How fluorescent lamp works?

How Fluorescent Lamp works?

What is Fluorescent Tube (Tube Light)?
Fluorescent tube is a gas discharge lamp. Since the fluorescent power is used in the tube, it is called as fluorescent tube.
Commercially it is available in 10, 20, 40 and 80 watts. They are widely used in domestic and street lighting.

Important accessories required for Fluorescent Lamp:

(1) Starter:
The  starter has two functions. They are

  • To complete the circuit initially for preheating the electrodes.
  • When high voltage is induced across the choke, starter opens the circuit.

There are two type of starters used in tube lights. Namely
[1] Glow starter
[2] Thermal starter

(2) Choke:

  • It is an iron cored inductance coil.
  • It will be connected in series with the tube.
  • The choke acts as s ballast in running condition and it provides a voltage impulse for starting.
  • The choke has two functions. They are
    (a) To provide a very high voltage ( many times higher than supply voltage) to start the ionization process in the lamp.
    (b) To limit the current through the circuit when the tube is opening.

(3) Condenser ( capacitor):

  • The condenser is incorporated in the fluorescent lamp to improve the power factor.
  • It also reduces the radio interference.

Fluorescent tube - Construction:

  • The tube contains organ gas with a small quantity of Mercury under low pressure.
  • A thin layer of fluorescent material in power form is coated inner side of the tube.
  • Various fluorescent materials(like phosphor) give different color light.

Principle of working of Fluorescent Lamp:


  • The glow starter is a voltage operated device.
  • It consists of two bimetallic electrodes enclosed in a glass bulb.
    One electrode is in fixed manner and another one is movable in nature which is in U shape.
    The glass bulb is filled with a mixture of helium and hydrogen at low pressure.
  • At the normal condition(ie during power OFF condition), the starter terminals are opened ( as shown in the figure).
  • When the power supply is turned ON, the phase of the supply reaches the starter through choke and tube element.
  • The applied voltage appears across the glow starter.
    It will start to glow discharge between two electrodes.
    The heat generated will bend the bimetallic strip. Consequently it makes contact with the fixed electrode and thus close the switch.
  • The circuit is completed through  choke, tube electrode(A), starter and tube electrode(B).
  • Now as the glow between starter electrodes shorted out, the bimetallic strip cools down and contacts open.
  • Due to this sudden contacts open ( breaking of circuit) a high voltage (around 1000V) is induced in the choke.
    NOTE: It is the property of inductor to oppose sudden change in the current flow.
  • The self-induced voltage of 1000V appears across the tube electrodes.
  • This initiates the discharge in the tube. The heat is produced.
    This heat is enough to vaporize mercury and potential difference between across tube falls around 110V.
    This voltage is insufficient to restart the glow starter.
  • Thus, the fluorescent tube is used to change the ultraviolet light into the visible radiation.

You may also like to read:

How Fluorescent Lamp (Tube Light) works on DC Supply?
Tube Light (Fluorescent Lamp) Troubleshooting Tips
How to Control the Speed of DC Shunt Motors?

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