Electronic Ballast Control Circuit

A ballast for operating a first lamp set and for selectively operating a second lamp set in combination therewith, said ballast comprising:

• a rectifier configured to receive alternating current power from a power supply and to provide a direct current (DC) voltage to a DC voltage bus;
• a first lamp driver circuit and a second lamp driver circuit, each configured to receive power from the DC voltage bus and to provide AC power to operate its lamp set when said lamp driver circuit is enabled;
• a controller configured to control the second lamp driver circuit, to monitor a first value corresponding to a current through a filament of a lamp in the second lamp set, to monitor a second value corresponding to a reference current, to determine a calculated ratio of the first value to the second value, and to enable the second lamp driver circuit based on the calculated ratio such that:
• the controller disables the second lamp driver circuit when the calculated ratio of the first value to the second value is less than a predetermined ratio, and
• the controller enables the second lamp driver circuit when the calculated ratio is more than the predetermined ratio;
• and
• a control circuit comprising a first input terminal and a second input terminal, the first input terminal and the second input terminal configured to selectively connect to the power supply, the control circuit configured to reduce the current through the filament of the lamp in the second set when a non-positive potential exists between the first input terminal and the second input terminal, such that the calculated ratio determined by the controller is less than the predetermined.

A method of operating a first lamp set via a first lamp driver circuit of a ballast and selectively operating, in combination therewith, a second lamp set via a second lamp driver circuit of the ballast, said method comprising:

• monitoring a first value via a first input line connected to a terminal of a lamp in the second lamp set and to a control circuit, said control circuit adapted for selectively connecting to a power supply of the ballast, said first value corresponding to a direct current (DC) through the lamp in the second lamp set;
• monitoring a second value via a second input line connected to the second lamp driver circuit, said second value corresponding to a reference current;
• determining a calculated ratio of the first value to the second value;
• controlling operation of the second lamp driver circuit based on the calculated ratio, said controlling comprising:
• enabling the second lamp driver circuit to operate the second lamp set when the calculated ratio is more than a predetermined ratio;
• and
• disabling the second lamp driver circuit to prevent operation the second lamp set when the calculated ratio is less the predetermined ratio;
• and
• reducing by the control circuit the current through the lamp in the second lamp set so that the calculated ratio falls below the predetermined ratio as a function of a connection state of the control circuit to the power supply.