Patent ID: 7557513

Claim:
A lighting circuit for turning on a discharge lamp, the lighting circuit comprising: a DC-AC converting circuit to convert an input DC voltage into an AC voltage in response to a control signal for controlling power to be applied to the discharge lamp; a transformer including a primary winding to receive the AC voltage from output of the DC-AC converting circuit, and further including a secondary winding; a capacitor on the primary side of the transformer; an inductor on the primary side of the transformer; first and second outputs for supplying a power from the secondary winding to the discharge lamp; a resistor having one end connected to the second output and grounded and the other end connected to one end of the secondary winding; and a detecting circuit including a current monitor circuit for monitoring a current flowing to the discharge lamp by using a signal from the other end of the resistor, wherein the capacitor, the inductor and the primary winding are connected in series, and wherein: the secondary winding of the transformer has an intermediate tap, the detecting circuit has a first generating circuit having an input connected to the other end of the resistor, and the detecting circuit has a voltage monitor circuit, the first generating circuit is operable to generate a first signal corresponding to an amplitude of the AC voltage at the input, and the voltage monitor circuit includes: a second generating circuit having an input connected to the intermediate tap and operable to generate a second signal corresponding to the amplitude of the AC voltage at the input; and a first arithmetic circuit for processing the first signal and the second signal to output a lamp voltage equivalent signal.