Abstract:
A system and method for controlling a matrix of light emitting diodes (LED) connected to an input line comprises a power converter for connecting to the matrix of LEDs and to the input line therebetween and for receiving from the input line an input current and an input voltage characterized by a shape and a frequency and for providing a direct current (D.C.) output for powering up the LEDs, yielding an operating current through the LEDs. The power converter includes a first current sensor for sensing the input current and a second current sensor for sensing the operating current. The system further comprises a controller for connecting to both the input line and to the power converter. The controller includes a voltage sensor for sensing the input voltage and a pre-regulator i) for receiving the operating current, the input current and the input voltage, ii) for biasing the operating current towards a target current value, and iii) for regulating the power converter to cause the input current to follow the shape and frequency of the input voltage, yielding a unity power factor and minimizing the input current harmonic distortion. The present method and system allows maximizing the energy savings, controlling current flowing in the diodes so as to obtain the maximum flux of light with the minimum energy and also allows meeting all safety, EMI, reliability and robustness requirements.

Description:
FIELD OF THE INVENTION 
     The present invention relates to streetlights or the like provided with a matrix of light emitting diodes. More specifically, the present invention is concerned with a system and method for controlling such matrix. 
     BACKGROUND OF THE INVENTION 
     The conventional streetlight, provided with metal halide, mercury or sodium filled bulb suffers from few disadvantages. A first disadvantage is the relatively high energy consumption. Another one is the relatively short life of the bulb. Indeed, after a few years of operation the bulb fails and needs to be replaced. 
     Matrices of light emitting diodes (LEDs) have been introduced in streetlights as a replacement solution to the conventional bulbs. However, the power controlling of current LED matrix in streetlight has been found inefficient, resulting in lost of energy and of light flux for a given input power consumption. 
     More efficient system and method for controlling a matrix of light emitting diodes are thus desirable. 
     OBJECTS OF THE INVENTION 
     An object of the present invention is therefore to provide an improved system and method for controlling a matrix of light emitting diodes. 
     Another object of the present invention is to provide improved streetlights or improved lights provided with a light emitting diode matrix. 
     SUMMARY OF THE INVENTION 
     According to the present invention, there is provided a method and system for controlling a matrix of light emitting diodes in a streetlight or the like. The present method and system allows maximizing the energy savings. Moreover, it allows controlling current flowing in the diodes so as to obtain the maximum flux of light with the minimum energy and also allows meeting all safety, EMI, reliability and robustness requirements. 
     For example, a streetlight provided with a matrix of light emitting diodes and a system for controlling such a matrix according to the present invention provides significant energy savings and a useful life that is more then 10 times higher compared to the conventional high pressure sodium or mercury lamps. One major advantage is that the light efficiency is much higher. Therefore a streetlight according to the present invention generates a large economy of energy in the order of 80% compared to streetlights provided with bulb lamps. A second advantage is the longer life of the diodes matrix. A high pressure sodium bulb has only a few years of useful life while the light emitting diode has more then 20 years of useful life. This allows significantly reducing the maintenance cost, reducing the scrap and increasing the road safety. 
     More specifically, in accordance with the present invention, there is provided a system for controlling a matrix of light emitting diodes (LED) connected to an input line, the system comprising: 
     a power converter for connecting to the matrix of LEDs and to the input line there between and for receiving from the input line an input current and an input voltage characterized by a shape and a frequency and for providing a direct current (D.C.) output for powering up the LEDs, yielding an operating current through the LEDs; the power converter including a first current sensor for sensing the input current and a second current sensor for sensing the operating current; 
     a controller for connecting to both the input line and to the power converter; the controller including a voltage sensor for sensing the input voltage and a pre-regulator i) for receiving the operating current, the input current and the input voltage, ii) for biasing the operating current towards a target current, and iii) for regulating the power converter to cause the input current to follow the shape and frequency of the input voltage, yielding a unity power factor and minimizing the input current harmonic distortion. 
     According to a second aspect of the present invention, there is provided a system for controlling a matrix of light emitting diodes (LEDs) connected to an input line, the system comprising: 
     converter means for connecting to the matrix of LEDs and to the input line there between and for receiving from the input line an input current and an input voltage characterized by a shape and a frequency and for providing a direct current (D.C.) output for powering up the LEDs, yielding an operating current through the LEDs; 
     first sensing means for sensing the input current; 
     second sensing means for sensing the operating current; 
     third sensing means for sensing the input voltage; and 
     controller means for connecting to both the input line and to the converter means i) for receiving the operating current, the input current and the input voltage, ii) for biasing the operating current towards a target current, and iii) for regulating the converter means to cause the input current to follow the shape and frequency of the input voltage, yielding a unity power factor and minimizing the input current harmonic distortion. 
     According to a third aspect of the present invention, there is provided a method for controlling a matrix of light emitting diodes (LED) connected to an input line, the method comprising: 
     measuring from the input line an input current; 
     measuring from the input line an input voltage characterized by a shape and a frequency; 
     providing a LED target current; 
     converting the input line voltage into a direct current (D.C.) output voltage for powering up the LEDs, yielding an operating current through the LEDs, by forcing the input current to follow the shape and frequency of the input voltage, yielding a unity power factor and minimizing the input current harmonic distortion; 
     measuring an operating current through the LEDs; and 
     biasing the operating current towards the LED target current. 
     Other objects, advantages and features of the present invention will become more apparent upon reading the following non restrictive description of illustrated embodiments 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 view of a streetlight unit according to a first illustrative embodiment of the present invention; 
         FIG. 2  is a circuit diagram illustrating the electromagnetic interference (EMI) filter of the streetlight unit from  FIG. 1 ; 
         FIG. 3  is a circuit diagram illustrating the power converter of the streetlight unit from  FIG. 1 ; 
         FIG. 4  is a circuit diagram illustrating an auxiliary power supply of the streetlight unit from  FIG. 1 ; 
         FIGS. 5A-5B  are circuit diagrams illustrating the power converter controller of the streetlight unit from  FIG. 1 ; 
         FIGS. 6A ,  6 B,  6 C and  6 D are graphs illustrating respectively the steady state wave forms at nominal input utility line, the start up wave forms at low utility line, the load transient wave forms and the utility line drop out wave forms of the streetlight unit from  FIG. 1 ; channel  1  representing the input voltage measurement, channel  2  representing the output voltage measurement, channel  3  representing the input current measurement and channel  4  representing the output current measurement; 
         FIG. 7  is a circuit diagram illustrating an electromagnetic interference (EMI) filter part of a system for controlling a matrix of light emitting diodes according to a second illustrative embodiment of the present invention; 
         FIG. 8  is a circuit diagram illustrating a power converter part of the system for controlling a matrix of light emitting diodes according to the second illustrative embodiment of the present invention; 
         FIGS. 9A-9B  are circuit diagrams illustrating a power converter controller part of the system for controlling a matrix of light emitting diodes according to the second illustrative embodiment of the present invention; and 
         FIGS. 10A ,  10 B and  10 C are graphs illustrating respectively the steady state wave forms at nominal input utility line (input current and voltage), the start up wave forms at low utility line (input voltage and output current) and the flyback main transistor wave forms (voltage and current) of the streetlight according to the second illustrative embodiment of the present invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A streetlight unit  10  according to a first illustrative embodiment of the present invention will now be described with reference to  FIG. 1  of the appended drawings. 
     The streetlight unit  10  comprises a matrix of light emitting diodes (LEDs) 12  connected to the A.C. (alternating current) utility network  14  via a power converter  16 , and a controller  18  for the power converter  16 . 
     The matrix of LEDs  12  includes a combination of diodes connected in series and in parallel (not shown). This connection arrangement of diodes provides a significant improvement to the reliability and life of the streetlight  10  compare with a conventional streetlight provided with a matrix of LEDs. Indeed, the parallel connection of the diodes (for example 2 to 20) assures that even if one diode is failing short or open, the remaining matrix is not affected by the failure; the streetlight  10  can still operate, with only a small degradation of luminescence. The streetlight  10  can however operate beyond its stated and rated life if the LEDs would all have been connected in series. 
     The series connections (for example 2 to 250) allow driving the LEDs  12  with a high DC voltage and therefore simplifying the power converter  16  and improving its efficiency. 
     The streetlight unit  10  will now be described in more detail with reference to  FIGS. 2 to 6 . 
     The streetlight unit  10  further includes an electromagnetic interference (EMI) filter  20 , which is illustrated in  FIG. 2 , connected to the A.C. utility  14  at the input of the power converter  16 . The EMI filter  20  together with the power converter  16  and controller  18  define a system for controlling a matrix of LEDs. 
     The filter  20  includes two differential mode capacitors C 2  and C 3 , five common mode capacitors C 9 , C 10 , C 11 , C 12  and C 13  and a common mode inductor L 2 , the leakage inductor of this magnetic element L 2  further acting as a differential mode filter. It is to be noted that the capacitor C 4  and C 5  of the power converter  18  are also used for the EMI concerns. The EMI filter  20 , in association with the proper layout, such as the one described in  FIGS. 1-4 , renders the unit  10  conformed to the EMI American and European specifications (FCC part 15, EN55022/CISPR 22 and CSA C108). Since such specifications are believed to be well known in the art, and for concision purposes, they will not be described herein in more detail. 
     The unit  10  is also designed to be conformed to the well-known IEEE C62.41 specifications allowing it to handle most type of utility disturbances without any damage, including lightning strikes (typically 6000V, 3000 A, 50 microseconds). For that purposes, the EMI filter  20  includes three transient voltage suppressors MOV 1 , MOV 2  and MOV 3  (see  FIG. 2 ) which are coupled to a diode D 1  of the power converter  16 . The diode D 1  helps transferring some of the lightning energy to the output capacitor formed by C 1  and C 6  in series (see  FIG. 3 ). This allows increasing the MOV&#39;s life time and decreasing the over voltage stress on all the power converter semiconductors including its input diode bridge D 4 , D 5 , D 8  and D 9 . Indeed, decreasing the maximum voltage constraint on the power semiconductor contribute to increasing their life time and also the overall efficiency of the converter  16 . 
     Returning to  FIG. 2 , two input line fuses F 1  and F 2  are used to prevent damage inside the unit  10 . A gas arrester GA 1  is also provided to minimize the leakage current of the transient voltage suppressors MOV 2  and MOV 3 , thereby increasing their life time and permitting to test the line to chassis isolation without damaging the MOVs. Then, for the safety, the converter further has the VDE, CSA and UL certifications. 
     Finally, the input  22  of the power converter  16  includes a negative temperature coefficient (NTC) resistor to control the inrush current during the start-up. The unit  10  is configured conformably to the specifications IEC-1000-2-3 and EN60555 part 2, regarding the quality of the input current wave form. Since such specifications are believed to be well known in the art, and for concision purposes, they will no be described herein in more detail. 
     The input  22  of the power converter  16  is connected at the AC utility  14  (VAC 1 , VAC 2 ). The converter  16  provides a DC output that is used to power up the LEDs  12 . The input frequency and input voltage is converted into DC voltage and current to properly drive the LEDs  12  to maximize the luminescence. As will be explained herein below in more detail, measures of both the input voltage and current are sent to the controller  18  to allow for a unity power factor and to minimize the input current harmonic distortion. The controller  18  forces the input current to follow the input voltage, forces also the LEDs current set pointo extract a maximum luminescence and manages all the utility  14  disturbances (Start-Stop, Swell, Sag and Surge). This provides the robustness to withstand the utility transient. 
     Turning now to  FIG. 3 , the power converter  16  will now be described in more detail. As will become more apparent upon reading the following description, the converter  16  is in the form of a boost converter, adapted for a matrix of LEDs including a large number of LEDs, such as 200 or more. In addition to a streetlight, applications for a matrix including such a large number of LEDs includes without limitations lights for a highway, a play-ground, a monument, an indoor parking, pathways, building, and flood and area type lighting fixtures and luminaries. 
     The power converter  16  includes an input diode bridge formed by diodes D 4 , D 5 , D 8  and D 9 , the primary of a transformer L 1 , an active switch M 1  and a boost converter output diode D 2 . Any transistor technology, such as IGBT (insulated gate bipolar transistor), MOSFET (metal-oxide semiconductor field-effect transistor) or bipolar transistor (BIPOLAR) can be used for the active switch M 1 . 
     The switch M 1  is modulated at high fixed frequency to force the input current to follow the input voltage. The current for the LEDs is set for maximum luminescence and minimum input power. The input current is sensed by three resistors connected in parallel R 16 , R 17  and R 18 , while the LEDs operating current is sensed by R 11  and R 12  in parallel. Both current measurements are sent to the controller  18 . 
       FIG. 4  illustrates a low cost high frequency auxiliary power supply  24  including the L 1  secondary winding associated with the network, resistor devices R 22 , R 23 , R 26 , R 27  and R 33 , diodes D 12  and D 15 , capacitors C 20 , C 21 , C 22  and C 23 . The power supply  24  is configured so that its output voltage is automatically regulated proportionally to the output voltage of the power converter  16 . 
     The controller  18  of the converter  16  will now be described in more detail with reference to  FIGS. 5A-5B . 
     The controller  18  includes a power factor pre-regulator  26  and an input line voltage sensor  28  in the form of three resistors in series (R 38 , R 39  and R 40 ) connected to the pre-regulator  26  as an input thereof. The controller  18  biased the power converter  16  towards a unity power factor and a low THD (total harmonic distortion). The controller  18  senses via the sensor  28  the input line voltage and regulates the converter  16  to cause the input current to follow the shape and frequency of the input voltage. It is to be noted that the zero and pole for the input current controller are fixed by R 24 , R 31 , R 34 , C 15  and C 17 . This yields a unity power factor (higher then 0.99 at nominal AC line input voltage, higher then 0.97 for all input voltage range “nominal voltage±15%”) and also a low THD, which is less then 5% at nominal AC line input voltage. 
     Even though the illustrative embodiment of  FIG. 5A  includes a UCC3817 from Texas Instrument as the pre-regulator  26 , any power factor pre-regulator can be used to control the input current wave shape and to regulate the input power. 
     As described hereinabove, the output voltage (+VDC) is in the form of a high voltage DC output. One conventional way to drive the LEDs  12  is to insert a resistor in series with the diodes  12  and then to drive the LEDs  12  by a voltage source. The disadvantage of such method is a variation of current through the LEDs  12  with the input voltage, the component variations and the temperature. This variation of current through the LEDs  12  would cause a variation of luminescence from the diodes  12 . The flux of light would then vary with some internal and external parameters. Since the voltage drop of the LEDs  12  varies with the temperature, the resulting current would then vary accordingly. Also the luminescence of the diode decreases with temperature. 
     Since the LEDs  12  require a specific current to generate the light, the controller  18  according to the present invention is configured to drive the LEDs  12  with a precise current as opposed to a precise voltage. 
       FIG. 5B  illustrates a LEDs voltage and current controller  30 , part of the power converter controller  18 . In a nutshell, the current of the LEDs matrix  12  is monitored as well as the temperature of the diodes. The controller  18  processes this information and controls the converter  16  to assure that the LEDs  12  are driven by a DC current with a maximum of luminescence. This allows optimizing the light output of the LEDs  12  while taking a minimum input power. 
     The zero and pole for the LEDs voltage and current controller  30  are determined by R 30 , R 43 , C 24  and C 27  from the controller  18 . 
     Turning back briefly to  FIG. 3 , a measure of the current is performed at R 11  and R 12  in parallel and transmitted to UlA  32  by V_IOUT. The output voltage of UlA  32  is proportional to the LEDs current [IOUT×(1+R 29 /R 28 )]. UlA  32  allows the controller  18  to maintain the current to a very stable nominal target value. 
     A temperature sensor  33  (see  FIG. 1 ) detects the operating temperature of the LEDs  12  and a modification to the nominal target current is done to assure the optimum luminescence of the LEDs  12  is achieved with different ambient temperature. The temperature sensor  33  may take measures at fixed or variable time intervals. Those intervals may also vary depending on the climate where the light  10  is installed. Of course, more precise temperature measurements may yield both a better luminescence and a better life time of the light  10 . 
     The resistor R 28  can be replaced by a digitally controlled variable resistor EEPOT (Electrically Erasable Potentiometers), allowing to selectively increase the LEDs current by increasing the variable resistor. 
     In addition, the nominal target current may be adjusted with time to cope with the aging of the LED matrix  12 . The target values or a predetermined algorithm allowing to obtained such values may be stored in a memory (not shown) coupled with the controller  18 . The time adjustment may be based on the number of powering ups of the matrix  10 . This feature allows uniform luminescence over time even though the luminescence of the diodes may vary with time. 
     The controller  18  offers a dual mode of regulation. Indeed, as described hereinabove, the normal regulation is with the LEDs  12  current. But to protect the LEDs  12  from failing and to avoid a high voltage thereon, which can damage them, the controller  18  is configured to switch over a voltage regulation mode. UlB  34  (see  FIG. 5B ) then regulates the controller  18  to assure a selected voltage is not surpassed and indeed will protect the LEDs  12  if multiple failures occur. UlA  32  sends the information to the controller  18  when the output voltage reaches a pre-determined safety value. 
     As stated hereinabove, the power converter  16  is rugged under AC line voltage disturbances. Indeed, the controller  18  offers protection in case of high voltage present on the input or high current being drawn from the line  14 . In such cases the switch Ml momentarily stops functioning to assure the disturbance is passing through without overstressing any components. 
     Experimental wave form results obtained using the streetlight unit  10  are shown in  FIGS. 6A-6D . 
     More precisely,  FIGS. 6A ,  6 B,  6 C and  6 D are graphs illustrating respectively the steady state wave forms at nominal input utility line, the start up wave forms at low utility line, the load transient wave forms and the utility line drop out wave forms of the streetlight unit  10 . 
     In  FIGS. 6A-6D , channel  1  represents the input voltage measurement, channel  2  represents the output voltage measurement, channel  3  represents the input current measurement and channel  4  represents the output current measurement. 
     The experimental values have been obtained using a system for controlling a matrix of LEDs according to the first illustrative embodiment of the present invention similar to the system  10 , configured to control a matrix of LEDs of 90 Watts and having an operating range between 176 Vrms and 295 Vrms. 
       FIG. 6A  shows that the waveforms of the input current (channel  3 ) and of the input voltage (channel  1 ) are identical, yielding a unity power factor and allowing to minimize the harmonic distortion.  FIG. 6A  also shows that the output current (channel  4 ) is a well-controlled D.C. current. 
       FIG. 6B  shows a minimum of a bout 10 to 20 minutes are required, in the case of sodium or mercury-based bulb, to achieve a maximum of illumination intensity when an input voltage is applied. Less than two (2) seconds are required to achieve maximum illumination using a controlling system according to the present invention. 
       FIG. 6C  shows that both the input and output currents remain under control even when the matrix of LEDs is connected or disconnected while the converter remains alive. 
     Finally,  FIG. 6D  illustrates the controlled extinction of the matrix during a utility power outage 
     A system for controlling a matrix of LEDs according to a second illustrative embodiment of the present invention will now be described with reference to  FIGS. 7 to 9B . Since the LEDs matrix controlling system according to this second illustrative embodiment is similar to the one described in reference to the streetlight  10 , and for concision purposes, only the differences between the two systems will be described herein in more detail. 
     The LEDs matrix controlling system according to the second illustrative embodiment shares the same general layout as the unit  10  as described shown in  FIG. 1 . It includes an EMI filter  36  (see  FIG. 7 ) at the input stage, which, in association with proper layout, allows the unit to be conformed to the EMI American and the European specifications (FCC part 15, EN55022/CISPR 22 and CSA C108, a power converter  38  (see  FIG. 8 ), in the form of a flyback converter, and a controller  40  for the converter (see  FIGS. 9A-9B ). While the filter  20 , power converter  16  and controller  18  are together particularly suitable for controlling a matrix having a large number of LEDs  12 , such as 200 or more, the filter  36 , power converter  38  and converter  40  are together particularly suitable for controlling a matrix having a number of LEDs lower than 5000. Applications for such a controlling system includes traffic signal lights, train signalization lights, residential lights, industrial building lights, office lights, etc. 
     The filter  36  includes two differential mode capacitors C 1  and C 2 , and a differential mode inductor L 1 . The capacitors C 10  and C 5 , which are part of the converter  40  (see  FIG. 8 ) are also used for the EMI concerns. The unit is designed to prevent damage under utility disturbances. More specifically, the filter  36  includes two transient voltage suppressors MOV 1 , MOV 2  coupled to the resistors R 1 , R 2 , R 5  and R 6 , which would generate for example less then a quarter watt losses for a matrix including  400  LEDs. These resistors limit lightning current circulating into MOV 1  and MOV 2 . This technique allows decreasing the over voltage stress on all the semiconductors of the power converter  38 . Two input line fuse F 1  and F 2  are used to prevent any catastrophic damage inside the LEDs controlling system. For further safety purposes, the converter can have the VDE, CSA and UL certifications. Since VDE, CSA and UL certifications are believed to be well known in the art, and for concision purposes, they will not be described herein in more detail. 
     The power converter  38  will now be described in more detail with reference to  FIG. 8 . The power converter is in the form of a flyback converter having an input diode bridge  42  (D 1 , D 2 , D 6  and D 7 ), a transformer T 1 , an active switch Q 1  and two output diodes D 3  and D 4 . The active switch Q 1  can take many forms, including without limitations IGBT, MOSFET and BIPOLAR. 
     The transformer T 1  extra secondary winding associated with D 5  and C 8  represents a low cost high frequency auxiliary power supply. According to this configuration, the output voltage of the auxiliary power supply is automatically regulated proportionally to the output voltage. 
     The network formed by D 8 , D 9 , R 7 , R 10 , R 12 , R 14  and C 6  helps to clamp the voltage across the switch Q 1 ; the transformer leakage inductor energy being damped by this network. 
     The switch Q 1  is modulated at a high predetermined frequency to force the input current, in association with the input EMI filter  36 , to follow the input voltage. The current for the LEDs is set at the optimal point for maximum luminescence and minimum input power. 
     The converter controller  40  will now be described with reference to  FIGS. 9A-9B . 
     The controller  40  is configured so as to yield a unity power factor and a low THD. Considering a maximum duty cycle of 50% and that this duty cycle is fixed for at least half period of the utility frequency (10 or 8.33 milliseconds for 50 Hz or 60 Hz respective frequency), this yields a unity power factor (higher then 0.97 at nominal AC line input voltage, higher then 0.95 for all input voltage range “nominal voltage±15%”) and also a low THD, which will be less then 10% at nominal AC line input voltage. To achieve these performances, any fixed frequency pulse width modulator with 50% maximum duty cycle can be used to control the input current wave shape and to regulate the output current. For example, the UCC3851 from Texas Instrument 44 can be used for such purposes. 
     To ensure high robustness against line disturbances some extra protections are implemented. Then to avoid transformer saturation, the transistor peak current limit is implemented. More specifically, a measurement network is formed in the power converter  38  by R 17 , R 16 , C 9 , and the threshold is set by Vref, R 41 , R 42  and C 27 . To keep the main transistor  44  in a safe operating area, fast high input voltages detect is implemented via R 28 , R 29 , R 30  and C 21 . It is to be noted that the duty cycle can be limited cycle by cycle. 
     Experimental wave form results obtained using the LEDs matrix controlling system according to the second illustrative embodiment of the present invention are illustrated in  FIGS. 10A-10C . 
       FIGS. 10A ,  10 B and  10 C are graphs illustrating respectively the steady state wave forms at nominal input utility line (input current and voltage), the start up wave forms at low utility line (input voltage and output current) and the flyback main transistor wave forms (voltage and current) of the streetlight according to the second illustrative embodiment of the present invention. 
     The experimental values have been obtained using a controlling system according to the present invention having components similar to those described with reference to  FIGS. 7-9B  configured to control a matrix of LED of 16 Watts and having an operating range between 176 Vrms and 300 Vrms. 
       FIG. 10A  shows that the waveforms of the input current (channel  2 ) and of the input voltage (channel  1 ) are identical, yielding a unity power factor and allowing to minimize the harmonic distortion. 
       FIG. 10B  shows that a maximum delay of about 0.3 second is required to achieve maximum illumination after applying the input voltage. This is one of the reasons why the system for controlling a matrix of LEDs according to the second illustrative embodiment of the present invention is particularly interesting in signalization applications (including road, railway and ocean signalization). 
       FIG. 10C  shows that the cycle ratio is fixed and inferior to 50% (on at least half a cycle), that the current is discontinuous, and that the voltage at the transistor&#39;s terminal is clamped. 
     Even though the present invention has been described by way of reference to illustrative embodiments wherein the input line has been in the form of an A.C. utility line, it can be connected to any type of input line, including a D.C. line. 
     Although the present invention has been described hereinabove by way of illustrative embodiments thereof, it can be modified without departing from the spirit and nature of the subject invention, as defined in the appended claims.