摘要 |
A controller (5) for a high intensity discharge lamp (4) includes a primary ballast for providing a primary current to the lamp (4), and a current injector which in the case shown comprises a control ballast (8) and a switc h (9) (e.g. a triac) controlled by a control unit (12). The current injector injects a secondary current into the lamp (4) that is less than the primary current and can be varied by the control unit (12), e.g. by varying the amou nt of time within an a.c. cycle that the control ballast (8) is switched into t he circuit. The controller (5) allows the lamp (4) to be controlled, for exampl e, to operate at a constant power (variable light output) or at a constant ligh t output (variable power).
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