuit  24  also comprises a capacitor  30  connected between terminal  29  of the resistor  27  and the ground. To transform the non linear relation between the LED supply dc voltage at the output  26  of the power converter  20  and the dc current supplied to the set  2  of light emitting diodes  3  into a linear relation, the values of the resistor  27  and capacitor  30  must be precisely and carefully adjusted in relation to the current-to-voltage converting characteristic of the converter  10  and the voltage/current characteristic of the type of diode  3  being used. 
     By means of a simple filter circuit (linearizing circuit  24 ) integrated into the current feedback loop, the non linear charge (set  2  of light emitting diodes  3 ) is sensed by the controller  23  as a linear charge. More specifically, the input voltage feedback signal on the input  25  of the controller  23  varies linearly with the LED supply dc voltage at the output  26  of the power converter  20 . To current feedback control the supply of the set  2  of light emitting diodes  3 , the controller  23  requires on its input  25  a current-representative voltage feedback signal which varies linearly with the LED supply dc voltage at the output  26  of the power converter  20 . 
     Still referring to FIG. 1, the power converter  20  comprises an inductor device  31  having a core  32 , and a coil  33  supplied with full-wave rectified voltage and current from the rectifier bridge  15 . A second multi-tap coil  34  is wound onto the core  32  of the inductor device  31 . The coils  33  and  34  act as primary and secondary windings, respectively, of a transformer. Rectified voltage and current applied to the coil  33  will induce in the coil  34  rectified voltage and current transmitted to a capacitor  35  through a diode  36 . Electrical energy is stored in the capacitor  35  to convert the full-wave rectified voltage and current induced in the coil  34  to dc voltage and current supplied to the output  26  of the power converter  20  and therefore to the set  2  of light emitting diodes  3 . Diode  36  prevents return of the electrical energy stored in the capacitor  35  toward the coil  34 . The level of the dc voltage across the capacitor  35  and therefore the level of the LED supply dc voltage on the output  26  can be adjusted by selecting the appropriate tap  37  of the coil  34 . 
     Supply of coil  33  of the inductor device  31  is controlled by an output  38  of the controller  20  through a MOSFET power transistor  39 . The current supplying the coil  33  is converted to a voltage signal by a current-to-voltage converter  40  connected between transistor  39  and the ground. The current-to-voltage converter  40  comprises an output for supplying the voltage signal to an input  41  of the controller  23 . 
     The current through the coil  33  is also measured through an additional coil  42  also wound on the core  32  of the inductor device  31 . The current-representative voltage appearing across the additional coil  42  is supplied to an input  43  of the controller  23  through a resistor  44 . 
     The additional coil  42  is also connected to an accumulator, formed by a capacitor  48 , through a diode  46 . The function of the accumulator  48  is to supply an input  49  of the controller  23  with a dc voltage amplitude higher than a given minimum voltage reference to enable operation of the controller  23 . 
     The controller  23  is therefore responsive to the current-representative voltage feedback signal on its input  25 , to the voltage signal on its input  41 , to the current-representative voltage on its input  43 , and to the voltage across the capacitor  48  (input  49 ) to regulate the amplitude of the dc current supplied to the set  2  of light emitting diodes  3 . Referring to appended FIG. 1, the controller  23  has its output  38  connected to the MOSFET transistor  39  to control, through this MOSFET transistor, the level of the current through the coil  33  and thus the amplitude of the dc voltage on the output  26 . In particular, the controller  23  changes the amplitude of the dc voltage on the output  26  of the power converter  20  so as to maintain the dc current through the light emitting diodes  3  below a predetermined threshold. The durability of the light emitting diodes will not be prejudiced as long as the dc current through the light emitting diodes  3  is lower than this predetermined threshold. 
     The full-wave rectified current drawn through the coil  33  by the MOSFET transistor  39 , under the control of the power factor controller  23 , is proportional to the full-wave rectified voltage at the output of the full-wave rectifier bridge  15 . More specifically, the current waveform is sinusoidal and in phase with the voltage waveform so that the power factor is, if not equal to, close to unity. 
     Operation of the power factor controller MC34262 is believed to be otherwise well known to those of ordinary skill in the art and, accordingly, will not be further described in the present specification. 
     The conflict monitor  16  comprises a lamp controller  47  to control opening and closing of the power switch  21  to turn on or turn off the lamp  1 . 
     When the switch  21  passes from an open to a closed position, the lamp  1  is instantaneously supplied with electric power but the set  2  of light emitting diodes is supplied only after a start time. The duration of this start time is determined by the time required to charge the accumulator, more specifically the capacitor  48  to reach a minimum operation voltage of the power factor controller  23 . Indeed, as already mentioned in the foregoing description, the voltage across capacitor  48  is supplied to an input  49  of the power factor controller  23  and this power factor controller will not operate as long as the voltage applied to its input  49  has not reached this minimum operation voltage. When the voltage across the capacitor  48  has reached the minimum operation voltage, operation of the power factor controller  23  is authorized and power is transmitted to the set  2  of light emitting diodes  3  through the converter  20 . 
     Fast charging of the capacitor  48  is enabled by a low-impedance shunt element, for example a low-impedance resistor  50 . More specifically, capacitor  48  is charged through the low-impedance resistor  50  and a normally closed switching device  51 . Of course, the low-impedance resistor  50  accelerates charging of the capacitor  48  to reduce the start time. 
     As will be seen in more detail in the following description, a second function of the shunt circuit formed by the low-impedance resistor  50 , the s witching device  51  and the capacitor  48  is to establishing a low input impedance circuit of the lamp  1  when the power switch  21  of the load switching device  17  is open. The impedance of the amp  1  is sensed, when the power switch  21  is open, by detecting the voltage across the lamp  1  through a volmeter  52  of the conflict monitor  16 . The voltage detected by the voltmeter  52  is applied to the non-inverting input  53  of a comparator  54 . If the voltage across the lamp  1  exceeds a voltage threshold  55  applied to the inverting input  56  of the comparator  54 , a fault-indicating signal is produced on the output  57  of the comparator  54  to indicate to a safety system (not shown) that the lamp  1  has failed. In response to the fault-indicating signal from the output  57  of the comparator  54 , the safety system (not shown) causes all the red and yellow lamps of the traffic light to flash for thereby warning the automobilists crossing the corresponding junction. 
     Closing and opening of the switching device  51  is controlled by the output  59  of an “OR” gate  58 . More specifically, a low logic level on the output  59  will close the switching device  51  while a high logic level on the output  59  of the “OR” gate  58  will open the switching device  51 . As will be seen in the following description, the outputs of a flip-flop  60  and a comparator  61  are low following turning on of the lamp  1  to produce on the output  59  of the “OR” gate  58  a low logic level which closes the switching device  51 . 
     The voltage across the capacitor  48  is applied to the non-inverting input of the comparator  61 . When the amplitude of the voltage across the capacitor  48  exceeds the amplitude of a reference voltage  64  (for example equal to 1.5 time the above mentioned minimum operating voltage of the power factor controller  23 ) applied to the inverting input of the comparator  61 , a high logic level signal is produced on the output  62  of the comparator  61 . This high logic level signal is also supplied to a first input  63  of the “OR” gate  58  and transmitted to the switching device  51  to open the latter switching device. When the switching device  51  is open, the coil  42  of the inductor device  31  then forms an auxiliary supply to charge the capacitor  48  through the diode  46 . Coil  42  will maintain the charge of the capacitor  48  to a voltage higher than the reference voltage  64 . The switching device  51  remains open as long as the lamp  1  is turned on. During this period, the low-impedance resistor  50  is disconnected and supplied with no power to prevent a useless consumption of electric power. 
     When the lamp controller  47  opens the power switch  21 , the lamp  1  is still supplied through a shunt impedance element  65  connected in parallel to the power switch  21 . However, because of the voltage drop across the shunt impedance element  65 , the auxiliary supply (coil  42 ) is no longer capable of maintaining across the capacitor  48  a voltage higher than the minimum operating voltage of the power factor controller  23  and the lamp  1  turns off; of course the capacitor  48  discharge in the surrounding circuits. Then, the signal on the non-inverting input of the comparator  61  falls under the reference voltage  64  and the signal on the output  62  passes from a high to a low logic level to close the switching device  51 . As the impedance of the resistor  50  is low compared to the impedance of the shunt element  65 , the voltage measured by the conflict monitor  16  through the voltmeter  52  is lower than the voltage threshold  55  whereby the conflict monitor  16  detects a good condition of the lamp  1 . 
     Of course, the lamp  1  has been designed to ensure safety of the automobilists. In particular, the lamp  1  has been designed to detect the failure of a given number of light emitting diodes  3  to thereby ensure constant visibility of the lamp  1 . Experiments have demonstrated that following failure of more than 20% of the light emitting diodes  3 , that is loss of more than 20% of the luminous surface, the lamp  1  is no longer safe. Accordingly, the conflict monitor  16  must detect a failure of the lamp  1  when more than 20% of the light emitting diodes  3  have failed. 
     To be reliable, the circuit for detecting failure of more than 20% of the light emitting diodes  3  must be capable of operating within a temperature range located between −40° C. and +85° C. As light emitting diodes are very sensitive to variations of temperature and, for that reason, must be current-feedback controlled, a reliable manner to detect failure of the light emitting diodes  3  is to sense the current flowing therethrough. Selection of the number of subsets  4  of series-connected diodes  3  thus becomes an important design parameter. Obviously, those of ordinary skill in the art will appreciate that failure of a single light emitting diode  3  causes complete failure of the corresponding subset. 
     The maximum current the light emitting diodes  3  can withstand is 1.7 time the nominal current of these light emitting diodes. Since failure of a subset  4  of series-connected light emitting diodes  3  causes the dc current through the remaining subsets  4  to increase, another important design parameter is the maximum current that will be allowed to flow through the diodes  3 ; this upper limit has been fixed to 1.5 time the nominal current of the light emitting diodes. This design parameter has led to selection of a number of six subsets  4  of series-connected light emitting diodes  3 . 
     With a number of six subsets  4  of light emitting diodes  3 , failure of a first subset  4  causes loss of about 16% of the light emitting diodes  3 ; such a failure is acceptable. Then, the dc current amplitude in the remaining five subsets  4  of light emitting diodes  3  is equal to 1.2 time the nominal current. 
     Failure of a second subset  4  of light emitting diodes  3  causes loss of more than 20% of the light emitting diodes and the luminous surface; the dc current through the light emitting diodes  3  of the remaining subsets  4  is then 1.5 time the nominal current. Failure of more than 20% of the light emitting diodes  3  must be detected by the conflict monitor  16  since, in such a situation, the LED lamp  1  is no longer safe. 
     It should be pointed out that the luminous intensity produced by the light emitting diodes  3  is directly proportional to the magnitude of the dc current flowing through these diodes. Upon failure of a subset  4 , redistribution of the dc current in the remaining subsets  4  of light emitting diodes  3  prevents reduction of the total luminous intensity produced by the lamp  1 . Accordingly, safety of the lamp  1  upon failure of a subset  4  of light emitting diodes  3  is ensured by current-feedback control of the set  2  of light emitting diodes  3 . 
     The circuit for detecting failure of the subsets  4  of light emitting diodes  3  will now be described. 
     This failure detecting circuit comprises, for each subset  4 , a comparator such as  66  having an inverting input connected to the terminal  6  of the resistor  5  associated to the corresponding subset, a non-inverting input connected to a reference voltage  67 , and an output  68  connected to an input  69  of an adder  70 . The adder  70  has an output  71  connected to the non-inverting input of a comparator  72 . The inverting input of the comparator  72  is supplied with a reference voltage  73 , and the output  74  of the comparator  72  is connected to an input  75  of an “AND” gate  76 . The “AND” gate  76  has an output  81  connected to an input  77  of an “OR” gate  78 . The flip-flop  60  has a “Reset” input  80  connected to the ground and a “Set” input  82  connected to an output  79  of the “OR” gate  78 . 
     A comparator  83  has a non-inverting input connected to the output  22  of the current-to-voltage converter  10 , an inverting input supplied with a reference voltage  84 , and an output  85  connected to both an input  86  of the “AND” gate  76  and an input  87  of an “AND” gate  88  through an inverter  89 . The “AND” gate  88  has an output  90  connected to an input  91  of the “OR” gate  78 , and an input  97  connected to an output  96  of a comparator  94  having an inverting input supplied with a reference voltage  95 . The alternating voltage at the input of the full-wave rectifier bridge  15  is also rectified by a diode  92 , and this half-wave rectified voltage is supplied to the non-inverting input of the comparator  94  through a voltage divider  93 . 
     Upon failure of a subset  4  of light emitting diodes  3 , no current is flowing through the corresponding resistor  5  and voltage is no longer generated across this resistor  5 . Therefore the voltage supplied to the inverting input of the corresponding comparator  66 , which is higher than the reference voltage  67  as long as current is flowing through the subset  4 , lowers under this reference voltage  67  to produce on the output  68  a high logic level signal supplied to the associated input  69  of the adder  70 . 
     When two subsets  4  of series-connected light emitting diodes  3  fail, the two high logic level signals on the outputs  68  of the two corresponding comparators  66  are summed by the adder  70 . Then, the adder  70  delivers on the output  71  a signal having an amplitude higher than the reference voltage  73 . The comparator  72  then produces a high logic level signal supplied to the input  75  of the “AND” gate  76 . At that time, the current-representative voltage signal on the output  22  has an amplitude higher than the reference voltage  84  since feedback-controlled current is supplied to the set  2  of light emitting diodes  3 . A high logic level signal is therefore supplied to the other input  86  of the “AND” gate  76 . In response to the high logic level signals on its inputs  75  and  86 , the “AND” gate  76  produces on its output  81  a high logic level signal transmitted to the “Set” input  82  of the flip-flop  60  through the “OR” gate  78 . Flip-flop  60  then produces a high logic level signal on its output  98 , which high logic level signal is stored by the flip-flop  60  and transmitted to the switching device  51  through the “OR” gate  58  to lock this switching device  51  in the open position. When the power switch  21  is subsequently opened by the lamp controller  47 , the switching device  51  is locked in the open position whereby the lamp  1  presents a high input impedance. The voltage measured through the voltmeter  52  is then higher than the voltage threshold  55  so that the comparator  54  produces a high logic level signal on its output  57  to signal to the safety system (not shown) failure of the lamp  1 . Even if the lamp controller  47  subsequently closes the power switch  21 , the switching device  51  remains open to prevent turning on of the lamp  1 . As explained in the foregoing description, the switching device  51  must be closed to enable the accumulator (capacitor  48 ) to charge to the minimum operating voltage of the power factor controller  23 . 
     Accordingly, a failure of more than 20% of the light emitting diodes  3  is detected to lock the switching device  51  in the open position. This allows the conflict monitor  16  to detect failure of the lamp  1  and to signal this failure to the safety system (not shown). 
     In the same manner, failure of a component of the power supply circuit of the lamp  1  will be detected by the conflict monitor  16 . 
     The amplitude of the alternating voltage at the input of the full-wave rectifier bridge  15  is first detected. For that purpose, this alternating voltage is half-wave rectified by the diode  92  and supplied to the non-inverting input of the comparator  94  through the voltage divider  93 . The reference voltage  95  has an amplitude representative of a minimum alternating voltage amplitude required to operate the lamp  1 . If the amplitude of the alternating voltage at the input of the full-wave rectifier bridge  15  is higher than this minimum alternating voltage amplitude, the comparator  94  produces on its output  96  a high logic level signal supplied to the input  97  of the “AND” gate  88 . On the contrary, if the amplitude of the alternating voltage at the input of the full-wave rectifier bridge  15  is lower than the minimum alternating voltage amplitude required to operate the lamp  1 , the comparator  94  produces on its output  96  a low logic level signal supplied to the input  97  of the “AND” gate  88 . Normally, the minimum voltage amplitude corresponds to 70% of the nominal alternating voltage of the network. 
     Supply of the set  2  of light emitting diodes  3  is also detected. As indicated in the foregoing description, the current-representative voltage signal on the output  22  has an amplitude higher than the reference voltage  84  when the set  2  of light emitting diodes  3  is supplied with feedback controlled current. The output  85  of the comparator  83  then delivers a high logic level signal to the inverter  89  to thereby supply the input  87  of the “AND” gate  88  with a low logic level signal. On the contrary, the current-representative voltage signal on the output  22  of the current-to-voltage converter  10  has an amplitude substantially equal to zero when no current is supplied to the set  2  of light emitting diodes  3 . The output  85  of the comparator  83  then delivers a low logic level signal supplied to the inverter  89  to thereby supply the input  87  of the “AND” gate  88  with a high logic level signal. 
     When no current is supplied to the set  2  of light emitting diodes  3  (high logic level signal on the input  87  of the “AND” gate  88 ) and the amplitude of the alternating voltage at the input of the rectifier bridge  15  is higher than the minimum alternating voltage amplitude required to turn the lamp  1  on (high logic level signal on the input  97  of the “AND” gate  88 ), failure of the power supply circuit of the lamp  1  should be signalled to the conflict monitor  16 . Since a high logic level signal appears on both the inputs  87  and  97 , the “AND” gate  88  produces on its output  90  a high logic level signal transmitted to the “Set” input  82  of the flip-flop  60  through the “OR” gate  78 . Flip-flop  60  then produces a high logic level signal on its output  98 , which high logic level signal is stored by the flip-flop  60  and transmitted to the switching device  51  through the “OR” gate  58  to lock this switching device  51  in the open position. When the switch  21  is subsequently opened by the lamp controller  47 , the switching device  51  is locked in the open position whereby the lamp  1  presents a high input impedance. The voltage measured through the voltmeter  52  is then higher than the reference voltage  55  so that the comparator  54  produces a high logic level signal on its output  57  to signal to the safety system (not shown) failure of the lamp  1 . 
     Although the present invention has been described hereinabove by way of a preferred embodiment thereof, this embodiment can be modified at will, within the scope of the appended claims, without departing from the spirit and nature of the subject invention.