Abstract:
A system is described for remotely controlling the dimming level of ballasts for fluorescent lamps through standard two-wire AC power wiring normally used within buildings. The system makes use of a unique dimming control that creates a small positive-negative voltage asymmetry, or DC offset, and encodes this control signal onto the power lines that supply the ballasts to be dimmed. The control unit can be integrated with a wall switch as part of a variable dimming control to supply “local” remote control. It can be interfaced to a building control computer. A small decoding module preferably located in the lighting fixture near the ballast recovers the control signal from the applied asymmetry of the power voltage, processes it if necessary, and feeds it to the ballast. To minimize power losses in the dimming control, a DC offset of zero volts, or no asymmetry, produces full light output. For full dimming, an offset of about 15 volts is used. The resulting maximum dissipation is only a few watts when dimming up to six, 32-watt fluorescent lamps. The advantages include savings in re-wiring costs, higher power factor levels, lower harmonic distortion and lower radio frequency interference compared to other two-wire dimming methods.

Description:
FIELD OF THE INVENTION 
     The present invention relates to an electronic system for dimming fluorescent lamps and, more particularly, to a control system which utilizes existing building wiring to transmit dimming control signals to an electronic ballast for use in a fluorescent lighting system. 
     BACKGROUND OF THE INVENTION 
     Lighting applications account for about 30% of the electrical energy consumption in the United States. With increasing interest in energy conservation, lighting systems that use less energy and are easy and cost-effective to install are becoming more important. One effective way to reduce energy consumption of lighting is to use dimmable fluorescent lighting systems. Newer lighting systems can control light output and energy consumption by adjustment of lighting levels throughout the day, reducing energy usage when light is not needed. Existing dimmable fluorescent lighting systems require extra, low-voltage control wiring to provide control signals to the ballasts contained within lighting luminaires. The present invention allows the lighting control signals to be sent over the same wiring that is used to provide AC power to the ballasts. By using the existing wiring, the changeover from conventional lighting is greatly simplified and installation costs are significantly reduced. 
     Presently there are two types of dimmable ballasts for fluorescent lighting. The first type utilizes a phase-controlled thyristor dimming control to reduce the AC current provided to a special lighting ballast. These ballasts respond by creating suitable internal dimming signals that are then used to vary light intensity. This type of ballast can use conventional, two-wire AC power wiring, allowing them to be installed easily. However, the abrupt modulation of the AC line current causes severe distortion, greatly reducing the power factor, and increasing the harmonic content of the line current. This type of ballast, therefore, can introduce significant problems into the power distribution when a significant number of the ballasts are installed within a large building. 
     The second type of dimming ballast uses separate low-voltage control wiring to provide dimming signals to the ballast. The control voltages are typically DC signals that may vary from 0 volts (fully dimmed) to 10 volts (full brightness). The AC power is handled on separate conventional AC power wiring. Because the control function is separated from the power line, no distortion is introduced by the control system to the AC power, making this type of ballast suitable for use in large installations. The drawback to their use is the added complexity of installation, since the additional low-voltage control wiring is needed. 
     DISCUSSION OF THE RELATED ART 
     U.S. Pat. Nos. 4,876,498 and 4,954,768 for TWO-WIRE LOW VOLTAGE DIMMER, issued Oct. 24, 1989 and Sep. 4, 1990, respectively, to David G. Luchaco, et al. describe a dimming control system of the first type described above. Luchaco, et al. teach a low-voltage, two-wire dimming circuit comprising a voltage compensating circuit for regulating the RMS value of an AC voltage applied to a load and a correcting circuit for eliminating DC current that may flow through the load. The desired dimming level is accomplished by varying the phase angle of the applied AC voltage. In contradistinction, the dimming control system of the present invention does not utilize phase control of the AC power line supplying the ballast, but rather encodes dimming control information as a slight asymmetry in the AC waveform without any of the waveform distortion problems described hereinabove. The power factor of the AC supply to even a great number of ballasts is unaffected by the control system of the present invention. 
     Another prior art ballast control system is described in U.S. Pat. No. 5,107,184 for REMOTE CONTROL OF FLUORESCENT LAMP BALLAST USING POWER FLOW INTERRUPTION CODING WITH MEANS TO MAINTAIN FILAMENT VOLTAGE SUBSTANTIALLY CONSTANT AS THE LAMP VOLTAGE DECREASES, issued Apr. 21, 1992 to Feng-Kang Hu, et al. A dimming ballast system allows the light output of the lamp to be controlled by a remote source. The system encodes the remote control signal by interrupting the current flow to the ballast. On the other hand, the control system of the present invention does not interrupt the current flow to the ballast and, consequently, does not introduce power line distortion and power factor alteration. The control system of the instant invention encodes a DC control signal onto the AC power lines by means of a slight asymmetry of the AC waveform. 
     U.S. Pat. No. 5,675,221 for APPARATUS AND METHOD FOR TRANSMITTING FORWARD/RECEIVING DIMMING CONTROL SIGNAL AND UP/DOWN ENCODING MANNER USING A COMMON USER POWER LINE; issued Oct. 7, 1997 to Hong K. Yoo et al. describes an apparatus and a method for transmitting a dimming control signal in an up/down encoding manner. The encoded binary data is set as transmission data on the AC power line. The Yoo, et al. system transmits a series of bits at zero-crossing time in the AC waveform. Unlike the Yoo system, the inventive system requires no microcomputer, zero-crossing detector or pulse train generator. 
     U.S. Pat. No. 5,691,605 for ELECTRONIC BALLAST WITH INTERFACE CIRCUITRY FOR MULTIPLE DIMMING INPUTS, issued Nov. 25, 1997 to Yongping Xia, et al. describes a lamp controller (including a receiver) which receives an input signal and decodes control signals supplied from a transmitting device such as a power line wall controller. The system operates in accordance with at least two of the following communication techniques: phase angle control, step control, and coded control. Each of these control types is subject to the problems described hereinabove. The inventive system, on the other hand, relies on none of these techniques with their attendant problems, but rather utilizes a control signal encoded onto the AC power line by a slight asymmetry in the AC waveshape. 
     Accordingly, it is an object of the invention to provide a fluorescent lighting system that can simply control the lighting level through signals sent over the conventional two wire AC power wiring of a building. 
     It is another object of the invention to provide a dimming control that uses minimal power and has a low cost. 
     It is yet another object of this invention to provide a dimming control system that also allows for the use of standard 0-10 volt dimming electronic ballasts. 
     It is a still further object of the invention to provide a dimming control system that produces minimal disturbances of the AC power quality by maintaining high power factor and low harmonic distortion of the AC power line current. 
     It is an additional object of the invention to provide a dimming control system that can offer remote control of a device from more than one location. 
     It is a still further object of the invention to provide a dimming control system that may be readily integrated into overall building energy management systems. 
     SUMMARY OF THE INVENTION 
     The present invention features a new type of control system for a dimmable fluorescent ballast that inserts a slight voltage asymmetry or DC offset into an AC power supply line to an electronic ballast. The asymmetry or offset is inserted only during one half cycle of the power voltage alternation. This asymmetry or DC offset is “transmitted” over the power wiring to the device(s) being controlled where it is decoded by a small circuit module preferably located in the lighting luminaire adjacent to the dimming ballast. A low voltage control signal in the range of approximately the 0-10 volts is derived by the circuit module and is fed to the low voltage control lines of the ballast. According to standard industry practice, at maximum light output the low voltage ballast control signal is set to 10 volts. To minimize power dissipation in the dimmer, the DC offset is required to be zero at maximum lighting levels, so that the full offset is transmitted. 
     In the embodiment chosen for purposes of disclosure, the DC offset is introduced by a variable voltage reference circuit that emulates a programmable zener diode connected across a bypass diode. The voltage reference produces a given voltage drop of up to 4 volts during one half of the power line alternation cycle. The bypass diode conducts during the other alternation, producing the desired asymmetrical voltage waveform. A simple DC restore circuit at the load end of the wiring recovers the offset. It is then inverted and level-shifted to the voltage swing required by the ballast control input. The actual ballast drive is generated by an open emitter voltage follower, as the control input of a dimmable ballast is a clamped current source. With this type of circuit architecture, power dissipation is minimized over that which would occur if the standard 0-10 volt swing itself were used as the asymmetry or DC offset to be transmitted over the wiring to the lighting fixture. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A complete understanding of the present invention may be obtained by reference to the accompanying drawings, when taken in conjunction with the detail description thereof and in which: 
     FIG. 1 is a schematic block diagram of a lighting system incorporating the control system of the present invention; 
     FIG. 2 is a schematic block diagram of a lighting system including room occupancy sensing; 
     FIG. 3 is a schematic diagram of the dimming control signal generator of the present invention; 
     FIG. 4 is a single cycle of AC power showing the imposed DC offset used for control signal transmission; 
     FIG. 5 is a schematic diagram of the decoding module of the present invention; 
     FIGS. 6 a  and  6   b  are a diagrams showing the voltage transformation of the DC offset voltage into a conventional 0-10 volt dimming control signal; 
     FIG. 7 is a schematic diagram of an alternate embodiment of a decoding circuit for use in the present invention; 
     FIG. 8 is a schematic diagram of another embodiment of the present invention with a polarity-insensitive circuit; 
     FIG. 9 is a schematic diagram of an alternate embodiment of the inventive dimming control which incorporates a regulation function to compensate for extraneous asymmetry of the AC signal; 
     FIG. 10 is a schematic diagram of an alternate embodiment showing the control unit interfaced directly to a building control computer network; and 
     FIG. 11 is a schematic diagram of an alternate embodiment with a multi-access control unit directly interfaced to a building control computer network. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to FIG. 1, there is shown generally at reference number  10 , a block diagram of a lighting system incorporating the two-wire control system of the present invention. AC power  12  is applied to lighting (dimming) controller  14  which is, in turn, connected to a two-wire lighting control circuit bus  16 . Circuit bus  16  connects one or more lighting luminaires  18  that each contains lamps  20 , dimmable ballasts  22 , and a decoding module  24 . 
     The decoding module  24  generates low voltage control signals  26  needed to control the ballasts  22 , while power wiring  28  provides AC power to the ballasts  22 . One decoding module  24  can supply control signals  26  to many ballasts  22  that may be located in nearby luminaires  18 , but separate low-voltage control wiring and power wiring must be connected to each luminaire  18 . The total electrical load that can be driven by lighting control module  14  is limited by its electrical, mechanical, and thermal design. A typical unit  14  may drive up to six luminaires  18 , each containing two 32-watt lamps  20 . 
     Referring now to FIG. 2, there is shown generally at reference number  40 , a block diagram of the lighting system of FIG.  1 . In addition, the light level from lamps  20  in luminaires  18  is under control of a number of occupancy sensors  42  connected together with a three-wire bus  44  which connects to the two-wire lighting control circuit bus  16 . When the presence of an occupant (not shown) is sensed by any of the sensors  42 , indicating that a person is in the room, an associated relay contact  46  closes. The current normally flowing through dimming controller  14  is then bypassed through wiring  48  and the closed contact(s)  46 . The bypassing of dimming controller  14  by contacts  46  permits full, uncontrolled AC power to be applied to the luminaires  18 , which then operate at their full brightness. 
     If no occupant is sensed by any of the sensors  42  (i.e., no person is in the room), then after a predetermined delay, all relay contacts  46  open, and current again passes through dimming controller  14 . In the embodiment chosen for purposes of disclosure, a delay in the range of approximately  1  to  10  minutes has been found satisfactory. This controller  14  adjusts the output brightness of luminaires  18 , returning them to their dimmed state, in which they draw less power. The advantage to this approach is safety. The lighting system  10  may be designed so that at no time are any of the luminaires  18  completely shut off; a low lighting level always remains. This is an important concern for an installation, for example, in a public hallway. Also, this function can be easily implemented in a building in which the lighting is already installed. 
     Most existing wiring in buildings is similar to that shown in FIGS. 1 and 2. Luminaries  18  are supplied with AC voltage by means of two-wire lighting circuits  12 . In most installations, the function of lighting controller  14  is already performed by an on/off switch. A retrofit to the dimming/occupancy sensor architecture needs only the installation of sensors  42  and their associated wiring  44 . If on/off control is required, an additional on/off switch may be added, possibly as part of lighting controller  14 . 
     Referring now to FIG. 3, there is shown a schematic diagram of the circuit residing in lighting controller  14 . The circuit consists of a variable-voltage reference multiplier circuit with a shunting bypass diode  50 . This type of circuit is well known within the art and is commonly used within integrated circuit chips to provide voltage level shifting. A reference to this circuit is in BASIC INTEGRATED CIRCUIT ENGINEERING, by D. J. Hamilton and W. G. Howard, McGraw-Hill, 1975, pp. 314-316. 
     In operation, node  52  is connected to the AC supply line side input  10  (FIGS. 1 and 2) and node  54  is connected to the load side  12 . During the half cycle when node  52  is driven in the positive direction and current flow results along arrow  56 , the multiplier circuit becomes active and forces a constant voltage drop across transistor  60 . The amount of voltage drop is set by the dividing ratio of resistors  62   a ,  62   b  and  64 . The circuit sets the voltage drop so that the base emitter voltage of transistor  60  remains at a constant value. During the half cycle when node  52  is driven in the negative direction and current flow results along arrow  58 , current passes through bypass diode  50 , contained inside the case  66 . The voltage drop when current flows in this direction (arrow  58 ) is much lower than the voltage drop which occurs when current flows in the first, opposite direction (arrow  56 ). Thus, the controller circuit  14  forces a slight asymmetry to the output voltage waveform as observed at node  54 . A decoding unit  80  (FIG.  5 ), generally placed within a luminaire  18 , generates a DC dimming control voltage from this induced voltage waveform asymmetry, as described in detail hereinbelow. 
     Referring now to FIG. 4, the voltage waveform asymmetry created by the controller circuit  14  of FIG. 3 is shown. A normal voltage waveform has symmetrical positive  70  and negative  72  half cycles. When in a dimmed state, controller circuit  14  introduces an asymmetry so that positive half cycle  70  is reduced by a voltage offset  74  to half cycle  76 . This reduction and the associated voltage drop across controller  14  causes a dissipative power loss in controller  14 . By arranging the polarities so that the maximum voltage drop is associated with the dimmed state, the maximum voltage drop occurs with the minimum current. Dissipative heating in controller  14  is thereby minimized. 
     Referring now to FIG. 5, there is shown a schematic diagram of a decoding module  80  for use cooperatively with dimming controller  14 . AC input power  16  (FIGS. 1 and 2) is applied to nodes  82  and  84 . The low voltage control signals, generated by decoding module  80 , are connected to output nodes  86  and  88 . The main structural blocks of the circuit include half wave DC supply  90 , dual peak detector and difference generator  92 , inverting level shifter  94 , voltage following buffer  96 , and output voltage follower  98 . 
     In operation the difference between the peak AC voltages in the positive and negative directions is determined in block  92  at node  100 . This difference signal is filtered and buffered by block  96 , which contains operational amplifier  102 . The output signal  104  from block  96  is one input  114  to the inverting adder in block  94 . The adder, consisting of operational amplifier  106  and its associated circuitry, level-shifts signal  104  from block  96  to form the proper voltage swing and polarity required by the dimming ballast  22 . 
     Referring now also to FIGS. 6 a  and  6   b , there are shown diagrams illustrating the voltage transformation. Maximum dimming to full light output requires a 0-10 volt signal  108  to the ballast (FIG. 6 b ), but an inverted positive to zero swing of different magnitude (typically in the range of 2-7 volts)  110  (FIG. 6 a ) is produced at node  100 . The voltage inversion and level shifting is accomplished in block  94 . The control circuit of a dimming ballast  22  is a current source. Transistor  112  is configured as a voltage follower in block  98  so that the voltage between output nodes  86  and  88  corresponds to the proper dimming level, regardless of the number of ballasts connected in parallel across output nodes  86  and  88 . 
     The decoding module circuit  80  uses a differential peak detector to determine the degree of asymmetry of the waveform. An alternative approach is to average the signal with RC filtering. An example of this type of circuit is shown in FIG. 7, reference numeral  120 . The averaging circuit is an RC filter (capacitor  122 , resistor  124 ) in block  126 . A lower voltage is obtained with this circuit than with the circuit of FIG. 5, but it has been proven to be less susceptible to electrical interference and thus useful in noisy electrical environments. A two-stage filter is shown in the circuit of FIG.  7 . Note that the input voltage for most AC power situations is relatively free of DC since power distribution transformers do not couple DC current, and have output voltages, the average value of which is zero. It should be noted, however, that some electrical appliances use an asymmetric switching device such as a silicon controlled rectifier (SCR), which produces an asymmetric voltage drop due to electrical resistance in the AC power line. If one or more of these appliances is connected to nodes  82  and  84 , the AC input nodes to decoding module  80  as described in FIGS. 5,  7  and  8 , then a DC signal will be generated that may affect the dimming level of the lamps. A solution for this case will be described in an alternate embodiment to be described in detail hereinbelow with reference to FIG.  9 . 
     The circuits of both FIGS. 5 and 7 are sensitive to the polarity of the AC power wiring connected to controller  14  and to the decoder module  80 . This occurs because of the direct conversion of the controller signal to the dimming control voltage, which must be in the range of 0-10 volts only. It would be desirable to have a system which is insensitive to the polarity of connections of both the controller  14  and the decoder module  80  but which still produces a 0-10 volt signal with correct polarity. 
     Referring now to FIG. 8, there is shown a schematic diagram of a polarity-insensitive circuit which overcomes this difficulty. Circuit  140  uses a full wave rectifier or absolute value operational amplifier circuit  142  in a manner well known to those skilled in the circuit design art. In block  142  signals in the positive direction at input node  144  result in positive signals at the output node  146 , since diode  148  conducts for positive inputs. For negative inputs at node  144 , diode  148  blocks current flow while diode  150  conducts. The circuit acts as an inverting amplifier, so that negative inputs result in positive signals at output node  146 . 
     A circuit of this type that is polarity insensitive has important practical advantages. Because of insensitivity to polarity, the lighting (dimming) controller  14  (FIG. 3) may be connected in the circuit in either of two possible ways, with either node  52  or node  54  connected to the incoming AC line. Thus, the pigtail wiring used by the electrician to connect the control can have the same coloring; which wire is used for the incoming AC line does not matter. This allows a dimming controller  14  to be installed as if it were a conventional on/off switch in which the wiring order of line side/load side is not specified. This offers a safety advantage, because a dimming controller  14  cannot inadvertently be installed in a reversed fashion. In a conventional polarity-sensitive controller, such an error might result either in a safety hazard or in damage to the controller. 
     There is another, more subtle advantage. A small DC and 120 Hz harmonic current is introduced into the AC line by this type of dimming controller. However, the phase of the harmonics depends on the polarity with which the controller  14  is installed in the AC line. If a great number of controllers  14  is randomly installed, the probability is high that approximately 50% of the controllers will be installed with each polarity, resulting in out-of-phase cancellation of a significant portion of the generated harmonics. In a large lighting installation where power quality and harmonic content is an important concern, such a feature may be quite important. Also, because of the signal polarities chosen, harmonics are created only at the lower dimming levels at which the AC current and its harmonic content are already low. Total harmonic distortion (THD) of the present invention has been found to be in the range of 5% and never exceeding 15%. On the other hand, conventional phase controllers typically introduce distortions greater than 50%. 
     Referring now to FIG. 9, there is shown an alternate embodiment of the dimming control. This circuit utilizes negative feedback regulating properties to correct for small residual asymmetry of the incoming AC line voltage that might otherwise disturb the dimming function. An example of such a disturbing perturbation is the asymmetry that can be introduced onto an AC line by a variable speed motorized appliance. Such appliances sometimes make use of a half-wave SCR type of variable speed control which draws power preferentially from one-half of each AC cycle. If such an appliance is plugged into the same circuit as the controlled lighting ballasts, then resistive voltage drop in the supply line can be sufficient to cause an asymmetric voltage drop which can ultimately affect the lighting levels when the motorized appliance is turned on or off. For electric circuits that consist of purely lighting devices, a regulation circuit is not required since lighting loads are symmetrical in nature. In practice, the intentional asymmetry control voltage is set so that the average of the positive and negative voltages typically varies in the range of approximately 2-10 volts, even as high as 15 volts. It has been found, however, that voltages above 15 volts lead to excessive power dissipation in the dimming controller  14 . In comparison, the incoming voltage asymmetry of the power system is typically about a volt for most power systems supplying power to a small asymmetrical load. This type of incoming offset can be corrected by using a regulating circuit. 
     The dimming control of FIG. 9 alleviates this problem by means of such a regulation circuit. In this embodiment, input power on the “hot” AC conductor  500  of ac supply line  162  is supplied to node  500 , and delivered to the load at node  502 . The neutral return connection necessary to complete the circuit is not shown. Conductor  504  is the green safety ground wire and is used to provide a potential reference for the circuit, and to provide a drain for the small amount of power used by the circuit. This illustrates a standard connection method used, for instance, to connect a wall light switch to a lamp load. The regulation op-amp  510  controls p-channel MOSFET  506  to produce a small voltage drop over one half of the ac voltage waveform. Bypass diode  504  shunts the load current over the opposite half of the cycle, producing the required asymmetry. The RC circuit composed of capacitor  512  and resistor  514  are used to provide an average of the outgoing voltage waveform. The voltage at sense node  516  is a measure of the outgoing asymmetry. It is a typically few volts offset from the voltage at node  502 , depending on the amount of asymmetry present. The circuitry within box  518  forms a conventional half wave power supply which provides a small amount of power for the regulator circuit. 
     Amplifier  510  is provided a constant reference voltage at its positive input  520 , while its negative input  522  provides the summing point for the regulator circuit. By servo action, the system will attempt to keep summing point  522  at the same potential as its reference positive input  520 . In doing so, the gate drive voltage will become adjusted so that sense node  516  is held at the desired offset level, a constant voltage proportional to the setting of control potentiometer  524 . 
     The dimming controller  14  (FIGS. 1 and 2) may be implemented with many different package form factors, each of which is optimized for a particular application. For example, the dimming controller  14  can be integrated with a wall switch as part of a variable dimming control. This type of package is intended to provide a direct replacement for an on/off wall switch or an incandescent dimmer. This package would typically be used in applications where a small number of ballasts (up to six, for example) are to be dimmed and only local remote control is required. The term “local” indicates the idea that the control unit and the devices to be controlled are generally in the same room, or at least in the same general area. 
     In another case where local control is not needed or desired, the controller  14  may be interfaced to a building control computer network either directly, or through an intermediate computerized control unit. Referring now to FIG. 10, there is shown generally at reference numeral  180 , a schematic diagram of such a configuration. This configuration  180  allows remote control through a direct connection to the network (not shown). AC power  12  is applied to dimming controller  14  and also to a normally open (n/o) contact of relay  182 . The coil of relay  182  is connected to the output of network interface module  184  by means of interface wiring  186 . The input of network interface module  184  is connected to a building control computer network through network interface nodes  188 . Network interface module  184  also may have connections (not shown) that allow it to sense events through the use of sensors and to enable functions such as controlling relay  182 . 
     In operation, the dimming control unit  180  allows lighting controller  14  to be active or to be bypassed by relay  182 . This allows the building control computer to set the control signal to one of two dimming levels. It will be apparent to those skilled in the art that additional monitoring and/or controlling functions may be added to the basic dimming control circuit. Such functions include the ability: to turn off the power to the controlled devices; to provide a plurality of different dimming levels by replacing the variable resistor  64  (FIG. 3) with a digitally-controlled potentiometer (not shown) such as the Model No. DS1804 semiconductor manufactured by Dallas Semiconductor; to sense.whether the devices are actually drawing power; and to determine how much power the devices are drawing. It should be obvious that the controlled ballast itself forms a typical device for which power status monitoring is useful. 
     The inventive two-wire controller may be easily interfaced with building control computer networks, such as Lonworks® from Echelon Corporation of Palo Alto, Calif. and CEBus® from the Electronic Industries Association. 
     A hotel room is a typical example of an application that could use an intermediate computerized control unit to connect to various sensors and control functions. In this example it might be cost-prohibitive to attach every sensor and control signal in every hotel room directly to a building control computer network, but it may be highly desirable to have certain devices in each room in communication with the network. An intermediate control computer could handle “local” control functions and export certain, preselected functions to the building network computer. 
     Referring now to FIG. 11, there is shown generally at reference numeral  200 , a schematic diagram of another embodiment of a dimming control unit that allows local on/off as well as dimming control. It may also be interfaced to a building control computer network either directly, or through an intermediate computerized control unit. AC power  12  is applied to the input of dimming controller  14 . The output of dimming controller  14  is applied to the common terminal  202  of a conventional three-way s.p.d.t. toggle switch  204 . The remaining terminals  206 ,  208  of switch  204  are connected to contacts  210 ,  212  of relay  214 , respectively. The common contact  216  of relay  214  provides controlled output AC power  16 . 
     The coil of relay  214  is connected to the output of network interface module  184  by relay interface wiring  186 . Network interface module  184  is connected to a building network (not shown) through network interface nodes  188 . Voltage sensor  218  is connected across the controlled output power source  16  and provides an input to network interface module  184  through voltage sense interface wiring  220 . Network interface module  184  allows the building control computer (not shown) to determine if power is being applied to the two-wire lighting power circuit  16  by monitoring voltage sensor  218 . Relay  214  is preferably of the latching variety, since no energy is required to maintain a state once it has changed states. 
     Relay  214  and switch  204  act as a three-wire (three-way) circuit often found in household lighting applications where, for example, one switch may be located at the top of a stairwell while a second switch may be located at the bottom; both can control the same light. This arrangement allows either the local switch or the network to control the load. That is, the lamp may be turned on or off either locally or by the building control computer. A typical application for this type of control strategy is a conference room. During the day, people may enter and leave the room, turning the lights on or off as required. At the end of the day or at other times as required, the building network control computer ensures that the lights are turned off. If an employee happens to be working late in that room, he or she could still override the building computer with the local switch. As mentioned above, it will be apparent to those skilled in the art that additional monitoring and/or controlling functions may be added. Occupancy sensing and/or daylight sensing could also readily be added. 
     The two-wire control systems described so far can be used to sense a parameter at a remote device and to send the sensed parameter information back over the same AC power circuit providing power and control signals to the device, to the controller. The control unit may be configured to provide either local and/or remote (i.e., proximate a room or small area or at a remote central monitoring facility). So although the locations of the control module and the decoding module have been reversed compared to examples previously described, the heart of the invention (i.e., encoding a control signal onto an AC voltage by adding a direct current offset to at least one-half of the AC voltage and sending the information over the power line) is essentially the same. An example of the monitoring function could be to monitor the temperature inside an electronic ballast. A thermistor or similar element, which is a two-terminal device that varies its resistance with changes in temperature, located in a ballast enclosure could be used to sense the temperature in the ballast. With the addition of a small number of inexpensive components, the thermistor can be used as part of a circuit to generate an appropriate offset voltage that can be encoded onto the AC voltage and sent over the two-wire control circuit bus to a decoding module. The output of the decoding module could then be used to display the ballast temperature locally or the signal could be provided to a building control computer network such as Lonworks® for monitoring purposes. 
     Since other modifications and changes varied to fit particular operating requirements and environments will be apparent to those skilled in the art, the invention is not considered limited to the example chosen for purposes of disclosure, and covers all changes and modifications which do not constitute departures from the true spirit and scope of this invention. 
     For example, while ballasts for gas-discharge lamps have been used for purposes of disclosure, it will be obvious to those skilled in the art that the disclosed apparatus and method of the present invention may be used to control other types of loads, including, but not limited to small motors (e.g., fans, curtains, blinds, or the like), small incandescent light loads, household appliances, low-voltage halogen lighting systems, etc. 
     Having thus described the invention, what is desired to be protected by Letters Patent is presented in the subsequently appended claims.