ELECTRONIC DEVICE FOR CONTROLLING THE LIGHT INTENSITY OF A GASEOUS DISCHARGE LAMP WITHOUT A HEATED CATHODE

For the purpose of controlling the electrical energy fed to an electrical gas-discharge lamp (20), without an incandescent cathode, as a function of a d.c. control voltage (US), there is a circuit arrangement (IC1, R3-R11, C4-C6, D-1), by means of which the ignition timing or angle of a triac (TR1) may be varied within each alternating-voltage half-wave. In order to prevent the lamp (20) from being completely extinguished unintentionally, when adjusted to minimal brightness, an additional circuit arrangement (30, 42, R12-R20, C7, D3, T1) is provided, ensuring a given minimal intensity of the current, flowing through said lamp (20), regardless of the level of said d.c. control voltage (US). Said additional circuit arrangement comprises a current transformer (30) and a rectifier arrangement (42) for producing a regulating voltage (UR) dependent upon the intensity of the lamp current which controls an electronically variable resistance (T1). The latter influences the critical voltage for the ignition-angle-control whenever the intensity of the lamp current drops below a permissible minimal value, in such a manner that any further decrease in current intensity is automatically counteracted. The device is intended, for example, for controlling the brightness of lighting units and of mercury-vapor lamps or high-pressure sodium-vapor lamps used for interiors, highways and tunnels.