Fluorescent lamp energizing circuit

Claim 1

An energizing circuit for operating a fluorescent lamp at a predetermined constant current comprising: a rectifier adapted to be connected across an AC voltage source, an electronic switching means having an input and an output, a current sensing means, a smoothing filter, said lamp being directly DC coupled across said rectifier through the series combination of the output of said switching means, said filter and said current sensing means, a switching regulator having an output for producing a pulse width modulated pulse train having a variable duty cycle, a reference input having a reference voltage applied thereto to establish a predetermined duty cycle corresponding to said constant current, said regulator also having a signal input for varying said duty cycle in accordance with the difference between the voltage applied thereto and said reference voltage, said signal input being coupled with said current sensing means, said switching means having its input coupled with the output of said switching regulator whereby the switching means produces a DC pulse train having a duty cycle which increases when the current through said current sensing means in less than said predetermined constant current whereby the switching means is operated at a higher duty cycle for producing a high starting voltage for said lamp and whereby the duty cycle decreases when the current through said current sensing means is greater than said predetermined constant current whereby the switching means is operated at a lower duty cycle for maintaining said constant current when the lamp is started.