GB720279A - Improvements in or relating to circuits for operating electric discharge lamps - Google Patents

Abstract

720,279. Supply systems for space-discharge apparatus. GENERAL ELECTRIC CO., Ltd., and CATES, J. Oct. 31, 1952 [Aug. 9, 1951], No. 18797/51. Class 38 (4). Dimming of one or more electric-discharge lamps 1 is effected by a circuit arrangement supplied with A.C. at variable frequency and including a frequency-dependent ballast impedance comprising effectively a capacity C1 and an inductance L in series with each other and with the lamps and a reactor, in the embodiment shown a capacitor C2, in parallel with one or both the capacity and inductance, the one being of opposite sign to the reactor, the arrangement being such that the lamp is dim at the supply frequency at which the ballast impedance is parallel-resonant, and operates at high brightness, in series with the ballast impedance which is then effectively of opposite sign from the reactor, at supply frequencies which are either more or less than the resonant frequency depending upon whether the reactor is inductive or capacitive respectively. In the inductive ballast impedance shown in Fig. 3, the inductance L may be split into two parts with the capacitor C1 interposed, and complementary circuits are described for capacitive ballast impedances. The addition of capacitor C1 or inductor L permits an increase in the ballast inductance value relative to that of the shunting reactor, thereby decreasing the resonant circuit current and hence the minimum lamp current, which latter may be further decreased by the provision of a seriesconnected capacitor 12 and resistor 13 to shunt the lamp 1. Dimming may additionally be effected by conventional variation of voltage amplitude. The arrangement is applicable to cold-cathode as well as to the hot-cathode lamp shown, and a plurality of lamps may be series or parallel connected, having a common or separate ballast impedances respectively. Power-factor correction is effected in a capacitive circuit by a transductor, and in an inductive circuit by a transductor in series with a condenser, the current in the transductor control windings being arranged to vary with variations of supply frequency.