Multiple Failure Detection Shutdown Protection Circuit for an Electronic Ballast

Abstract

A shut-down circuit configured for use with an electronic ballast coupled to a lamp in a control path includes a device for sensing the electrical energy associated with the control path, and a sensing circuit for shutting down the ballast in the event that the energy does not conform to a predetermined condition. The sensed energy may be current indicative of lamp installation or short circuit, or voltage indicative of arcing or open circuit. The device for sensing the electrical energy associated with the control path may be an isolation transformer, or alternative devices such as optical isolators may be employed. The circuitry may further include electronic componentry to disable the sensing circuit during initial energization of the lamp. In one disclosed example, the sensing circuit includes a node that should be at or near a predetermined electrical potential when the lamp is operating properly, and a switch such as a Schmitt trigger coupled to the node that turns on or off to shut down the ballast if the node is not at or near the predetermined electrical potential. In an implementation used to detect voltage fluctuations indicative of arcing, the circuitry may include a high-pass filter or differentiator and detector to detect high-frequency noise. Alternatively, a phase-locked loop may be coupled to a low-pass filter to detect high-frequency noise indicative of arcing. In another alternative implementation, a microprocessor performing a digital signal processing algorithm may determine the presence of an abnormal condition.

1 . An electrical circuit including a lamp, the electrical circuit comprising:
 a ballast inverter adapted to receive an input electrical power and adapted to provide a relatively controlled electrical current to the lamp;   an output transformer connected to said ballast inverter adapted to receive the controlled electrical current and to transform the controlled electrical current into alternating current to drive the lamp;   a lamp current path including said output transformer, the lamp, and an step-up transformer;   wherein said step-up transformer detects high-frequency noise component on the lamp current path; and   a sensing circuit connected to said step-up transformer and connected to said ballast inverter, said sensing circuit adapted to receive the detected noise component and adapted to, when an undesired level of noise is sensed, cause said ballast inverter to cease providing the controlled electrical current.   
     
     
         2 . The electrical circuit recited in  claim 1  wherein the noise component indicates lamp installation or removal. 
     
     
         3 . The electrical circuit recited in  claim 1  wherein the noise component indicates electrical arcing on the lamp current path. 
     
     
         4 . The circuit recited in  claim 1  wherein said sensing circuit comprises an optical isolator. 
     
     
         5 . The circuit recited in  claim 1  further comprising electronic componentry adapted to disable the sensing circuit during initial energization of the lamp. 
     
     
         6 . The circuit recited in  claim 1  wherein the sensing circuit comprises:
 a node substantially at a predetermined electrical potential when the lamp is operating; and 
 a switch coupled to said node adapted to turns on or off to shut down said ballast inverter when said node drafts away from the predetermined electrical potential. 
 
     
     
         7 . The circuit recited in  claim 6  wherein said switch comprises a Schmitt trigger. 
     
     
         8 . A method of operating an electrical circuit including a lamp, the method comprising:
 receiving input electrical power;   providing a relatively controlled electrical current to the lamp by processing the input electrical power;   transforming the controlled electrical current into alternating current to drive the lamp;   operating the lamp by supplying the alternating current to the lamp;   detecting high-frequency noise component of the alternating current of the lamp; and   ceasing the provision of the relatively controlled electrical current to the lamp when the detected noise component is beyond a threshold level.   
     
     
         9 . The method recited in  claim 8  wherein the noise component indicates lamp installation or removal. 
     
     
         10 . The method recited in  claim 8  wherein the noise component indicates electrical arcing on the lamp current path. 
     
     
         11 . The method recited in  claim 8  further comprising a step of disabling the detecting step during initial energization of the lamp. 
     
     
         12 . An abnormal load condition detection circuit configured for use with an electronic ballast, the circuit comprising:
 a device for sensing electrical energy associated with a lamp load path;   a switch element having a switch output that acts as a one-way diode when not actuated, and as a negligible resistance when actuated;   a coupling circuit to transfer energy from said sensing device element to said switch output, whereby a rectified DC voltage is present at a node when the switch is unactuated and load path electrical energy is present; and   a high pass filter for actuating the series pass switch element when the harmonic content of the electrical energy is above a predetermined threshold, thus reducing the DC voltage present at the node.   
     
     
         13 . The electrical circuit recited in  claim 12 , wherein the series pass switch element is a field-effect-transistor with an intrinsic diode between source and drain. 
     
     
         14 . The electrical circuit recited in  claim 12 , said detection circuit adapted, when the harmonic content of the electrical energy is above said predetermined threshold, to cause said ballast inverter to cease providing electrical current.