Patent Publication Number: US-2005134196-A1

Title: Fluorescent dimming system

Description:
BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The invention relates to fluorescent dimming systems, particularly to a system that allows stepped light output from fluorescent lighting between a normal output and a lower or dimmed output.  
      2. Background Art  
      Fluorescent lamps provide efficient lighting. However, each lamp usually is controlled at a single level of light output. Recently, dimming ballasts have been developed that allow the reducing of power applied to the fluorescent tube. However, there remains a need for a dimming system that allows for efficient stepping down and up of the light output, with substantially no power loss.  
      An example of such a need is in the area of public transportation buses. As the bus travels at night, only a low level of lighting is desired, so as to not distract the driver and save energy. However, when the bus stops, there is an immediate need for a higher level of lighting so that the passengers may safely get on and off the bus.  
      A goal of the present invention is to provide an efficient fluorescent dimming system that provides for efficient stepped light output from the fluorescent lighting between a normal output and a lower or dimmed output. It is also a goal of the present invention that the dimming system be operable with standard ballast systems now on the market. It is another goal that the dimming be accomplished with substantially no power loss.  
     SUMMARY OF THE INVENTION  
      An aspect of the present invention is to provide a system for dimming fluorescent lighting. The system includes fluorescent lighting, ballast for the fluorescent lighting and a dimmer. The dimmer raises and lowers the amount of light from the fluorescent lighting in a stepped fashion between two levels—a bright level and a dimmed level. The dimmer has two circuits electrically isolated from one another, including a dimmer circuit for controlling the circuit flowing through the fluorescent lighting at two levels, one resulting in a bright level of lighting and the second a dimmed level of lighting. The dimmer also includes a control circuit for causing the current in the dimmer circuit to move between the two levels in a stepped fashion. The dimmer circuit may have two parallel branches connected in series to the fluorescent lighting. The first branch has a low impedance and the second branch has a high impedance. The control circuit switches the low impedance branch on and off to provide movement between the two levels in a stepped fashion. The low impedance branch may comprise a diode bridge and transistor such that no current flows through the low impedance branch when the transistor is in the off state, but rather flows through the high impedance branch. When the transistor is in the on state, there is flow through the low impedance branch and no flow through the high impedance branch. The high impedance branch may include a reactive component, such as in the form of a capacitor. The control circuit moves the transistor between the on and off states. The control circuit may be electronically isolated from the dimmer circuit by the interaction of a light emitting diode in the control circuit and an optical transistor in the dimmer circuit.  
      A further aspect of the present invention is to provide a system particularly suited for dimming fluorescent lighting in the passenger area of public transportation. It includes fluorescent lighting in the passenger area, a ballast for the fluorescent lighting and a dimmer. The dimmer raises and lowers the amount of light from the fluorescent lighting in a stepped fashion between a bright level suitable for allowing substantially safe movement by passengers when the public transportation is stopped and a dimmed level suitable for when the public transportation is moving. The dimmer has two circuits electronically isolated from one another, including a dimmer circuit for controlling the current flowing through the fluorescent lighting at two levels corresponding to the bright level of lighting and the dimmed level of lighting. The dimmer also has a control circuit for causing the current in the dimmer circuit to move between the two levels in a stepped fashion. The control circuit has a control switch that moves between two states, the first state resulting in the stepped change in the current in the dimmer circuit and the second state resulting in a reversal of the stepped change. The control switch may change states in response to the opening and closing of a door of the public transportation. It may include a second manually operated control switch in parallel with the first control switch.  
      Yet another aspect of the present invention is to provide a system for causing the stepped change of current in a device. The system has two parallel branches connected in series to the device. The first branch has a low impedance and the second branch has a high impedance. The low impedance branch comprises a diode bridge and transistor such that no current flows through the low impedance branch when the transistor is in the off state, but rather flows through the high impedance branch. When the transistor is in the on state, there is flow through the low impedance branch and no flow through the high impedance branch. A control switches moves the transistor between the on and off states. The control circuit may be electronically isolated from the dimmer circuit by the interaction of a light emitting diode in the control circuit and an optical transistor in the dimmer circuit. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a schematic of the fluorescent dimming system of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)  
      Referring to  FIG. 1 , there is shown a dimming system for fluorescent lighting  10  including a fluorescent tube  12 , a conventional ballast  14  and a dimmer  16 . This system is designed for use on fluorescent lighting illuminating the passenger area of the public transportation, such as a bus, but use with other fluorescent lighting is envisioned. Power is supplied to the system through a red plus input  18  from the bus&#39; 24 volt electrical power supply mains and a black negative input  20  from the bus&#39;s electrical power supply ground. The red plus power input  18  is supplied by suitable wiring  22 ,  24 ,  26  to dimmer control switch  28 , fluorescent tube  12  and ballast  14 , respectively.  
      The control switch  28  is part of a control circuit  30  which is grounded to the black negative input  20  and includes diode D 1  and resistor R 1 . Also part of the control circuit is a light emitting diode  32  which is part of optical coupler  34 .  
      A dimmer circuit  36  is connected to fluorescent tube  12 . It has two parallel branches  37 ,  38  connected in series with the fluorescent tube  12 . The first branch  37  provides high impedance to current flow and includes capacitors C 1 , C 2 . The second branch  38  is configured in series with the fluorescent tube and includes diodes D 3 , D 4 , D 5 , D 6 . These diodes D 3 , D 4 , D 5 , D 6  are connected in the configuration of a bridge  39  in parallel with the capacitors C 1 , C 2 . The diode bridge  39  is configured such that its AC inputs are connected to capacitors D 3 , D 4  and its DC side is connected to transistor Q 1 . The transistor Q 1  collector is wired to the positive output of the bridge  38 . The transistor Q 1  emitter is wired to the negative output of the bridge  38 . This enables the DC controlled transistor Q 1  to perform as a dimming switch controlling the AC current flow through the bridge. With the transistor in the off state, there is no current flow through the bridge, thus placing a high impedance in parallel with the capacitors C 1 , C 2 . This action has the capacitors C 1 , C 2  remain in series with the fluorescent tube, thus dimming tube  12 . Since the dimming is accomplished through use of capacitors, which are reactive components, there is substantially no power loss. The dimmer circuit  36  is connected to ballast  14  by wire  40 .  
      The control switch  28  has two states, in this embodiment, two positions, open and closed. When the control switch  28  is in the closed position, the control circuit  30  is energized. This energizes optical diode  32 . The light from optical diode  32  causes optical transistor  42  of optical coupler  34  to be turned on. The output from optical transistor  42  turns on a second optical coupler transistor  44 . Optical coupler  34  provides forward transfer gain while electrically isolating the two circuits. When transistor  44  is turned on, transistor Q 1  is turned on. AC current then passes through the bridge  39 , which rectifies the current to pulsating DC and passes through transistor Q 1 , and thus not through capacitors C 1 , C 2 . This enables the fluorescent tube  12  to operate at maximum brightness.  
      Optical coupler  34  enables safe control operation of transistor Q 1  which is floating at high voltage. The use of an optical coupler electronically isolates voltage of the control circuit  30  from the dimmer circuit  36 .  
      DC supply voltage, used by the dimmer circuit  36  operation, is obtained from the tube  12  current flowing through transformer TR 1 . The secondary AC voltage of transformer TR 1  is rectified by diode D 2  and filtered by capacitor C 4  to produce the DC voltage to operate the dimmer circuit. Resistor R 5  bleeds off charge from capacitor C 4  at shutdown. A bias network consisting of resistors R 2 , R 3 , R 4  and capacitor C 3  generally provide current flow during the operation of optical coupler  34  and transistor Q 1 .  
      Although the preferred embodiment shows control of a single fluorescent tube  12 , the switch  28  is suitable for controlling a fluorescent light source of multiple fluorescent tubes so to allow concurrent stepped dimming and brightening. When used on a bus, the bright level of lighting is envisioned as that desirable for easy movement around the bus by passengers, while the dimmed level is envisioned for when the bus is in motion, so that the driver is not unduly distracted.  
      In one example mode of operation the fluorescent lighting is started in its non-dimmed position with switch  28  closed. The electronic ballast provides an AC voltage (e.g., a 30 KHz AC voltage). This voltage has sufficient power to start and maintain current through the fluorescent tube  12 . The fluorescent tube  12  performs as an open current until it initially conducts, then it changes characteristics to that of a constant voltage drop device and the current through it is limited. When the ballast  14  initially starts, its output voltage, across a typical 40 inch to 60 inch long fluorescent tube, will generally be 1000 to 2000 volts. After the tube conducts, its voltage will drop to its Zener voltage, typically 80 to 150 volts. Since control switch  28  is closed, the optical coupler  34  is activated and transistor Q 1  will be on. The fluorescent lighting will be at its normal bright condition.  
      Secondary devices may be used to control switch  28 . Thus, there may be a manually activated switch used by the driver or cleaning crew to allow manual control of whether the lighting is dimmed or not. There could also be a parallel device to control switch  28  tied to when the doors of the bus open or close. Multiple switches in series or parallel may be used. During use of the bus for transportation, it is desirable that whenever the doors of the bus are open, the switch is open and the fluorescent lights are at their bright condition.  
      However, when the bus is moving it is desirable for the lights to be in a dimmed position. Switch  28  is moved to its open position to dim the flourescent lighting. This de-energizes optical coupler  34 , resulting in transistor Q 1  being off. Since the current can no longer flow through bridge  39 , the current passes through the capacitors C 1 , C 2 , i.e., a branch of higher impedance. This reduces the current flow through the flourescent tube, thus dimming the flourescent lighting.  
      The dimmer  16  is low cost since it is designed to use off the shelf items. In the preferred embodiment, the following parts are used.  
                                                   Part   Description                          D1   1N4004 diode           D2   1N5819 diode           D3   1N4004 diode           D4   1N4004 diode           D5   1N4004 diode           D6   1N4004 diode           R1     2700 ohm resistor           R2      390 ohm resistor           R3    10,000 ohm resistor           R4      470 ohm resistor           R5   100,000 ohm resistor           C1      33 nF capacitor           C2      33 nF capacitor           C3      22 nF capacitor           C4      22 mF capacitor           Q1   BU1507AX transistor by Phillips           ICI   6N139 optical coupler                      
 
      The transformer TR 1  uses a SP269 core with 4 turns of 24 awg wire (magwire) on the primary coil and 33 turns of 24 awg wire (magwire) on the secondary coil.  
      The dimmer  16  is power efficient and does no require heat sinking. The preferred embodiment works over a wide temperature range, −30° F. to 165° F.  
      While embodiments of the invention have been illustrated and described, it is not intended that these embodiments illustrate and describe all possible forms of the invention. Rather, the words used in the specification are words of description rather than limitation, and it is understood that various changes may be made without departing from the spirit and scope of the invention.