| 摘要 |
A circuit 21, 23 applies to lamp 34 a time varying first current that is adjustable for lamp dimming, and a circuit 25, 27, 29, 31 provides a second current to sustain the discharge in which the first current alone is insufficient. Dimming may be achieved by phase control, reverse phase control, notch phase control, use of a PWM d.c. chopper, or an adjustable autotransformer and the sustaining second current may be applied continuously or only during off periods of the first current. The second current and heating current to lamp filaments 35, 35' may be d.c. or at line frequency, but more particularly is at high frequency produced by frequency converters 29, 39 and transformers 31, 37 or by one converter and one transformer (Figs 7, 8). A circuit (117), (121), (Fig 6), may disable the converter 39 if excess current is sensed. H.F. discharge sustaining current may be provided via an LC resonant circuit (223), (225), (Fig 8), (333), (335) (Fig 11), which is also operative prior to lamp starting to provide high voltage to strike the lamp. Starting may be achieved by applying high frequency to strike a low level glow current while simultaneously applying a line frequency voltage below the Townsend glow-transition voltage (VI), (Fig 10). A circuit (337), (339), (Fig 11) may respond to excess voltage in the resonant circuit by moving the driving frequency away from the resonant frequency. The first current from the dimming circuit may be applied to two lamps in parallel, whereas the HF second current passes through the lamps in series (Figs 8, 11).
<IMAGE>
|
