Fluorescent lamp-dimming circuit

1,103,540. Dimming arrangements for fluorescent lamps. BRITISH LIGHTING INDUSTRIES Ltd. 18 Oct., 1966, No. 46548/66. Heading H2H. A circuit for operating parallel-connected filament-heated fluorescent lamps 10, 11 at normal or reduced illumination comprises a three-position switch 12 controlling a relay coil R having contacts rl, r2, r3, the arrangement being such that in one position (bright) of the switch 12 the coil is de-energized and the lamps are supplied from an auto-transformer winding 17 through contacts r2, respective ballast members 19, 21, 22 and respective contacts rl, r3, while in another position (dim) the coil is energized to insert resistors 18, 20 in series with the respective lamps and to change over the contacts r2so that the lamps are supplied with increased voltage from the auto-transformer 17 to maintain the stability of operation at low current densities. Capacitors 24, 25 reduce lamp shimmer in the dim condition and a resistor 23 prevents excess current should contacts rl, r3 fail to open. Interference suppression capacitors 26, 27 are connected so as to be effective in both conditions of the circuit. In an arrangement to supply a single lamp, Fig. 2 (not shown), the auto-transformer (171) supplies the filament heating current and a third set of relay contacts (r13) controls a power factor capacitor (28) connected across the supply terminals L, N. The circuit of Fig. 1 may be modified by connecting the members 19, 21, 22 in series with the respective contacts rl, r2 across respective resistors 18, 20 so that these members are out of circuit in the dim condition.