Electric lamp and discharge devices: systems – With automatic shunt and/or cutout
Reexamination Certificate
1999-04-06
2001-01-16
Wong, Don (Department: 2821)
Electric lamp and discharge devices: systems
With automatic shunt and/or cutout
C315S246000
Reexamination Certificate
active
06175189
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. Aircraft vibration may shake individual lamps loose from their sockets exposing the metal contacts and resulting in arcing. Electrical arcs are clearly undesirable in an environment exposed to combustible fumes from jet fuel.
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. This type of output protection circuit works well in ballasts intended for use with a known lamp load, i.e. a given lamp wattage. If the lamp wattage is known in advance, then the normal operating lamp voltage can be anticipated and a reliable reference voltage can be preset for the comparator circuit.
The conventional idea of sampling the output voltage or current of the lamp load and comparing that information against a fixed reference for sensing an over voltage or over current condition is sound. However, the use of a fixed reference will not work when the output lamp load changes, or the ballast is designed for bright and dim lamp operation. This shortcoming is particularly evident with so called universal ballasts designed to supply a range of lamp loads, allowing an end user to install lamps of different wattage as needed. Universal ballasts simplify inventories of electrical supply vendors and facilitate ballast replacement in installed light fixtures, and have come into widespread usage.
What is needed is an output protection circuit which can sense abnormal or unsafe operating conditions at the output of an arc discharge lamp ballast intended for operation with variable lamp loads. Such a protection circuit should be reliable, relatively simple and economical.
SUMMARY OF THE INVENTION
The aforementioned need is addressed by this invention which provides a floating reference fault detection circuit for arc discharge lamp ballasts useful with ballasts intended for supplying variable lamp loads. The improved fault detection circuit has a comparator with first and second inputs of opposite polarity and a comparator output, and a sampling circuit connected for deriving a sample signal related to the lamp voltage or lamp current delivered by the ballast output The sample signal is provided to both of the comparator inputs, and a biasing circuit is connected for holding one of the inputs at a bias above the sample signal relative to the other of the comparator inputs, thereby to hold the comparator output in a normal state. A discriminator circuit is connected for passing relatively fast rise-time waveforms only and not slower change sin the waveforms to the other of the comparator inputs, such that a relatively fast rise-time component waveform of sufficient amplitude in the sample signal overcomes the bias and changes the comparator output to a shut down state. The bias voltage is summed to the sample signal voltage at the comparator input and consequently provides a floating reference voltage independent of the absolute value of the voltage or current at the ballast output. As a result, abnormal fast rise-time components of sufficient amplitude of the sample signal can be reliably detected relative to the bias level irrespective of slower fluctuations and changes in lamp voltage or current, and irrespective of changes in lamp load wattage.
The sample signal may be a rectified A.C. signal derived from the voltage or current supplied at the ballast output to a lamp load. The sampling circuit may be a voltage divider, such as a resistive voltage divider, connected for sampling the alternating voltage at the ballast output and a full wave rectifier for rectifying the sampled voltage to derive the sample signal. Alternately, the sampling circuit may include a current transformer connected for sampling an alternating lamp current and a circuit for converting the sampled current to a sample voltage, which is then rectified by a full wave rectifier to derive the sample signal. The discriminator circuit may be a low pass filter, such as a low pass capacitor connected across; the comparator input to remove higher frequency components in the sample signal. The biasing circuit may be a resistive voltage divider connected for holding one of the comparator inputs at a bias voltage relative to the other comparator input. A latching diode may be connected between the comparator output and a comparator input for latching the output in the shut down state.
The bias between the comparator inputs may be fixed, but since it is added to the sample signal at the comparator inputs the bias provides a floating reference level substantially independent of the voltage or current through the lamp load.
It is contemplated that the fault protection circuit is to be part of a ballast unit, and therefore the invention contemplates the combination of the floating reference fault protection circuit with a ballast circuit intended for powering an arc discharge lamp load, to provide in combination an output protected ballast.
In a broader sense, the invention is a fault detection circuit for detecting relatively fast rise-time changes in either current or voltage output of the ballast, the fault detection circuit having an output connected for turning off electrical power from the ballast to the lamp load in response to the fast rise-time changes. The fault detection circuit includes a filter, such as a low pass capacitor, selected to pass waveforms representative of undesirable lamp conditions to an input, such as a comparator input of the fault detection circuit. One comparator input is held at a bias relative to the other comparator input, and the filter is connected such that the bias is overcome by fast rise-time cha
Beehler & Pavitt
Bruce Industries, Inc.
D Chuc Tran
Epstein Natan
Wong Don
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