A neon tube cannot be dimmed as easily as an incandescent lamp because the tube can start only at a voltage much higher than the mains, after which it will remain lit at the mains voltage. The level of both the starting voltage and the working voltage depends on the temperature of the tube.
Normally, the high starting voltage is obtained by interrupting the current through a choke. This is usually done by the starter, which also ensures that a fairly large current flows through the filaments of the tube. This heats the ends of the tube, which makes starting easier.
These tasks of the starter are taken over by the circuit shown in the diagram, which also enables the tube to be dimmed.
During the zero voltage crossings of the applied mains voltage, the triac will instantaneously switch off. At those instants, capacitor C3 will be charged rapidly, which results in the instantaneous voltage, whose phase has shifted relative to that of the current, being applied across the tube. Capacitor C3 and the choke form a resonant circuit that raises the sudden voltage across the tube to a very high whereupon the tube starts.
The larger the angle of the mains voltage ring which the triac conducts, the larger the current through the tube filaments, which results in a lower starting voltage. At the same time, since a larger part of the current flows through the triac, that through the tube will be reduced, so that the tube will light more faintly.
When the tube is first switched on, the dimmer control, P1, should be set for maximum brightness of the tube to facilitate starting.
The triac used should have a high dμ/dt value, otherwise, the steep voltage transitions occurring across the tube, and thus across the triac, during the zero voltage crossings would cause the triac to remain on.