Abstract:
An LED traffic signal having a single switching power supply placed inside the load switch to supply power to each of the aspect signals. The traffic signal has several aspects, such as red, yellow and green. The load switch comprises a switching power supply to supply power to the signals, an output selection circuit to select the desired aspect, and a conflict monitor interface circuit to mimic an incandescent circuit.

Description:
[0001]    This application claims the benefit of U.S. Provisional Application No. 60/383262 filed May 24, 2002. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention relates to traffic signals and the electrical power control circuitry driving them. In particular, this invention deals with light emitting diode (LED) traffic signals controlled via standardized NEMA TS2 load switches. More specifically, this invention introduces a system where a single power supply placed inside the load switch replaces the three power supplies of three conventional LED traffic signals connected to the load switch.  
           [0004]    2. Description of Related Art  
           [0005]    LED traffic signals are gaining in popularity, replacing the prior designs using incandescent bulbs. Driven by the stable current and voltage levels produced by switching power supplies, LED traffic signals consume relatively low amounts of power and have extremely long lifetimes compared to standard incandescent light bulbs.  
           [0006]    Previously, LED traffic lights have been used as a direct retrofit for previously existing incandescent light bulbs. Because most signal installations normally have at least three traffic signals per load switch (red, yellow and green signals), at least three LED power supplies are required. Considering that there is only one traffic signal activated at a time, it is possible, via the present invention, to use only a single power supply per load switch, resulting in significant manufacturing and operating power consumption cost savings.  
         SUMMARY OF THE INVENTION  
         [0007]    The present invention is composed of two main parts: the load switch and the traffic signals. The load switch has a standard NEMA TS2 type connector, a switching power supply, an output selection circuit and a conflict monitor interface circuit. The power supply turns on as soon as the load switch is plugged into the intersection control cabinet. The output selection circuit is activated by the intersection controller whenever it wants to switch a traffic signal on or off. The conflict monitor interface circuit monitors the current through the traffic signal to relay the desired “incandescent bulb” state to the load switch outputs.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]    In order to fully describe the invention, a specific embodiment is provided in schematic form.  
         [0009]    [0009]FIG. 1 is a block diagram of the invention.  
         [0010]    [0010]FIG. 2 is an electrical circuit diagram of the power supply.  
         [0011]    [0011]FIG. 3 is an electrical circuit diagram of the output selection circuit.  
         [0012]    [0012]FIG. 4 is an electrical circuit diagram of the conflict monitor interface circuit.  
         [0013]    [0013]FIG. 5 is an electrical circuit diagram of the traffic signal to be used with the LED traffic signal load switch. 
     
    
     DETAILED DESCRIPTION  
       [0014]    [0014]FIG. 1 shows one possible embodiment of the invention in block form. The power supply  11  is directly connected to the mains line. Therefore, as soon as the load switch  10  is connected, the power supply  11  will be on and ready for use. The output Vp of the power supply  11 , is connected to the output selection interface  12 . The output selection interface  12  is controlled by the intersection controller (existing) through load switch  10 . Whenever the intersection controller asks that a traffic signal be activated, the output selection interface  12  applies the power supply  11  output Vp to the selected traffic signal  14 ,  15  or  16 . More than one traffic signal may be hooked up to a single output, for example in the case of opposing intersection signals. Because the number of signals driven by the same switch is unlimited, the single power supply per switch is sized for the resulting load. The conflict monitor interface circuit  13  will then operate its circuitry to make sure that the conflict monitor receives feedback as to which traffic signal is on and which is off.  
         [0015]    [0015]FIG. 2 shows a sample embodiment of the power supply  11 . It is a switching power supply, having a high power factor, low harmonic distortion and an output regulated at 48 Vdc. When the intersection controller does not activate any output, the amount of power drawn from the main line is very small. The power supply is also fitted with a circuit to control the main voltage at which it turns on. If the line voltage is below a certain point, it is considered that the power supply will not work properly and therefore it is self de-activating. Whenever the main line voltage is high enough, the power supply  11  is allowed to function normally. Protection from transitory voltage spikes is provided by metal oxide varistors. A fuse provides over-current protection at the mains line connection.  
         [0016]    [0016]FIG. 3 shows a sample embodiment of the output selection interface  12 . The same circuit is repeated three times, once for each output. Therefore, only one of the three circuits will be explained in detail. In order to preserve isolation between the intersection controller and the load switch  10 , an opto-MOS U 101  is used. An opto-MOS is an opto-isolator driving a MOSFET all in one package. This enables easy switching of the positive output of the power supply to the selected traffic signal with very low power loss in the switch itself. Resistor R 101  is selected to limit the current through the opto-MOS emitter portion. LED LD 101  is used to give a visual feedback of the state of the load switch to a person working at the intersection controller cabinet. Resistor R 102  is used to sense the current through the traffic signal.  
         [0017]    [0017]FIG. 4 is a sample embodiment of the conflict monitor interface circuit  13 . Here again, the same circuit is repeated three times. The purpose of this circuit is to make the conflict monitor believe that an incandescent bulb is connected to the intersection controller terminals. From an electrical point of view, an incandescent light bulb is a resistor. Therefore, the conflict monitor expects to see more than 80 Vac when a lamp is on and less than 15 Vac when a lamp is off. To mimic this result, resistors are switched from the output terminals to either the live main wire or the neutral main wire.  
         [0018]    [0018]FIG. 5 is a sample embodiment of the traffic signal circuit. Because the power supply  11  generates a 48 Vdc bus, the traffic signal only has to regulate the current through the LEDs. This is accomplished using a current regulator  50 , for example, a LM317K. A diode bridge is used at the input of the traffic signal to allow for the possible inversion of the wire coming from the load switch to the traffic signal during installation. The diode bridge will condition the input voltage so that even if the positive output of the power supply  11  is applied to Vn and the negative output power supply  11  is applied to Vp, the traffic signal will still operate. LED array  52  may be composed of at least one chain of at least one LED. The only restriction on the number of LEDs in a chain is that the sum of the forward voltage of the LED(s) of the chain be less than 48V.  
         [0019]    The traffic signal circuit may also incorporate light degradation sensing and/or visible fault mode circuits as described in U.S. patent application No. 10/039407, “Light Degradation Sensing LED Signal with Visible Fault Mode” filed Jul. 11, 2001, and hereby incorporated by reference in its entirety.  
         [0020]    When used as a retrofit to existing intersection controls, the invention only requires that the existing incandescent light bulbs be replaced with suitable LED arrays mounted in housings adapted for placement into the existing signals. The controlling circuitry described herein may be fitted into the existing intersection control cabinet, interconnected via the existing load switch pin-outs and the intersection controller outputs. The existing power supply wires for the original incandescent lights may be used to connect the replacement LED arrays without requiring new wires to be pulled throughout the intersection.