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What is CFL – How it Works, Circuit Explanation, Advantages & Disadvantages

We all have witnessed the era when light bulbs were replaced by a better alternate known as Compact Fluorescent Lamp (CFL). A CFL works in an energy efficient way. This post will discuss about What is CFL, How it works, Phase wise Circuit Explanation, Advantages and Disadvantages

What is Compact Fluorescent Lamp (CFL)

The term ‘CFL’ stands for Compact Fluorescent Lamp. It is also known as compact fluorescent light, energy-saving light, and compact fluorescent tube.

Different type of CFLs around us

The CFL was initially designed to replace the incandescent lamp in terms of its compactness as well as energy efficiency. The basic construction of a CFL consists a tube which is curved/spiraled to fit into the space of an incandescent bulb, and a compact electronic ballast in the base of the lamp.

How Compact Fluorescent Lamp (CFL) works – Working Principle

A CFL uses vacuum pipe which is principle wise same to the strip lamps (commonly known as Tube light) . Tube has two electrodes on both ends which is treated with Barium. Cathode is having a temperature of about 900º C and generates a beam of electrons which is further accelerated by potential difference between electrodes.

Inner view of a CFL

These accelerated electrons strike Mercury and Argon atoms which in turn results in the arise of a low temperature plasma. This process initiates the radiation of Mercury in Ultra violet form. Tube’s inside face contains ‘Luminophore’ whose function is to convert Ultra violet light into visible light.

This tube is fed with AC power supply which facilitate the changing functionality of Anode and Cathode. The CFL also consists a switched mode converter. It functions on a very high frequency and acts as a replacement of ballast (choke) and starter assembly.

Circuit Explanation of CFL

The CFL PCB is quite compact and fits in the holder base. Though being compact, it efficiently performs the requirements as a choke. The circuit of CFL is explained in succeeding paragraphs.

Key components of PCB of CFL

The PCB of a CFL contains the following key components :

  • Bridge rectifier made up of diode 1N-4007
  • Suppressor for suppressing interference
  • Filter capacitor
  • Fuse point
  • Supply point

Phase wise Circuit Explanation of CFL

The working of a CFL can be divided into two broad phases: –

  • Starting Phase
  • Normal Phase

Starting Phase

The starter segment comprise of a Diac, C2, D1 and R6. The components D3, R3, D2 and R1 work as a protection circuit and the rest as normal operation circuit. You have to keep the following terminology in mind :

  • D refers for diode
  • R refers for Resistor
  • C refers for Capacitor, and
  • Q stands for Transistor

it diagram of PCB of CFL

The Diac, C2 and R6 send a voltage pulse to Q2 Transistor’s base which causes it to get its threshold value and it starts operating. As soon as the operation starts, the D1 diode blocks the whole section. Capacitor C2 is also discharged (after charged to its full value) each time the transistor Q2 operates.

Therefore after its first start, There is not enough energy left for reopening of Diac. Further, the transistors are excited with the help of TR1 transformer. As the voltage is raised from resonant circuit (L1, TR1, C3 and C6), the tube lights up as soon as resonant voltage is specified by capacitor C3 (which feeds Filaments). At this moment the voltage C3 is over 600V.

Normal Phase

Just after the ionisation of gas present in the vacuum-pipe, the practical shorting of capacitor C3 is carried out. This results in stepping down of the voltage. After that C6 starts driving the changer. A very small voltage is generated by this changer, yet it is sufficient to run the lamp in ‘ON’ condition.

Under regular running condition, if the transistor comes in OPEN state, The current being fed to TR1 keeps increasing till the core of transformer saturates, and thus the feed to base drops down resulting it to close the transistor.

Immediately after this process, the second transistor is excited by reverse winding of TR1 and so the process goes on.

Advantages of Compact Fluorescent Lamp (CFL)

The advantages of CFL are as follows: –

  • It is energy efficiency
  • It has a higher life span ( nearly five to fifteen times) compared to the old filament bulbs.
  • It has lesser power rating (almost 80 percent) compared to the old filament bulbs.
  • It is low life cycle cost. Though, it has a higher purchase price than an incandescent lamp, it can save over five times its purchase price in electricity costs over the lamp’s lifetime.

Disadvantages of Compact Fluorescent Lamp (CFL)

  • It takes more time to start
  • The initial Purchase cost is high.
  • It does not comes in dark shades too.
  • Like all other fluorescent lamps, CFLs contain mercury, which complicates their disposal.
Read How to Re-utilize  damaged CFL : How to Easily Reuse/ Repair Damaged CFL - Step By Step Method
Ratna is a B.E (Computer Science) and has work experience in UK Mainframe IT industry. She is also an active Web Designer. She is an author, editor and core partner at Electricalfundablog.


  1. Hey, do you even understand what are you talking about ? Provided schematic DOES not contains DIAC !
    In this schematic C1 is used to start operation not the diac ( which is absent )…. Explanation of the schematic is taken from complete different schematic
    This is sad 🙁
    Copy-past plagiarism
    Please correct this garbage and not mislead people.

    • Hi dear, got your comment. Thanx a lot for the kind information as such comments improve us. Normally a post goes through various verification stages before being published in our website. This is an old post and we need some time to verify the stage clearances it has got before publishing. Will surely update it soon.


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