Under normal working conditions, the startup process of high-pressure sodium lamps takes about 10 minutes. When the HPS lamp is connected to the power supply, the current passes through the ballast, thermal resistor, and bimetallic moving contact to form a path. At this time, there is no current in the discharge tube. After a while, the thermal resistor heats up, causing the bimetallic thermal relay to disconnect. The ballast coil generates a high self-induced electromotive force at the moment of disconnection. This ionizes and discharges the nitrogen in the tube. The increase in temperature, in turn, causes the mercury to turn into vapor and discharge. When the temperature inside the tube further increases, the sodium also changes into a vapor state, begins to discharge, and emits strong visible light.
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