Question Corner: Tube light flicker

Photo: R. Ragu  

Why does a tube light not glow immediately on switching on like a CFL bulb?



Flicker start is a very common phenomenon in the conventional tube lights. However, the modern versions of tube lights (TL) and the compact fluorescent lamps (CFLs) do not exhibit this, but they start rather immediately on turning them on.

Both the tube lights and the CFLs work by the same principle. These lamps consist of a fluorescent phosphor coated glass tube filled with a mixture of the inert gas argon and mercury vapour. This gas is excited by the energetic electrons emitted from the cathodes provided at the ends of the tube.

These excited gas atoms interact with the phosphor material coated on the walls and we receive the light from this glowing phosphor material. Once this process is started, it sustains itself because both the excited gas atoms and the accompanying electrons are capable of repeating the excitation process further.

However, the initiation of the gas excitation is accomplished by extracting electrons from a heated cathode, by using an instantaneous high voltage pulse generated by a ballast circuit. The conventional TLs use a magnetic ballast circuit which makes use of self inductance of an iron core choke coil along with a discharge lamp type automatic starter switch to trigger the ballast.

And an inductance has a longer time constant and a resultant slower voltage build up for extraction of electrons from the cathode. These features of the ballast circuit often require more than one attempt for the gaseous excitation. This leads to slower start and start-up flickering of the tube lights. Because this arrangement has the above problems and also consumes more power, electronic ballasts have been subsequently developed which do not use the magnetic chokes, but employ semiconductor devices in an electronics circuit for the purpose and avoid the use of a discharge lamp type of starter switch.

They generate higher voltage to extract high energy electrons from the heated cathodes and so the extracted electrons excite the gas atoms without fail. Thus, these ballasts are called “rapid start ballasts” and do not exhibit delay or flickers of the lamp.

Prof. H. K. SAHU

Chennai Mathematical Institute, Chennai

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Printable version | Oct 25, 2020 12:15:42 AM |

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