Low pressure sodium vapour lamp

[Sparky_Wario]

I have searched the web and end up getting confused. Thus thought I should pop in and pick the brain of the knowledgeable mentors and trainees. I understand the requirements of ignitor in the circuit is to break the resistance of the gases and kick start the process of discharge. What I can't wrap my head around is, after excitation, when and how does the ignitor drops out?
Can someone talk me through its principle of operation please?
Thanks in advance.

 

[Lucien Nunes]

There are different connection modes for different types of ignitor, but SOX ignitors are usually connected in parallel with the lamp. When the lamp is quiescent, there is no current through the ballast and no voltage drop across it, so the full mains voltage reaches the ignitor, causing it to operate. Once the lamp is alight, the voltage across it falls to a value determined by the lamp type, the difference being dropped by the ballast. At this voltage the ignitor stops generating impulses.

The same threshold voltage concept is used in a switch-start fluorescent tube, although that employs series-mode ignition instead of parallel. When the tube is quiescent, the glow starter sees the full mains voltage through the heaters, strikes and warms up, closing its contacts to complete the tube heater circuit, then opening again to generate the impulse. When the tube strikes, the voltage between the ends falls below the striking voltage of the glow starter, so the process stops.

 

[Sparky_Wario]

Thanks Lucien.