| 摘要 |
A combined ballast and dimmer for a low voltage halogen lamp (20) uses a step-down transformer (22) fed off the AC mains through an interference filter (1), a rectifier (2), a smoothing filter (3), a high frequency push-pull converter using power transistors (16,17) that have four-terminal networks (11,12) between base and emitter, condensers(25-27), a starting switch (4) and a control transformer (28). The transformer has a double hole ferrite core without an airgap or two single hole ferrite cores without an airgap. The primary winding (29,30) occupies both holes whereas the secondary (9,10) occupies just one. The current gradient direction in the primary is the same for both types. Pre-magnetisation of one primary core section (28.1,28.2) by an external DC magnetic field gives the necessary voltage dimming.
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