Abstract:
A high frequency control circuit for a gaseous discharge lamp which includes a mechanically variable reactance device coupled in series with the lamp to control the current fed to the lamp. The mechanically variable reactance device is adjustable by a user to alter the intensity of the light emitted by the lamp.

Description:
FIELD OF THE INVENTION 
     This invention relates to the control of gaseous discharge lamps, such as fluorescent lamps, and more particularly to the adjustment of the intensity of their light output. 
     BACKGROUND OF THE INVENTION 
     Fluorescent lamps have been the commonest method of lighting consumer, commercial and industrial areas for many years. In operation, a gas mixture enclosed in the glass tube of the lamp is ionised by means of a high voltage pulse applied between two heated electrodes at each end of the tube. In a conventional lighting system, the gas in the fluorescent tube is extinguished and then ionised again with each half cycle of the 50 Hz conventional line frequency. This system has the merit of low capital cost and simplicity, but whilst far superior to incandescent lamps in the conversion of energy to light, it is nonetheless an inefficient mechanism. The circuit watt losses are similar whatever the wattage of the lamp and range from about 66% for an 18 watt lamp to 20% for a 70 watt lamp. In addition, the flicker caused by the re-ionisation of the lamp every half cycle at 50 Hz is now recognized as a major cause of headaches amongst office workers. 
     In consequence, a number of improvements have been initiated over the years to reduce the inefficiency and the flicker associated with fluorescent lamps. 
     An electronic controller addresses a number of these problems. It supplies the gases in the tube with a high frequency AC current, preferably above 18 kHz. This type of controller typically reduces circuit losses from the range 20-66% to the range 4-8%. Owing to the high frequency refresh rate of the lamp, its light output is increased. Accordingly, lamps are commonly under-powered such that the same output is produced as that resulting when running the lamp with a standard mains frequency circuit. For example: 
     
       
         
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 Standard Circuit 
                   
               
               
                   
                 Lamp wattage: 
                 18 watts 
               
               
                   
                 Circuit losses: 
                 12 watts 
               
               
                   
                 Total power consumption: 
                 30 watts 
               
               
                   
                 High Frequency Circuit 
               
               
                   
                 Lamp wattage: 
                 16 watts 
               
               
                   
                 Circuit losses: 
                  2 watts 
               
               
                   
                 Total power consumption 
                 18 watts 
               
               
                   
                   
               
             
          
         
       
     
     The lumens output of each lamp in the above example would be identical. 
     The use of a high frequency controller is also beneficial as the refresh rate of the lamp is effectively 60,000 a second when running at 30 kHz, for example. Therefore, there is no flicker detectable by the human eye. Also the electronic controller unit can be less than half the weight of a standard circuit, and generate less heat. An electronic controller is also more versatile. For example, it can be interfaced with passive infra-red movement detectors or optical sensors which detect ambient light levels. 
     It is generally desirable to include a dimming facility in a lighting system, as the required lighting level may vary depending on various factors. For example, an office may be converted to intensive computer use, and a lower level of lighting is then appropriate owing to the relative dimness of a computer screen. Also, it has been found that the light tolerance and the amount of light needed or felt to be needed for given tasks varies greatly between individuals. In particular, it varies considerably between different age groups. 50-60 year olds will require substantially more light for the same range of tasks as 18-25 year olds. In addition, the light required within open plan and cellular offices varies greatly according to the type of partitioning system, colours and furniture used. Furthermore, office layout designs are changed frequently and in large organisations this can affect as much as 20% of the office space per annum. In consequence the original lighting can be either too bright or too dim in the revised spatial layout. 
     The abstract of JP-A-01084596 describes control circuitry for a discharge lamp, in which a variable inductance is provided to control the light output of the lamp. 
     Various forms of dimmable high frequency electronic controllers are available which can reduce their operating wattage from 100% to about 5%. Typically, a wall mounted potentiometer operable by a user is provided to send a control signal to each controller. Each controller accordingly alters the current and frequency which powers the discharge lamps of the respective luminaire. However, such controllers are expensive, typically costing 60% more than a conventional electronic controller. Therefore, it is only worthwhile to link at least ten and usually at least twenty-five luminaires in the dimming circuit, such that light levels can only be adjusted over large areas and in a uniform manner. Furthermore, in such a configuration, wiring needs to be routed from the wall mounted potentiometer to each luminaire in turn to carry the control signal. Installing this wiring is a time-consuming process, particularly when refurbishing a building having existing partitions, fixtures and the like. 
     SUMMARY OF THE INVENTION 
     The present invention provides high frequency control circuitry for a plurality of gaseous discharge lamps, comprising a plurality of mechanically variable inductive reactance means, each reactance means being adapted to be connected in series with at least one gaseous discharge lamp to control the current fed to said lamp, the reactance means being adjustable by a user to alter the intensity of the light emitted by said lamp, and wherein each reacance means comprises a coil having a plurality of tappings spaced along its length and a switch for selectively connecting to one of the tappings, the circuitry further comprising a linkage connecting the respective switches of the reactance means such that adjustment of one switch produces a corresponding adjustment of the other switches. 
     Accordingly, the invention enables individually adjustable control circuits to be produced with little additional cost compared to a circuit without an adjustment facility. The additional cost may therefore be recouped relatively quickly through energy saving by dimming lights as necessary. 
     It may also allow the lighting level of individual luminaires to be varied above and below their standard fluorescent lamp wattage. Conversely, known dimming systems can only be used to reduce light levels from the standard wattage. 
     In a preferred arrangement, the circuitry includes a drive oscillator and the high frequency output of the drive oscillator is applied across a two-wire bus bar. A respective sub-circuit comprising starting means, constant inductive reactance means, variable inductive reactance means and output means is provided for each lamp, each sub-circuit being connected across the bus bar. The lamps are individually controllable, but are driven from one control unit with only the sub-circuit being replicated for each lamp. 
     Embodiments of the invention will now be described by way of example with reference to the accompanying drawings in which: 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an overall block diagram for an electronic high frequency controller circuit according to the invention; 
     FIG. 2 is the output portion of the controller circuit of FIG. 1; 
     FIG. 3 is a series inductor configuration of the invention; 
     FIG. 4 is a preferred variable inductor for the circuit of FIG. 1; 
     FIG. 5 is another preferred variable inductor; 
     FIG. 6 is a control dial for a variable inductor; 
     FIG. 7 is a perspective view of a controller circuit of the invention; 
     FIG. 8 is a further preferred variable inductor for the circuit of FIG. 1; 
     FIG. 9 shows linkage of inductors of the type shown in FIG. 8; 
     FIG. 10 is a circuit diagram of a controller circuit of the invention in combination with a plurality of discharge lamps; 
     FIG. 11 is a circuit diagram of an alternative embodiment to that of FIG. 10; 
     FIG. 12 is a block diagram of another controller circuit of the invention; 
     FIG. 13 is a circuit diagram of sub-circuit  48  of FIG. 12; and 
     FIG. 14 is a plan view of sub-circuit  48  of FIG. 12 mounted on a circuit board, and a lamp  6 . 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 is a block diagram illustrating the primary features of a high frequency controller in accordance with the invention. The blocks representing features of a conventional electronic controller are enclosed by a dotted line  2 . In operation, the live, neutral and earth lines of an AC power supply are connected to respective inputs  4 . A discharge lamp  6 , such as a fluorescent lamp, is connected to the output of the controller. According to the invention, two additional inputs  8  and  10  to the output inductor and heater drive block are provided and a variable control inductor  12  is connected thereto. 
     In a conventional controller, the current input to the lamp  6  is controlled by the inductance of a fixed value inductor  14 , shown in FIG.  2 . Its value is normally dependent on the frequency and the nominal wattage required to operate the lamp  6 . A typical inductance value therefor is 3 mH. In the configuration of the invention shown in FIGS. 1 and 2, the current supplied to the lamp  6  is adjusted by varying the inductance of the inductor  12 . This varies the value of the total inductance of inductors  12  and  14  which is in series with the lamp. As a high frequency voltage is used, the inductors may be relatively small in size. 
     The light output level of the lamp  6  may be varied above and below its standard wattage. For example, a luminaire fitted with a single 58 watt fluorescent lamp using an electronic high frequency controller would normally be installed with the controller running the lamp at 52 watts. Its light output is therefore consistent with that produced by a 58 watt fluorescent lamp, run on a standard mains circuit for 50 Hz operation. If a luminaire is fitted with the controller of this invention, its light output can be increased to 64 watts, for example, that is, by nearly 25%. Thus fewer luminaires may be required to illuminate a given space. Alternatively, where appropriate, such as a change of use of an area from general office purposes to computer use, the variable control inductor  12  can be simply adjusted so that the effective wattage is only 42 watts, say, producing approximately a 20% reduction of the lighting levels. If required, the wattage could be reducible further, to as low as 28 watts, say. Nevertheless, this will still give individuals the option to use higher light levels if desired. The controller of the invention also enables a user to compensate for deterioration in the output of a lamp by increasing the input power. 
     In this way, the variable control inductor is capable of controlling a 58 watt fluorescent lamp between 42 and 64 watts, for example. Similarly, a range of control can be facilitated with any type of fluorescent lamp. 
     FIG. 3 shows an alternative inductor configuration to that of FIG. 2, wherein the variable inductor  12  is connected in series with the fixed value inductor  14 . This serves to reduce the wattage of the lamp  6  for energy saving applications, whereas the arrangement of FIG. 2 enables adjustment of the supplied power above and below the nominal lamp wattage. If a 3 mH fixed value inductor is used, for example, a variable inductor connected in series may be used having a maximum inductance of about ½ mH, or about 3 mH if connected in parallel. The values selected depend on the power rating of the lamp and the frequency of the applied voltage. 
     FIG. 4 shows the construction of a variable inductor  12  of the invention. It consists of a coil  16  and a ferrite rod  18  which are relatively movable to move the rod into or away from the coil in the direction A, increasing and decreasing the inductance of the device, respectively. 
     FIG. 5 shows an alternative variable inductor embodiment. It comprises two E-shaped ferrite cores  20  and  22 , a coil  24  and a mechanical linkage  26 . The core  22  is fixed, whilst the linkage is operable to move the core  20  relative thereto. Moving the core  20  closer to core  22  reduces the air gap therebetween and increases the inductance of the device, thus reducing the power fed to a lamp  6 . Conversely, moving the core  20  away from core  22  increases the power supplied. The linkage may enable adjustment of the core spacing either by movement thereof parallel to or about its axis  28 . 
     The variable control inductor  12  may be fitted to a luminaire internally or externally depending on the type of access required. It may be configured to provide linear or non-linear adjustment of the lamp light level. 
     Configurations other than those of FIGS. 4 and 5 are envisaged, for example using U- or I-shaped cores with, in each case, the inductance being varied by moving the ferrite material relative to a coil. 
     Adjustment of the level of power fed to the luminaire may be provided economically by a mechanical control. FIG. 6 illustrates a control dial for a variable inductor of the invention. Rotation of the dial  30  allows the power fed to a lamp and therefore its light output to be adjusted by ±20%, for example. Alternatively, control may be achieved electronically via a remote control and infra-red link, for example. 
     FIG. 7 shows a high frequency controller adapted in accordance with the invention. It consists of a circuit board  32  on which known high frequency controller circuitry  34  is mounted. A variable inductor  12  is appropriately connected to the circuitry  34  and provided on the board to form a single unit for controlling the lamp  6 . 
     A further preferred inductor configuration is shown in FIG.  8 . It consists of a core  62 , a tapped coil  64  and a selector switch  66 . One end of the coil  64  is connected to an input  68  and one terminal of the switch  66  is connected to an output  70 . Although the illustrated coil includes six tappings, the number of tappings “m” may be greater or fewer as appropriate to give finer or coarser control. The inductance between adjacent tappings may be varied by altering the number of turns of the coil in each section. Rotation of the switch  66  brings connector  72  into contact with each tapping in turn. Accordingly, the inductance connected between input  68  and output  70  is variable in intervals between a maximum at position “1” and zero at the last position, “m”. 
     FIG. 9 illustrates an arrangement in which inductors of the type shown in FIG. 8 are linked together. This may be desirable in applications where it is necessary to vary equally groups of lamps being run from a corresponding number, from “1” up to “N”, of high frequency controllers. The switches  66  of the inductors are connected by a linkage  74  which, economically, may operate mechanically. The linkage operates so that adjustment of one switch  66  produces a corresponding adjustment of the other switches linked thereto. 
     A lighting system is illustrated in FIG. 10 which enables a plurality of lamps to be individually adjustable. The controller  36  is of another known configuration and such controllers may be adapted to drive up to four lamps  6 . A variable inductor is connected between the controller  36  and each lamp  6 , allowing the current supplied to each lamp (and therefore its brightness) to be separately altered. 
     A similar arrangement to that of FIG. 10 is shown in FIG.  11 . In this case, the four lamps  6  have a common return line  38  to the controller  36 . A single variable inductor  12  is connected in the return line, such that the light level of all the lamps is simultaneously adjustable. 
     A further controller circuit configuration of the invention is shown in FIGS. 12 to  14 . It consists of a main control unit  40  which receives an AC supply on inputs  42  and  44  and provides an output across a two-wire high frequency bus bar  46 . Each of a plurality of lamps  6  has a respective sub-circuit  48  which is in turn connected across the bus bar  46 . 
     The sub-circuit  48  is shown in greater detail in FIGS. 13 and 14. FIG. 13 is a schematic circuit diagram, whereas FIG. 14 is a plan view of a circuit board  49  and lamp  6 . Sub-circuit  48  comprises inputs  50  and  52  for connection to the bus bar  46 . One input  50  is connected to constant and variable inductors  14  and  12 . Although the inductors are shown in series, they may be arranged in parallel, as discussed above. Lamp starting components, namely capacitors  54 ,  56  and a thermistor  58 , are also included in sub-circuit  48  and connected in a known manner across the lamp  6 . The capacitors provide a heater current to start the lamp. The thermistor is initially at a low temperature and therefore has a low resistance, such that the heater current is high. Once the lamp has started, the temperature is higher and the thermistor reduces the heater current. Output points  60  are connected to the lamp  6 . The other components of the controller are provided within the high frequency main control unit  40 . 
     Using the configuration of FIGS. 12 to  14 , a plurality of individually controlled lamps  6  may be driven from one control unit  40  with only the sub-circuit  48  being replicated for each lamp. Whilst the known controller configuration  36  of FIGS. 10 and 11 can only supply up to four lamps, as it includes only four outputs, the arrangement of FIGS. 12 and 14 allows a greater number of lamps to be supplied, within the constraints of the power supply used. It substantially reduces the amount of wiring required as it is only necessary to run two wires to each lamp, rather than four as shown in FIGS. 10 and 11, and is more versatile as sub-circuits  48  can be selectively connected to or disconnected from the bus bar  46 , as required. Although a linear tube  6  is shown in FIG. 14, the control circuitry of the invention may of course be connected to tubes of any shape, size or power rating.