Challenges in Dimming Neon Tubes
Dimming a neon tube poses more challenges compared to an incandescent lamp due to the neon tube’s requirement of a significantly higher starting voltage than the mains. Once started, the tube remains lit at mains voltage. Both the starting voltage and working voltage are contingent on the tube’s temperature.
Generating High Starting Voltage
The high starting voltage necessary for neon tubes is typically achieved by breaking the current flow through a choke. This interruption is managed by the starter, which also ensures a substantial current flows through the tube’s filaments. This current heats the tube’s ends, simplifying the starting process. The responsibilities of the starter are handled by the circuit depicted in the diagram, which not only takes on the tasks of the starter but also allows for dimming the neon tube.
Triac-Induced Voltage Transitions for Tube Ignition
During the zero voltage crossings of the applied mains voltage, the triac swiftly enters the off state. In these moments, capacitor C3 undergoes rapid charging, leading to the emergence of an instantaneous voltage across the tube. This voltage exhibits a phase shift relative to the current, and it is this shifted voltage that is applied to the tube. The collaboration of capacitor C3 and the choke constitutes a resonant circuit that elevates the voltage across the tube to an extremely high level, initiating the tube’s operation.
Adjusting Brightness through Voltage Control
The extent of the main voltage cycle during which the triac conducts influences the current passing through the tube filaments, subsequently affecting the starting voltage. Simultaneously, a larger portion of the current flows through the triac, diminishing the current flowing through the tube. As a result, the tube emits a less intense light.
Setting Optimal Brightness during Initial Activation
When initially activating the tube, it is advisable to configure the dimmer control, P1, for maximum tube brightness to facilitate the starting process. The chosen triac should possess a high dμ/dt value to prevent prolonged triac conduction caused by the abrupt voltage transitions occurring across the tube and, consequently, the triac during zero voltage crossings.