Patent Document

BACKGROUND OF THE INVENTION 
     This invention relates to a protection circuit for smaller diameter gas discharge lamps and for compact fluorescent tube designs in particular. The device acts to reduce power in an electronic ballast and keeps the power at a low level when sensing a fault condition occurring in the lamp. 
     As fluorescent lamps age, the emissive material is worn away so that any arc current must flow from the bare tungsten filament. The work functions of the tungsten filament is high, therefore the filament is quickly melted. If the voltage supplied by the ballast is high enough, the lamp can be started with a broken or missing filament. After the filament has evaporated, the lamp arc current can flow from one of the filament support wires. These wires also have a high work function and considerable heat is generated when arc current flows from one of them. In some cases the heat is high enough to crack or melt the glass wall of the lamp. Overheating is a particular problem with small diameter lamp tubes and compact fluorescent lamps especially toward the end of the lamp&#39;s life. In a compact fluorescent lamp both ends of the lamp are adjacent to each other and are encased in a plastic housing. The base of the lamps are mounted in a plastic housing to which either a connector or a ballast is attached. When the lamps overheat, the hot glass can cause the plastic to deform. The glass can reach temperatures of over 350 degrees Celsius on the outer surface during abnormal lamp operating modes. It is possible that operation at these high temperatures will cause the glass wall to crack. It is desirable for the ballast, when operating, to be able to reliably control all lamp operating modes, so that abnormal lamp failure modes are not permitted to develop. 
     Three test schemes have been proposed as an IEC standard by Osram Sylvania; first, a test for maximum current heating in the electrodes, second, a test for lamp overvoltage or voltage rise and third, a test for lamp rectification. Designing a cost effective and manufacturable circuit to perform these tests is challenging. Various shutdown sensing and control circuit schemes are possible. One method is to sense when the lamp voltage rises above a setpoint while the lamp arc is ignited and then issue a shutdown or power reduction command to shut down the supply of power from the ballast to the lamp. A second possible solution is to sense when the output power rises above a control setpoint and then issue a command to reduce the supply of power from the ballast to the lamp. A third possible solution is to sense the filament current level when the filament voltage is present and issue a command if the filament current does not flow or has a zero value. Each of these solutions pose a challenge to the circuit designer. It is difficult to build a circuit that does not suffer from being complicated to build, and degrading the reliability of the ballast system by adding additional components that are subject to failure. 
     Once the ballast power is reduced to a safe level, the circuit needs to be able to perform two functions. First, it needs to be able to sense when a lamp is behaving abnormally and automatically resume normal operation when the failed lamp is replaced. The second function of a shutdown circuit could be described as limiting the periodic attempts at restarting until the lamp is replaced to minimize annoying blinking of the lamps until they are replaced. Another solution for this function, would be for the ballast to sense proper lamp operation and resume normal operations without input voltage reset or lamp blinking. The circuits could be designed to allow resetting only when power is first removed and then reapplied to the ballast. This would work in some applications, but not in others where the lamp fixture is normally continuously powered. 
     Prior attempts at gas discharge lamp ballasts with shutdown circuits have resulted in implementations with various drawbacks. Such implementations are illustrated by U.S. Pat. No. RE 32,901, RE 32,953, and 5,004,955. These show ballast shutdown circuits that sense an overvoltage condition in the resonant output circuit. They require a voltage clamp (varistor) to activate before the shutdown circuit can be activated. Its purpose is to prevent a shock hazard at the output terminals and not to protect the lamp. This approach has several draw backs, first two of the abnormal lamp failure modes may not be detected. The missing filament and asymmetrical operation. Second, these circuits require turning the power off and on after a new lamp is installed before the lamps will strike. For many large fixture installations on one circuit this is not feasible. U.S. Pat. 4,667,131, and 4,503,363 disclose shock protection circuits that have a two stage turn off. They use a high voltage SCR and a transistor to disable the inverter. These additional and high voltage parts add additional cost to the circuit. These circuits will not detect asymmetrical operation of the lamp. U.S. Pat. 5,436,529 shows a circuit that disables an inverter in response to an overvoltage condition occurring at the lamp. Once turned off, it stays off until the defective lamp is replaced and then automatically restarts. It requires extra components such as DC blocking capacitors to allow the circuit to sense a broken filament and turn-off. 
     These prior art circuits require a large component count and their associated complexity. The prior art shutdown and lamp protection circuits are all directed toward series-resonant non-isolated ballast designs that do not have an isolation transformer between the lamp and the circuit. It is advantageous to be able to have a lamp protection circuit that operates with a parallel resonant isolated ballast. This is the preferred ballast and lamp configuration in the industry. 
     A currently unmet need exists for a simple circuit to work with electronic ballasts that are of a parallel resonant design with an isolation transformer to protect small diameter gas discharge lamps and compact fluorescent lamps from overheating and cracking. 
     SUMMARY 
     An object of the invention is to provide a lamp protection circuit for smaller diameter gas discharge lamps such as compact fluorescent lamps including both isolated and non-isolated ballast circuits. The protection circuit protects against several lamp failure modes that can cause filament overheating and cracking of the lamp. 
     Another object of the invention is to provide a lamp protection circuit that detects a lamp overvoltage condition and a lamp rectification condition. 
     Another object of the invention is to allow a protection circuit design that may include automatic restarting after a defective lamp is replaced. 
     Another object of the invention is to provide a protection circuit that is simple, easily fabricated, low in parts count and cost, light weight and reliable. 
     A protection circuit for at least one gas discharge lamp. The protection circuit senses several failure modes in the lamp and provides a control signal to a ballast circuit powering the lamp to reduce power being supplied to the lamp to prevent overheating of the lamp. The circuit includes a sensor means for detecting the lamp failure mode. The sensor means providing a sensor signal proportional to the lamp power. A control means is coupled to the sensor means. The control means has a control input terminal for receiving the sensor signal and a control output terminal. The control means provides the control signal at the output terminal in response to the sensor means. A boost converter is connected to the control means output terminal. The control means provides the control signal to the boost converter such that power to the lamp is reduced. A time delay means is also a feature of the protection circuit. The time delay means is connected to the control means such that the control means is unresponsive to a transient overvoltage prior to detecting the failure mode. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other features, aspects and advantages of the present invention will become better understood with regard to the following description, appended claims and accompanying drawings where: 
     FIG. 1 is a block diagram of a Lamp Protection Circuit for Isolated Ballasts Driving Gas Discharge Lamps. 
     FIG. 2 is a schematic diagram of a preferred embodiment of a Primary Side Lamp Protection Circuit for Isolated Ballasts Driving Gas Discharge Lamps. 
     FIG. 3 is a schematic diagram of another embodiment of a Primary Side Lamp Protection Circuit for Isolated Ballasts Driving Gas Discharge Lamps that utilizes a comparator. 
     FIG. 4 is a schematic diagram of another embodiment of a Primary Side Lamp Protection Circuit for Isolated Ballasts Driving Gas Discharge Lamps that connects directly to the inverter. 
     FIG. 5 is a schematic diagram of another embodiment of a Primary Side Lamp Protection Circuit for Isolated Ballasts Driving Gas Discharge Lamps that connects directly to the inverter which utilizes a comparator. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG. 1, a block diagram of a ballast and a lamp protection circuit of the invention is shown. A rectifier  12  receives low frequency AC power  10  and converts it into a pulsed DC voltage. The rectifier output is fed into a boost converter  20  and an inverter  14 . The boost converter  20  increases the DC voltage above the magnitude of the AC line voltage and achieves power factor correction. This increased voltage is called the DC bulk voltage. The boost converter is used to generate a higher output voltage while controlling the rectifier input current waveshape to achieve unity power factor with the current at a low distortion. An inverter circuit  14  converts the DC signal to a high frequency AC signal on the order of 30 Kilohertz which drives a load. In this case, the load of interest is a gas discharge lamp such as lamp  18 . A resonant LC circuit  15  provides a limited sine wave voltage. The lamp may be electrically isolated from the rest of the circuit for shock hazard and safety reasons by output transformer  16 . It is also possible to omit the isolation transformer and use other means for shock protection. A lamp protection circuit would also work with these ballast designs. 
     The failure sensing circuit  22  is connected to the inverter  14 . Circuit  22  senses the power level drawn by the lamp. This power level is reflected back into the inverter or the primary of the output transformer as the voltage and current product. Upon sensing the failure modes it issues a control signal to the boost converter. The control signal turns off the converter. This causes the magnitude of the DC bulk voltage to fall and conversely causes the magnitude of the high frequency AC voltage from the inverter to fall. The lower voltage transmitted through the output transformer to the lamp is then not of sufficient magnitude to cause the defective lamp to overheat. In some instances, the electric arc in the lamp will be extinguished depending upon the lamp condition. The protection circuit may be designed to periodically allow the converter to cycle in a restart attempt. In other instances, the power to the lamp is limited, until the lamp becomes deactivated. 
     The circuit could also be utilized to regulate the maximum output power level to a desired value, if so chosen. Preferably, the failure sensing circuit along with the boost converter, rectifier, inverter and output transformer would be manufactured as an integrated assembly. 
     Referring now to FIG. 2, a detailed schematic of a ballast and the protection circuit is shown. In the circuit of FIG. 2, input terminals E 7  and E 8  are connected to a source of low frequency AC power. Diodes D 1 ,D 2 ,D 3  and D 4  such as a 1N4004 form a diode bridge which receives the AC power and provides an output of pulsed DC. A 0.1 microfarad capacitor C 3  is connected across the output of the bridge. The diode bridge is connected with line DC+ and DC−. These carry the DC bulk voltage. Inductor L 3  is located in series with a blocking diode D 5 . The Diode D 5  utilized was a commmercially available UF4004. Connected between inductor L 3  and diode D 5  is the drain of a power factor correction switch (transistor) Q 1  such as a IRF730. The source of Q 1  is connected through a 0.47 ohm resistor R 3  to the DC− line. Inductor L 3 , transistor Q 1 , diode D 5 , resistor R 3 , and circuit  33  form the boost converter. Inductor L 3  stores energy which is periodically switched into the rest of the circuit by Q 1 . This increases the magnitude of the DC bulk voltage at C 14  with respect to the DC− buss above the magnitude of the AC input voltage. Transistors Q 4  and Q 5  form a push-pull inverter for transforming the DC bulk voltage into a high frequency sine wave AC signal. Transistors such as Motorola XJE 18204 could be used for Q 4  and Q 5 . A 330 K ohm resistor R 5 , a 1.6 K ohm resistor R 9 , a 1.6 K ohm resistor RS, a 1.5 ohm resistor R 10 , a diode D 6 , a capacitor C 12 , inductor L 4  and transformer winding T 1 B form a biasing network for applying the proper voltages to the bases of transistors Q 4  and Q 5 . The emitters of Q 4  and Q 5  are commoned at junction J 1 . Junction J 1  is also known as inverter terminal J 1 . The collector of transistor Q 4  is connected to one end of the primary of output transformer T 1 . The collector of transistor Q 5  is connected to the other end of the primary of output transformer T 1 . A 1.8 nanofarad capacitor C 6  is connected across the primary of T 1 . Inductor L 4  has winding L 4 A connected between diode D 5  and a center tap on the primary side of output transformer T 1 . Winding L 4 B is connected between diode D 6  and the DC− line. Capacitor C 12  is connected between diode D 6  and the DC− line. A Diode D 9  such as a SGS BZW06 is connected between inductor L 4  and the DC− line. Diode D 9  prevents excessive voltages from damaging transistors Q 4  and Q 5  when lamp B 1  is removed from the circuit or when other damaging conditions may occur. The cathode of diode D 9  is connected to inductor L 4  and the anode of D 9  is connected to the DC− line. A  47  microfarad electrolytic capacitor C 14  is connected between the DC+ and DC− lines. 
     Power factor correction circuit  33  is connected to the protection circuit via terminals P 2  through P 6 . Circuit  33 , transistor Q 1 , inductor L 3 , diode DS and resistor R 3  form the boost converter. Circuit  33  could be a power factor correction IC such as a Siemens 4817 or it could be a discrete implementation of such a device. Circuit  33  controls the power factor correction switch (transistor Q 1 ) to generate a higher output voltage while controlling the rectifier input current waveshape to achieve unity power factor with the current at a low distortion. Terminal P 2  is connected to the junction J 2  of resistors R 5 ,R 8  and R 9  for biasing purposes. Terminal P 3  is connected to the collector of transistor Q 31 . Terminal P 4  is connected to the DC− line. Terminal P 5  is connected ahead of inductor L 2 . Terminal P 6  is connected to the gate of power factor correction switch Q 1 . One or more lamp(s) B 1  are disconnectedly connected through a 2.0 nanofarad capacitor C 8  to output terminals E 1  and E 2  of the secondary of output transformer T 1 . 
     Protection circuit  30  contains a transistor Q 31 . Transistor Q 31  can be a commercially available transistor such as a 2N4401. Circuit  30  also has a 10 K ohm resistor R 33 , a 3.9 ohm resistor R 32 , a 100 K ohm resistor R 31  and a 4.7 microfarad capacitor C 31 . Resistor R 32  is connected between the junction of the emitters of transistors Q 4  and Q 5  (junction J 1 ) and the DC− line. Resistor R 33  is connected between junction J 1  and the base of transistor Q 31 . The collector of transistor Q 31  is connected to terminal P 3  of power factor correction circuit  33 . The emitter of transistor Q 31  is connected to the DC− line. Capacitor C 31  is connected between the base of transistor Q 31  and the DC− line. Resistor R 31  is connected between the DC− line and the base of Q 31 . 
     The operation of the protection circuit of FIG. 2 is as follows. The protection circuit  30  will sense the lamp overvoltage failure mode and the rectifying lamp failure mode. The first condition sensed is when the lamp voltage rises above a setpoint while the lamp arc is ignited. If the lamp voltage rises by about 20 percent, then the lamp power will rise by about 20 percent. The power level on the primary winding of transformer T 1  is proportional to the output power appearing at the lamp. Since the ballasting capacitor C 8  delivers an approximately constant current to the load, any increase in load voltage will result in an increase in load power. The load power will be reflected across the primary of T 1 . Similarly, the primary voltage of T 1  is fixed by the magnitude of the DC voltage across the bulk capacitor C 14 . Therefore, an increase in load power will result in an increase in current drawn by transistors Q 4  and Q 5 . The voltage at the junction J 1  of the emitters of transistor Q 4  and Q 5  with respect to the DC− buss will be proportional to the reflected power from the lamp with a constant bulk voltage. Resistor R 32  acts as a sensor means to sense this current and outputs a sensor signal. Adjusting the value of resistor R 32  changes the threshold value that indicates an overvoltage condition in the lamp which is indicative of an abnormal operating mode. Alternatively, the power level could be sensed directly from the primary winding of T 1 . When the voltage at J 1  goes high, resistor R 32  will cause transistor Q 31  to be properly biased to turn on. Here transistor Q 31  along with its biasing elements is the control means with its control input terminal being the base and its control output terminal being the collector. When transistor Q 31  turns on, terminal P 3  of the converter will be pulled low. A low state on terminal P 3  will cause the power factor correction circuit to turn terminal P 6  off and consequently keep transistor Q 1  turned off When transistor Q 1  is turned off none of the energy from inductor L 3  is switched into the circuit to boost the magnitude of the DC voltage. The DC bulk voltage available to inverter transistors Q 4  and Q 5  is greatly lowered. The lower voltage when transferred across transformer T 1  is of insufficient voltage and power to cause lamp B 1  to overheat. Resistor R 33  and capacitor C 31  provide a time delay means for turning on transistor Q 31 . Capacitor C 31  initially provides for a charge time to allow the protection circuit to be non-responsive during lamp start-up. When a lamp is first started, a high initial voltage is required to strike the arc. In order to prevent the protection circuit from detecting the starting voltage, a time delay is incorporated. The time delay allows the lamps to start without triggering the protection circuit. Using a 10 K ohm resistor R 33  and 4.7 microfarad capacitor C 31  gives a time constant of  43  milliseconds in conjunction with R 31 . After C 31  initially discharges and turns off Q 1 , transistor Q 31  will turn off, allowing transistor Q 1  to turn on and the DC voltage to begin to rise again. Resistor R 31  helps to discharge C 31 . With the defective lamp still in place, the power will rise to an excessive level and be detected. C 31  will then charge up again and turn-on transistor Q 31  which will turn off power factor correction switch Q 1 . This cycling action allows for periodic attempts to restart the lamp and resume normal operation once the defective lamp has been replaced. 
     The second failure mode sensed by the circuit of FIG. 2 is a rectifying lamp. If a lamp should start to rectify the AC current flowing through it, it will show a DC potential difference across the lamp. When this occurs, the lamp will require (draw) more power due to increased cathode fall. This increased power will be reflected in the primary winding of transformer T 1  and also in the emitter currents at junction J 1 . This will be detected by protection circuit  30 . Protection circuit  30  will then operate in the same mode as discussed previously for the overvoltage condition to reduce the power supplied to the lamp(s). 
     Referring to FIG. 3, another embodiment of a ballast and lamp protection circuit is shown. In the circuit of FIG. 3, all the components are identical to those shown and described in FIG. 2 except for the protection circuit  35  which is different. 
     Protection circuit  35  contains a comparator  38  such as a SGS TS372 and a voltage reference  37 . Also included are a 10 K ohm resistor R 33 , a 3.9 ohm resistor R 32 , a 100 K ohm resistor R 31  and a 4.7 micro farad capacitor C 31 . Resistor R 32  is connected between the junction of the emitters of transistors Q 4  and Q 5  (junction J 1 ) and the DC− line. Resistor R 33  is connected between junction J 1  and the negative input of comparator  38 . Comparator  38  also has inputs connected to the DC− line and voltage reference  37 . Comparator  38  has its output connected to terminal P 3  of power factor correction circuit  33 . Resistor R 31  is connected between the DC− line and the negative input of comparator  38 . Capacitor C 31  is connected between the negative input of comparator  38  and the DC− line. 
     The operation of the protection circuit of FIG. 3 is as follows. The protection circuit  35  will sense the lamp overvoltage failure mode and the rectifying lamp failure mode. Both of these failure modes result in increased output wattage. The first condition sensed is when the lamp voltage rises above a setpoint while the lamp arc is ignited. The wattage developed in the primary winding of transformer T 1  is proportional to the wattage appearing at the lamp. The voltage at the junction J 1  of the emitters of transistor Q 4  and Q 5  with respect to the DC− buss will be proportional to the wattage developed in the primary of T 1 . Resistor R 32  acts as the sensor means to sense this current and output a sensor signal. When the voltage at J 1  goes high, it will appear at the negative terminal of comparator  38 . This will be compared to voltage reference  37  causing comparator  38  to go high at its output. When comparator  38  goes high, terminal P 3  of the converter will be pulled low. Here comparator  38  is the control means with its control input terminal being the negative input pin and its control output terminal being the output pin. A low state on terminal P 3  will cause the power factor correction circuit to turn terminal P 6  off and consequently keep transistor Q 1  turned off. When transistor Q 1  is turned off, none of the energy from inductor L 3  is switched into the circuit to boost the magnitude of the DC voltage. The DC voltage available to inverter transistors Q 4  and Q 5  is greatly lowered. The lower voltage when transferred across transformer T 1  is insufficient power to cause lamps B 1  to overheat. Resistor R 33  and capacitor C 31  provide a time delay means for turning on comparator  38 . Capacitor C 31  initially provides for a charge time to allow the protection circuit to be non-responsive to the increased lamp voltage required during lamp starting. Using a 10 K ohm resistor R 33  and a 4.7 microfarad capacitor C 31  gives a time constant of 43 milliseconds. After C 31  initially discharges and turns off Q 1 , the comparator will turn off allowing transistor Q 1  to turn on and the DC voltage to begin to rise again. Resistor R 31  helps to discharge C 31 . With the defective lamp still in place, the power will rise to an excessive level and be detected. C 31  will then charge up again and tum-on comparator  38  which will turn off power factor correction switch Q 1 . This cycling action allows for periodic attempts to restart the lamp and resume normal operation once the defective lamp has been replaced. 
     The second failure mode sensed by the circuit is a rectifying lamp. If a lamp should start to rectify the AC current flowing through it, it will show a DC potential difference across the lamp. When this occurs, the lamp will require (draw) more power due to increased cathode fall. This increased power will be reflected in the primary winding of transformer T 1  and also in the emitter currents at junction J 1 . This will be detected by protection circuit  35 . Protection circuit  35  will then operate in the same mode as discussed previously for the overvoltage condition to reduce the power supplied to the lamp(s). 
     Referring to FIG. 4, another embodiment of a ballast and lamp protection circuit is shown. The circuit of FIG. 4 is similar to FIG.  2 . In FIG. 4, the failure sensing circuit is connected to the inverter instead of to the power factor correction circuit. The changes include having the collector of transistor Q 31  connected to the center tap of transformer winding T 1 B. Resistor R 8  is eliminated and resistor R 9  is connected between Junction J 2  and the T 1 B center tap connection for biasing purposes. In this configuration, the transistors required would need to be high gain transistors. To use the same transistor as in the other embodiments, the values of R 31 , R 33  and C 31  need to be changed. R 31  is now a 5 K ohm resistor. R 33  is a 500 ohm resistor and C 31  is a 100 uF capacitor. The resulting time constant is 45 milliseconds. The rest of the circuit is identical to that shown and described in FIG.  2 . 
     The operation of the protection circuit of FIG. 4 is as follows. The protection circuit  30  will sense the lamp overvoltage failure mode and the rectifying lamp failure mode. The first condition sensed is when the lamp voltage rises above a setpoint while the lamp arc is ignited. The power level on the primary winding of transformer T 1  is proportional to the output power appearing at the lamp. Since the ballasting capacitor C 8  delivers an approximately constant current to the load, any increase in load voltage will result in an increase in load power. The load power will be reflected across the primary of T 1 . Similarly, the primary voltage of T 1  is fixed by the magnitude of the DC voltage across the bulk capacitor C 14 . Therefore, an increase in load power will result in an increase in current drawn by transistors Q 4  and Q 5 . The voltage at the junction J 1  of the emitters of transistor Q 4  and Q 5  with respect to the DC− buss will be proportional to the reflected power from the lamp with a constant bulk voltage. Resistor R 32  acts as the sensor means to sense this current. Alternatively, the power level could be sensed directly from the primary winding of T 1 . When the voltage at J 1  goes high, resistor R 32  will cause transistor Q 31  to be properly biased to turn on. Here transistor Q 31  is the control means with its control input terminal being the base and its control output terminal being the collector. When transistor Q 31  turns on, winding T 1 B will be pulled low. A low state on winding T 1 B will cause the inverter circuit of transistors Q 4  and QS to reduce its output power. The lower voltage when transferred across transformer T 1  is of insufficient voltage and power to cause lamp B 1  to overheat. Resistor R 33  and capacitor C 31  again provide a time delay means. 
     The second failure mode sensed by the circuit of FIG. 4 is a rectifying lamp. If a lamp should start to rectify the AC current flowing through it, it will show a DC potential difference across the lamp. When this occurs, the lamp will require (draw) more power due to increased cathode fall. This increased power will be reflected in the primary winding of transformer T 1  and also in the emitter currents at junction J 1 . This will be detected by protection circuit  30 . Protection circuit  30  will then operate in the same mode as discussed previously for the overvoltage condition to reduce the power supplied to the lamp(s). 
     Referring to FIG. 5, another embodiment of a ballast and lamp protection circuit is shown. The circuit of FIG. 5 is similar to FIG.  3 . In FIG. 5, the failure sensing circuit is connected to the inverter instead of to the power factor correction circuit. The changes include having the collector of transistor Q 31  connected to the center tap of transformer winding T 1 B. Resistor R 8  is eliminated and resistor R 9  is connected between Junction J 2  and the T 1 B center tap connection for biasing purposes. The rest of the circuit is identical to that shown and described in FIG.  3 . 
     The operation of the protection circuit of FIG. 5 is as follows. The protection circuit  35  will sense the lamp overvoltage failure mode and the rectifying lamp failure mode. Both of these failure modes result in increased output wattage. The first condition sensed is when the lamp voltage rises above a setpoint while the lamp arc is ignited. The wattage developed in the primary winding of transformer T 1  is proportional to the wattage appearing at the lamp. The voltage at the junction J 1  of the emitters of transistor Q 4  and Q 5 , with respect to the DC− buss, will be proportional to the wattage developed in the primary of T 1 . Resistor R 32  acts as the sensor means to sense this current. When the voltage at J 1  goes high, it will appear at the negative terminal of comparator  38 . This will be compared to voltage reference  37  causing  38  to go high at its output. Here comparator  38  is the control means with its control input terminal being the negative input pin and its control output terminal being the output pin. When comparator  38  goes high, winding T 1 B will be pulled low. A low state on winding T 1 B will cause the inverter current and voltage to be reduced. The lower voltage when transferred across transformer T 1  is of insufficient power to cause lamps B 1  to overheat. Resistor R 33  and capacitor C 31  again provide a time delay means 
     The second failure mode sensed by the circuit is a rectifying lamp. If a lamp should start to rectify the AC current flowing through it, it will show a DC potential difference across the lamp. When this occurs, the lamp will require (draw) more power due to increased cathode fall. This increased power will be reflected in the primary winding of transformer T 1  and also in the emitter currents at junction J 1 . This will be detected by protection circuit  35 . Protection circuit  35  will then operate in the same mode as discussed previously for the overvoltage condition to reduce the power supplied to the lamp(s). 
     The present invention has been described in connection with a preferred embodiment. It will be understood that many modifications and variations will be readily apparent to those of ordinary skill in the art without departing from the spirit or scope of the invention and that the invention is not to be taken as limited to all of the details herein. Therefore, it is manifestly intended that this invention be limited only by the claims and the equivalents thereof

Technology Category: y