Electricity: electrical systems and devices – Safety and protection of systems and devices – With specific quantity comparison means
Reexamination Certificate
1999-03-23
2001-09-18
Sherry, Michael J. (Department: 2836)
Electricity: electrical systems and devices
Safety and protection of systems and devices
With specific quantity comparison means
C361S087000, C361S093600, C315S136000
Reexamination Certificate
active
06292339
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention pertains to the field of ballast circuits for powering arc discharge lamps, and is more particularly directed to a fault protection circuit for turning off ballast output power to the lamp load in the event of lamp malfunction.
2. State of the Prior Art
Arc discharge lamps such as fluorescent lamps require relatively high operating voltages, particularly when first initiating the arc discharge in the lamp. Once started, the lamp current must be externally limited to a normal level because arc lamp impedance characteristically drops after start-up. Many so called ballast circuits have been devised for supplying the necessary voltages and currents for powering such lamps.
Arc discharge lamps, for example fluorescent lamp tubes, are subject to certain types of malfunction characteristic of this type of lamp. If the lamp tube is loosened from its electrical socket the lamp connectors or pins may become sufficiently exposed to sustain an arc discharge between the exposed metal parts. Such an arc discharge is undesirable not only because it can severely stress the electrical components of the ballast circuit, but may also pose a fire and electrical shock hazard. Under such circumstances, it is, of course, desirable to shut down electrical power to the lamp immediately. Another type of lamp malfunction occurs with aging of the discharge lamp, manifested by partial rectification of the A.C. current delivered by the ballast to the lamp load. In this case too it is desirable to interrupt electrical power to the lamp load in order to call the attention of maintenance personnel to the need for replacing the lamp. Ballast output protection is particularly needed in arc discharge lamp fixtures installed in aircraft cabin lighting and other vehicles where lighting fixtures are subject to shock and vibration. Lamps shaken loose from their sockets frequently result in arcing and occasionally in fires when the lamp contacts become exposed. In the interest of safety it is desirable to minimize the possibility of lamp arcing, not only on public transportation of all kinds, but wherever arc discharge lighting is used.
Various schemes have been devised for sensing abnormal operating conditions of the lamp load in arc discharge fixtures and shutting down the output of the ballast in such event. For example, it is known that certain failure modes, particularly arcing, result in abnormally high transient voltages and currents through the lamp load. This knowledge has been exploited in the past to design protection circuits responsive to such higher than normal voltages or currents. In one known type of protection circuit a voltage comparator has one input connected for sensing the lamp voltage and the other input referenced to a fixed, preset voltage threshold. The output of the comparator is connected for switching off the ballast output in the event that the lamp voltage exceeds the treshhold voltage. An improved form of this type of output protection circuit, intended for use with variable lamp loads, is disclosed in a commonly assigned copending application.
In general the sensing of lamp fault conditions in arc discharge lighting has proven to be a challenging task. Although many solutions have been proposed many are too complex or costly to implement on a large commercial scale. A successful solution must be not only dependable but also economically viable in the highly competitive and cost sensitive interior lighting industry. Furthermore, the ballast output protection must work reliably with so called universal ballasts intended to supply lamp loads of different wattage, as well as with dimmable ballasts which deliver a variable lamp current for adjustable illumination.
SUMMARY OF THE INVENTION
The aforementioned need is addressed by this invention which provides a simple output protection circuit which can be economically incorporated into a wide variety of ballast circuit topologies. The novel output protection circuit for an arc discharge lamp ballast includes a winding arranged and configured for inductively sampling a high frequency output current of the ballast to derive a sample current. A burden capacitor is connected across the winding, the capacitor being selected to present a low impedance across the winding to a substantially symmetrical waveform of the sample current. A switch element is actuated by a charge buildup on the burden capacitor due to asymmetry in the sample current waveform, the switch element being connected for shutting-off power supplied by the ballast to an arc discharge lamp. The switch element may be a silicon controlled rectifier having a gate connected to the burden capacitor so that the silicon controlled rectifier turns on in response to the charge buildup on the burden capacitor. For example, the silicon controlled rectifier may be connected for bringing low the base of one or more drive transistors of the ballast thereby to switch-off the drive transistors and consequently shut-off power ballast to the lamp in response to a charge buildup on the burden capacitor.
The invention may also be understood as having circuit means connected for applying an unrectified alternating current sample waveform representative of an output waveform of the ballast to a burden capacitor, and switch means connected for shutting-down the ballast responsive to a sufficient charge buildup across the capacitor due to a degree of asymmetry between positive and negative half-cycles of the sample waveform indicative of an abnormal operating condition of a lamp load connected to the ballast. Alternatively, the invention may also be expressed as a method for protecting a ballast circuit against an abnormal operating condition of an arc lamp load powered by the ballast circuit, the method comprising the steps of applying a sample waveform representative of an output waveform of the ballast to a burden capacitor, and disabling the output of the ballast circuit responsive to a sufficient charge buildup across the capacitor due to a degree of asymmetry between positive and negative half-cycles of the sample waveform indicative of an abnormal operating condition of a lamp load connected to the ballast. The ballast disabling switch may be a silicon controlled rectifier connected for disabling output power from the ballast circuit and the sufficient charge buildup is the charge required to actuate said switch by changing the state of the silicon controlled rectifier.
In a broader sense the invention is an output protection circuit for an arc discharge lamp ballast includes a first element or circuit for sampling an output waveform of the ballast, a second element or circuit for sensing asymmetry in the output waveform and a switch connected for shutting-down the output of the ballast responsive to a sufficient degree of asymmetry in the output waveform. That is, the output protection circuit
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is stand-alone to the ballast circuit topology.
The invention may be also understood as a method for sensing an abnormal operating condition of an arc lamp load powered by a ballast circuit, comprising the steps of sampling an output waveform of the ballast, assessing the degree of asymmetry in the output waveform, and declaring an abnormal operating condition upon exceeding a predetermined degree of asymmetry. The step of assessing may be performed by charging a capacitor with the output waveform and the predetermined degree of asymmetry may be determined by a charge on the capacitor sufficient to actuate a ballast disabling switch connected to the capacitor. The ballast disabling switch may be a silicon controlled rectifier and the aforementioned sufficient charge the potential across the burden capacitor required to switch the state of the silicon controlled rectifier, for example, from an off state to an on state.
These and other improvements, features and advantages will be better understood from the following detailed description of the preferred embodiment taken in conjunction with the accompanying drawings.
REFERENCES:
p
Natan Epstein Beehler & Pavitt
Sherry Michael J.
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