Ballast shut-down circuit responsive to an unbalanced load condition in a single lamp ballast or in either lamp of a two-lamp ballast

An electronic ballast comprising, in combination:

• a power supply having first and second output terminals for applying operating power at an operating voltage amplitude and frequency to a gas discharge lamp of the type having first and second cathodes;
• a first conductor coupled between the first output terminal of the power supply and the first cathode of said lamp;
• a second conductor coupled between the second output terminal of the power supply and the second cathode of said lamp;
• first sensing means coupled to said first conductor for generating a first feedback signal proportional to electrical current flowing through said first conductor;
• second sensing means coupled to said second conductor for generating a second feedback signal proportional to electrical current flowing through said second conductor;
• a summing circuit having first and second input terminals and having an output terminal, said summing circuit including means for generating an output voltage that is proportional to an algebraic combination of the first and second analog feedback signals;
• means coupled to the output terminal of the summing circuit for comparing the magnitude of the output voltage with a reference voltage, said comparator means having an output terminal for generating a shut-down signal in response to the condition that the magnitude of the output voltage exceeds the magnitude of the reference voltage;
• and,
• a shut-down circuit coupled between the output terminal of the comparator and the power supply, said shut-down circuit being responsive to the shut-down signal for removing AC operating voltage from said power supply output terminals.