Why is a choke required in a tube light and not in a CFL?
RAM POOJAN CHAURASIA
Sultanpur, Uttar Pradesh
Both conventional fluorescent lamps (usually 4 feet long) and compact fluorescent lamps — CFLs ( much smaller both in length and diameter of the tube) used in lighting applications are low pressure mercury vapour discharge lamps.
These lamps generate light by the process of fluorescence (accomplishing conversion of invisible ultra-violet, UV to visible light) by electrical discharge-passage of electricity through gaseous-vapour medium along the column of the tube.
When electrical discharge could strike the column of the tube, lot of invisible UV radiation having wavelength dominantly at 254 nm is generated. This UV radiation when strikes the white coating inside the tube made of fluorescent material- phosphors gets converted to visible light with wavelengths in the region of 400-700 nm through the process of fluorescence.
The electrical resistance of the discharge column of the tube increases with dimensions and decreases with miniaturization of lamp dimensions.
For a conventional fluorescent lamp, the ballast used is a choke which essentially a leak transformer (made of bulk coil windings) which momentarily produces an inductive kick in the form of high voltage (approximately 1000 volts) so that the electrical discharge could be struck along the column of the tube. So in a conventional fluorescent lamp the role of the choke is to initiate the electrical discharge process.
Once the discharge is struck it can be sustained through the drop in electrical resistance of the column. But CFLs, being smaller in dimensions offering much lower electrical resistance do not require such bulky chokes. Instead the discharge in CFLs is initiated by much compact electronic circuits integrated into the CFL holder. Usually these electronic ballasts are small oscillator circuits producing high frequencies (approximately 10 kilo Hertz) facilitating flicker free quick start of lamp as electrical discharge strikes faster at such high frequencies.
R. JAGANNATHAN
Luminescence Group
CECRI
Karaikudi, Tamil Nadu
