| 摘要 |
The electrical energy supplied to the sodium-vapour high-pressure lamp (20) is controlled as a function of a DC control voltage (US). This control is achieved by means of a circuit arrangement (IC1, R3-R11, C4-C6, D1) with whose aid the time or angle of triggering of a triac (TR1) is variable within each AC voltage half-cycle. In order to prevent the lamp (20) being completely extinguished inadvertently when it is set to the minimum brightness, a second circuit arrangement (30, 42, R12-R18, C7, D3, T1, 52, R25-R27, C9, D3, D5) is provided by means of which the required minimum current intensity of the current flowing through the lamp (20) is ensured irrespective of the magnitude of the DC control voltage (US). The voltage applied to the lamp (20) and the required minimum current intensity of the lamp reduce in accordance with the special characteristic of a sodium-vapour high-pressure lamp when the temperature of the lamp falls following the reduction in the lamp brightness. The voltage across the lamp (20) reduces to the same extent when the voltage across the triac (TR1) increases, so that the minimum current intensity is controlled automatically and indirectly as a function of the lamp temperature. The device created is used, for example, for brightness control of sodium-vapour high-pressure lamps in roadway and tunnel illumination systems. <IMAGE>
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