Abstract:
A dimming control interface for dimming ballasts is disclosed. The control interface includes DC voltage source input terminals in series with a pair of input terminals and a high-voltage resistance element to form a current source. The dimming control interface includes an isolated coupling element connected between a pair of dimming control input terminals and a pair of dimming control output terminals, and is operative to generate an output control voltage in response to an input control voltage applied to the input terminals. The dimming control interface further includes voltage clamping element to limit voltages generated in the control interface to safe levels when an external AC power source is connected to the dimming control interface. Similarly, a second high voltage resistance element is connected between one of the dimming control input terminals and the isolated coupling element to limit currents generated by the AC power source to safe levels.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates generally to dimming electronic ballasts used to power gas discharge lamps. Specifically, this invention pertains to a 0-10 volt dimming control interface for an electronic ballast that includes components that will prevent the dimming control interface from being damaged if it is accidentally connected to an external AC power source. More specifically, the dimming control interface of the present invention includes circuits that limit voltages and currents present in the dimming control interface circuit to safe levels when the external AC power source is connected to the dimming interface circuit. 
     Generally, a 0-10 volt dimming control interface is designed to be connected to a DC control voltage, while the ballast itself is designed to be connected to an external AC power source. A conventional 0-10 volt control interface is designed to deliver a small current preferably ranging from about 300 μA to 500 μA. Consequently, a typical DC control voltage may be obtained from a variable resistor, a shunt controller, or a more sophisticated light-level controller. Shunt controllers control ballasts by setting the voltage between the control input terminals to a level ranging from 0 volts to 10 volts. If the shunt controller is disconnected from the dimming control interface, the open-circuit voltage between the control input terminals will rise to approximately 10 volts. 
     To allow these connections, dimming ballasts usually include a pair of dimming control input terminals designed to be connected to the DC control voltage, and a pair of AC power input terminals designed to be connected to the external AC power source. Sometimes, however, the AC power source is accidentally connected to the dimming control input terminals rather than the AC power input terminals. As a result, the dimming control interface and the ballast may be damaged by excessive currents and voltages generated by the AC power source. Replacement of the damaged dimming control interface and/or the ballast is an expensive proposition. Thus, there is a need for a dimming control interface that may be connected to a DC control voltage, as well as, an external AC power source without being damaged. 
     To complicate matters even more, a dimming control interface may be connected to several different ballasts through a single pair of dimming control input terminals. When the external AC power source is connected to such a dimming control interface, excessive currents and voltages generated by the AC power source may damage several ballasts. Replacement of multiple dimming control interfaces and/or ballasts is accordingly more expensive and undesirable. Thus, there is an additional need for a dimming control interface that may be connected to an external AC power source without being damaged. 
     Although dimming control interface circuits are well known in the art, there are no known dimming control interfaces that are capable of withstanding a sustained connection to an external AC power source without being damaged. For example, one known dimming control interface is described in U.S. Pat. No. 5,751,188 issued to Mortimer on May 12, 1998 and entitled “Universal Input Dimmer Interface.” This patent teaches a dimming control interface that may be connected to an external DC control voltage or an external pulse-width modulated control voltage. In either case, the dimming control interface includes circuitry for generating an electrically isolated control voltage at output terminals on the dimming control interface in response to the input control voltage applied at input terminals on the dimming control interface, whether the input control voltage is the DC control voltage or the pulse-width modulated control voltage. This patent fails to teach or suggest the use of any circuits for the purpose of limiting currents and voltages present in the dimming ballast when an external AC voltage is applied to the dimming control input terminals. 
     Another example is described in U.S. Pat. No. 5,583,402 issued to Moisin et al. on Dec. 10, 1996 and entitled “Symmetry Control Circuit and Method.” This patent also teaches a dimming interface circuit designed to be connected to either a DC control voltage or a pulse-width modulated voltage signal. As was the case with the &#39;188 patent, this patent fails to teach or suggest the use of circuits designed to limit currents and voltages present in the dimming ballast when an AC power source is connected. 
     Two additional examples of dimming control interfaces are described in U.S. Pat. No. 5,089,751 issued to Wong et al. on Feb. 18, 1992 and entitled “Fluorescent Lamp Controllers with Dimming Control,” and U.S. Pat. No. 5,003,230 issued to Wong et al. on Mar. 26, 1991 and entitled “Fluorescent Lamp Controllers with Dimming Control.” Neither of these patents teach a dimming control interface that includes protection from connection of the dimming input terminals to an external AC power source. 
     In addition, there are two known commercial dimming control interfaces. A portion of a one such prior-art dimming control interface circuit is shown in FIG. 1. A small-signal PNP transistor Q 1  is connected to a biasing circuit (not shown) so that the transistor will supply about 400 μA current to a shunt controller (not shown) connected between control input terminals J 1  and J 2 . A zener diode D 1  prevents the voltage between terminals J 1  and J 2  from rising above about 10 V if no shunt controller is connected to the circuit. An isolated coupling circuit (not shown) may be connected between terminals J 3  and J 5 . The isolated coupling circuit sends a dimming control signal to a ballast inverter circuit (not shown). A resistor R 1  provides protection for the interface circuit if the control terminals should accidentally be connected to a low voltage source such as a 24 VAC control line. R 1  typically has a value of about 1 kΩ, so the 0.4 V voltage drop due to the 400 μA current is small in comparison with 10 V, and can be compensated for. It is impractical to make R 1  large enough in value to withstand ac power line voltages because the voltage that would then be dropped across it due to the 400 μA current would be large in comparison with 10 V. 
     A portion of another commercial dimming control interface is shown in FIG. 2. A floating 12 V dc power supply (not shown) is connected between terminals J 14  and J 15 , with the positive terminal connected to J 14 . Resistors R 2  and R 3  have a value of 10 kΩ. They supply current to control input terminal J 11  through a high voltage diode D 2 . A high-voltage N-Channel power MOSFET transistor Q 2  is biased on by resistors R 2  and R 3  when terminal J 11  is positive with respect to J 12 , which is the case during normal operation. During normal operation, the voltage between terminals J 13  and J 15  is about 0.6 V less than the voltage between terminals J 11  and J 12 . The voltage between terminals J 13  and J 15  is coupled to a regulator circuit consisting of an operational amplifier U 1 , resistors, R 4 , R 5  and R 6 , a capacitor, C 1 , and a current sensor circuit  10 . The input terminals of an optocoupler (not shown) are connected between terminals J 16  and J 15 . The current sensor provides a signal that is proportional to the lamp current. This signal provides negative feedback even though it is connected to the positive terminal of U 1  because the optocoupler is connected to the ballast inverter (not shown) in a way that decreases lamp current when the input current to the optocoupler increases. 
     If the voltage between terminals J 11  and J 12  becomes negative, Q 2  is turned off, and this protects the regulator circuit. If the voltage between terminals J 11  and J 12  becomes greater than about 11 V, diode D 2  becomes reverse biased, and this also protects the regulator circuit. If the blocking capabilities of D 2  and Q 2  were each about 600 V, then the regulator circuit would be protected if terminals J 11  and J 12  were connected across a 277 V power line that had no transients. This dimming interface circuit is more robust than conventional 0-10 V dimming interfaces, but it is excessively costly because it includes a high-voltage diode and a high-voltage power MOSFET transistor. In addition, there are transient voltages that commonly occur on ac power lines, and diode D 2  and transistor Q 2  could be destroyed by these transients. Furthermore, the active dimming range is from 0.1 V to 6.6 V, while many shunt controllers, particularly manual types with slide controls, are designed to operate from about 1 to 9 V. Finally the current sourced by the control terminals is only 48 μA at 10 V, and some controls require 200 μA. 
     What is needed, then, is a dimming control interface that provides a current ranging from 300 μA to 500 μA in response to a DC control voltage ranging from 0-10 volts, and limits the voltages and currents present in the dimming control interface to safe levels when an external AC power source is accidentally connected to the dimming control input terminals. 
     SUMMARY OF THE INVENTION 
     The present invention includes a dimming control interface designed to be used with an external dimming control device, such as a shunt controller, to provide a dimming control output voltage to a ballast. In addition, the present invention includes additional circuitry designed to protect the dimming control interface from excessive currents and voltages that may be generated when the dimming control interface is connected to an external AC power source. As such, the dimming control interface includes a current source connected to a pair of input terminals and designed to provide a current to the input terminals when the external dimming control device is connected to the input terminals. The current source includes a high voltage resistor for limiting the current supplied to the input terminals when they are connected to an external AC power source. An isolated coupling circuit is connected between the input terminals and a pair of dimming control output terminals, and is operative to generate an output control voltage in response to an input control voltage applied to the input terminals. A clamping circuit and a second high voltage resistor are connected between the input terminals and the isolated coupling circuit, and are designed to limit the currents and voltages generated in the dimming control interface when an AC power source is connected to the input terminals. An alternative embodiment of the isolated coupling circuit is disclosed and includes an error amplifier that is connected to a current sensor circuit and to the input terminals. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a circuit diagram of a portion of a prior art 0-10 volt dimming control interface circuit. 
     FIG. 2 is a circuit diagram of a portion of another prior art 0-10 volt dimming interface circuit. 
     FIG. 3 is a simplified schematic diagram of a dimmable electronic ballast. 
     FIG. 4 is a circuit diagram of one embodiment of the present invention of a 0-10 volt dimming control interface. 
     FIG. 5 is a circuit diagram of a coupling circuit used in one embodiment of the present invention. 
     FIG. 6 is a circuit diagram of an alternative embodiment of the present invention of a 0-10 volt dimming control interface. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG. 3, a simplified schematic diagram of an electronic dimming ballast is shown. The ballast has an ac-dc power supply  100  that has ac input terminals J 101  and J 102  for connection to an ac power line. The main dc output of the power supply is provided between a first set of dc output terminals J 106  and J 107 . These output terminals are connected to dc input terminals J 306  and J 307  of a resonant inverter  300 . 
     An inductor L 3  is part of a typical boost power factor correction circuit (not shown) that provides power to terminals J 106  and J 107 . When the power supply is operating, an ac voltage is developed across winding L 3 A. This voltage is rectified by a charge-pump circuit comprising a resistor R 114 , a capacitor C 106 , and diodes D 101  and D 105 . The rectified output of the charge-pump circuit is filtered by a capacitor C 101 , and regulated to a value of about 32 V by a zener diode D 103 . This voltage is supplied to a second set of dc output terminals, J 101  and J 103  that are electrically isolated from terminals J 101  and J 102 . A 15 V regulator circuit is connected to the 32 V supply, and consists of a resistor R 101 , a zener diode D 102 , a small-signal NPN transistor Q 101 , and a bypass capacitor C 102 . The output of the 15 V regulator circuit is connected to terminal J 102 . The 32 V and 15 V power supplies described above are simple, low-cost circuits. They are not novel, and other known power supply circuits may be used. 
     An isolated control interface  200  (also referred to as a dimming control interface) has dc supply input terminals J 201 , J 202 , and J 203  that are connected, respectively, to terminals J 101 , J 102 , and J 103  of power supply  100 . Control interface  200  has control input terminals J 205  and J 204  that are intended to provide a current for operating a 0-10 V dimming control (not shown) that functions as a shunt controller. Control interface  200  provides a dimming control output signal at control output terminals J 215  and J 216 . The dimming control output signal at control output terminals J 215  and J 216  is controlled by the control input voltage between control input terminals J 205  and J 204 . Control output terminals J 215  and J 216  are electrically isolated from control input terminals J 205  and J 204 . 
     Resonant inverter  300  has dimming control input terminals J 315  and J 316  that are connected, respectively, to control output terminals J 215  and J 216 . A lamp  500  is connected to the resonant inverter through terminals J 310 , J 311 , J 312 , and J 313 . The resonant converter may be constructed to supply additional lamps (not shown). The voltage between dimming control input terminals J 315  and J 316  controls the lamp current supplied by the resonant inverter. Thus the isolated control interface allows the control voltage between control input terminals J 205  and J 204  to control the lamp current supplied by the resonant inverter. 
     Referring to FIG. 4, a 4.7 kΩ resistor R 218 , and two 33 kΩ resistors R 208  and R 217  are connected in series between the 32 V supply and control input terminal J 205  to form a simple current source. When an external dimming control is connected to the control input terminals, the current source provides a current to the dimming control that ranges from about 300 μA when the control voltage is 10 V to about 500 μA when the control voltage is zero. A 32 V supply is used so that the current source will be able to supply adequate current when the control voltage is 10V. Because the 32 V supply is higher than the desired maximum open-circuit control voltage level of 10 V, a clamping circuit must be utilized. The preferred place for the clamping to occur is at an intermediate terminal, junction J 214 , because this allows resistors R 208  and R 217  to prevent excessive current from flowing into control input terminals J 205  and J 204  when excessive voltages are applied. The clamping circuit cannot be just a zener diode as in FIG. 1, because junction J 214  must have a voltage greater than 10 V with respect to junction J 204  under normal operating conditions. 
     A clamping circuit  220  was developed as part of the present invention to allow terminal J 214  to be clamped only when necessary. Clamping circuit  220  has a common terminal J 222 , a clamp terminal J 221 , and a sense terminal J 223 . Clamping terminal J 221  is connected to an intermediate terminal J 214  in the current source resistor string. Common terminal J 222  is connected to control input terminal J 204 . Sense terminal J 223  is coupled through resistor R 209  to control terminal J 205 . Clamping circuit  220  consists of an NPN Darlington transistor Q 203  and a 9.1 V zener diode D 206 . Under open-circuit conditions, the control voltage rises until the voltage at sense terminal J 223  reaches about 10V, and Darlington transistor Q 203  turns on. This reduces the voltage at terminal J 214  to about 10 V as current flows from terminal J 214  through terminal J 221  to terminal J 222 . The clamping circuit therefore limits the open-circuit value of the control voltage to about 10 V while resistors R 209 , R 208 , and R 217  prevent excessive current from flowing into control input terminals J 205  and J 204  when excessive voltages are applied. 
     If a large ac voltage should inadvertently be applied between control input terminals J 205  and J 204 , the voltage at a modulator input terminal J 227  will be clamped in the positive direction to about 10 V because of the low impedance path between the sense and common terminals of clamp circuit  220  that consists of two forward-biased emitter-base junctions of transistor Q 203  and zener diode D 206 . Resistor R 209  consists of a high voltage resistor, or a series string of smaller resistors (such as three surface-mount resistors) having a total resistance value of about 240 kΩ. 
     This limits the currents flowing into terminals J 227  and J 223  to safe levels. 
     For large negative swings of the control voltage, the voltage at terminal J 227  is limited to about −0.7 V by a diode D 207  that is connected between terminals J 227  and J 204 . Again, resistor R 209  limits the currents to safe values. Resistors R 208  and R 217  are each rated at 1 W so that they can operated indefinitely with an ac voltage of 277 V rms applied between the control input terminals. A diode D 208  is connected in parallel with resistor R 218  so that the collector-to-emitter voltage of transistor Q 203  is limited to a safe value when a large ac voltage is applied between the control input terminals. The control interface  200  has been tested by connecting control input terminals J 204  and  205  to ac input terminals J 101 , and J 102 . The control interface is so robust that it is not damaged by 2.5 kV power line transients. In contrast, transistor Q 2  and diode D 2  in FIG. 2 would be destroyed by transients of that level. 
     Control input terminal J 205  is coupled to modulator input terminal J 227  of a pulse-width modulator circuit  230  through a resistor R 209 . The pulse-width modulator has output terminals, J 210  and J 211 , that are connected to input terminals, J 410  and J 411 , of an optocoupler U 401  that is shown in FIG.  5 . As is explained in further detail below, the optocoupler and other the circuitry shown in FIG. 5 produce a control output voltage between terminals J 215  and J 216  that is directly related to the control input voltage. The lamp current is therefore controlled by the voltage between the modulator input terminal and control input terminal J 204 . The lamp current will have its maximum value when the control input terminals are disconnected, and it will have a minimum value when the control input terminals are shorted together. 
     The control input voltage is ideally a dc signal, but noise signals may be present between control input terminals J 205  and J 204 . Consequently, a 0.1 μF capacitor C 204  is connected from J 227  to J 204  in order to filter out ac signals. If a large ac voltage is connected between control input terminals J 205  and J 204 , the voltage between J 227  to J 204  will be at a level that produces a lamp current that is about 30 percent of the maximum value. This level is much less than the maximum value, and much greater than the minimum value, typically 5 percent. Thus, if the control input terminals should accidentally be connected across an ac power line, the ballast will function, but the lamp current cannot be controlled. Instead, it will be at an intermediate value that will be distinguishable from fault conditions in which the control input terminals are shorted or opened by observation of the lamp brightness. 
     Pulse-width modulator  230  is comprised of an oscillator circuit  201  and a comparator circuit  210 . The oscillator circuit is comprised of an open-collector comparator U 201 A, capacitors C 203  and C 205 , and resistors R 202 , R 203 , R 204 , R 205 , and R 206 . The oscillator circuit produces a triangle wave with a frequency of about 240 Hz at an oscillator output terminal J 206 , which is connected to a negative comparator input terminal J 226 . Pulse-width modulator input terminal J 227  is connected to a positive comparator terminal J 225 . Terminals J 225  and J 226  are connected to the input terminals of a second open collector comparator, U 201 B. Comparators U 201 A and U 201 B may be contained within a single integrated circuit such as an LM 393  from National Semiconductor. Resistors R 211  and R 212  provide biasing to the positive input terminal of U 201 B to level-shift and scale the signal coupled through R 209  to match the oscillator output signal present at negative comparator input terminal J 226 . The duty cycle of the open-collector output of U 201 B is zero (the output voltage is high) when the control voltage present between terminals J 205  and J 204  is less than about 1 V, and it increases to 100 percent when the control voltage reaches about 9 V. 
     Referring to FIG. 5, a light-emmiting-diode (LED) D 401  of optocoupler U 401  is on when the open-collector output of U 201 B is on. Resistor R 213  limits the diode current to a safe level. A phototransistor Q 401  is on when diode D 401  is on. A 5 V voltage source (not shown) is connected between terminals J 420  and J 216 . A capacitor C 401  and resistors R 408 , R 409 , and R 410  are connected to transistor Q 401  and terminals J 430 , J 215 , and J 216  so that the voltage between control output terminals J 215 , and J 216  is directly related to the duty cycle of transistor Q 401 . 
     An alternative embodiment of the present invention of a dimming control interface is shown in FIG.  6 . In this embodiment, the optocoupler is operated in a linear mode as is done in the prior-art circuit shown in FIG.  2 . The current source and clamp circuits are the same as in FIG.  4 . The voltage between terminals J 227  and J 203  is proportional to the control voltage. The voltage at terminal J 227  is scaled by resistors R 502  and R 503 , and coupled to the positive input of an operational amplifier U 501 , which functions as an error amplifier. A current sensor circuit  501  produces a negative feedback signal at terminal J 517  that is directly related to the lamp current. The current sensor circuit may include a current transformer and a rectifier circuit. It may be also include a biasing circuit in the current sensor so that the output voltage will be greater than zero for zero lamp current. Typically the bias circuit would be adjusted so that the minimum lamp current occurs when the control voltage is about 1 V. The gain of the error amplifier is set with resistors R 501 , R 504  and C 501 . An optocoupler (not shown) can be connected between terminal J 511  and either terminal J 202  or J 203 . A resistor R 513  limits the current supplied to the optocoupler. 
     Thus, although there have been described particular embodiments of the present invention of a new and useful Dimming control interface, it is not intended that such references be construed as limitations upon the scope of this invention except as set forth in the following claims.