The capacitor does not allow AC through it then what is the role of capacitor & choke in tubelight circuit ?

Asked by DEEPAK KOTHARI | 19th Oct, 2010, 11:54: AM

Expert Answer:

Dear student
  • A fluorescent lamp basically consists of a long glass gas discharge tube. Its inner surface is coated with phosphorous and is filled with an inert gas, generally argon, with a trace of mercury.
  • The tube is then sealed at low pressure with two filament electrodes each at its both ends.
  • These electrode filaments are used to preheat the tube and initiate a rapid conduction of electrons between the two end electrodes. The process initially requires a relatively high amount of power.
  • The bombardment of these electrons on the inner phosphorous coating is instantly transformed into a visible bright light, when we ultimately find the tube glowing brightly.
  • Once the conduction of electrons between the electrodes is complete, no more heating of the filaments is required and whole system works at a much lower current.

How do Fluorescent Lights Function?

We have all seen a tube light fixture consisting of a large heavy square ”choke” or “ballast” and a small cylindrical “starter.” Let’s try to understand how the whole system works. Please refer to the circuit diagram at the left as you read the following points:

  • The choke is in fact a large inductor. It consists of a long copper winding over iron laminations.
  • An inductor by nature always has a tendency to throw back the stored current in it, every time the power through it is switched OFF. This principle of the choke is exploited in lighting a fluorescent tube light.
  • When an AC voltage is applied to a tube light fixture, the voltage passes through the choke(typically a inductor), the starter(typically a capacitor), and the filaments of the tube.
  • The filaments light up and instantly warm up the tube. The starter holds the current for a moment and then releases to break the circuit.
  • This forces the choke to” kick back" it’s stored current, which again passes through the filaments and ignites the tube light.
  • If the tube does not sufficiently charge up, subsequent kicks are delivered by the choke due to rapid switching of the starter, so that finally the tube strikes.
  • After this the choke only acts like a low impedance current limiter to the tube as long as the light is kept illuminated.

We hope your doubt is resolved.

Answered by  | 20th Oct, 2010, 09:12: AM

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