Multiple failure detection shutdown protection circuit for an electronic ballast

#This invention relates generally to electronic ballasts and, in particular, to improvements relating to shutdown of the lighting circuit with abnormal load conditions, such as short circuit, open circuit, or arcing.

#Existing ballast circuits of the type used with fluorescent and, in some cases, neon lamps suffer from certain technical problems. Particularly with respect to single-pin lamps in DC-powered mobile applications, a single interconnection carries both DC input current and AC output current. This causes AC noise and ripple to feed back into the DC power bus distribution system, which, in turn, can affect performance and operation of other systems connected to the DC buss.

#FIG. 1 is a diagram of an existing single-pin lamp system. A 24-volt DC supply is connected to a ballast 102 and one side of the lamp 110. Typical of such systems, a safety switch 112 is provided to remove the DC power from the ballast if the lamp is removed. The connections are on either side of the pin, such that when the lamp is removed, the circuit is broken. The other pin 114 of the lamp 110 is connected to the ballast through line 116.

#FIG. 1 is a schematic diagram illustrating the problem associated with existing ballast circuits where an electrical path is common to both the DC and AC circuits;

#FIG. 2 is a simplified block diagram illustrating important components associated with a preferred embodiment of this invention;

#FIG. 3A is a detailed schematic diagram of the preferred embodiment;

#The system of this invention broadly prevents ripple and ballast-generated noise from feeding back into the buss power system of a lamp control circuit. This is broadly carried out by eliminating the common DC path and lamp current hookup connection to the lamp. This is more particularly carried out by eliminating the safety switching action of the pin and socket interlock currently typical of single pin systems.

#According to the invention, ballast shutdown is performed electronically when the lamp is removed. The circuit of the invention detects the presence of the lamp current to keep the ballast running, but if the lamp is removed or short-circuited, the ballast shuts down to address safety issues. The invention further includes apparatus and methods to detect arcing that occur if a wire becomes loose, foreign material interferes with a good connection, or a socket becomes defective. In the preferred embodiment, the circuit detects arcing conditions that last longer than a predetermined amount, such as 500 milliseconds, or thereabouts and shuts down the ballast if arcing is detected for this extended time. Broadly, because the output of the ballast is entirely isolated from the DC input, filtering inside the ballast can now attenuate noise produced by the ballast to acceptable levels.

#Reference is now made to FIG. 2, which depicts in block-diagram form a circuit configuration according to the invention. 24 VDC is provided to a ballast inverter 210, preferably through a line filter 208. The output of the ballast inverter 210 is fed to an isolation output transformer 212, which forms a lamp current path 222 to operate lamp 220. This configuration allows relatively little noise to couple back into the DC line, because the inverter ripple and noise current 209 are separate from the lamp current path 222.