Fluorescent Lamp Circuit Working Explanation

In this post, you are going to learn about the fluorescent lamp, construction details, circuit diagram with working principle and much more.


The low-pressure mercury discharge emits a considerable amount of radiation at about 2,500 AU, while the high-pressure discharge emits a considerable amount at about 3,650 AU.

Both of these emissions being in the ultra-violet part of the spectrum and therefore of no direct use as a source of light.

Such radiations can, however, be used to excite certain materials.

The important feature of these materials being that when the excite molecules return to normal they emit radiation at a frequency different from that which caused the excitation.

The radiation emitted by the materials may, in fact, be within the visible spectrum so that the ultra-violet radiations are converted to light.

Materials which possess this property are said to be fluorescent.

If the radiation from the material persists for some time after the removal of the exciting radiation, the material is said to be Phosphorescent.

A lamp embodying this principle consists, therefore, of a tube in which takes place a mercury discharge as in the discharge lamp already described.

What is Fluorescent Lamp?

A fluorescent lamp or fluorescent tube is a low-pressure mercury-vapor gas-discharge lamp that coated on the inside with a fluorescent material in the form of a powder which is excited by the ultra-violet radiation of different colours, while by mixing the powders light of any desired colour, including daylight, can be obtained.

Some of the commonly used powders are shown in the table below.

All are excited by radiations having wavelengths between 2,200 and 3,600 AU.

Fluorescent Powders

Powder Wavelength of maximum emitted radiation Colour
Calcium tungstate 4,400 Blue
Magnesium tungstate 4,800 Blue-white
Zinc silicate 5,250 Green
Zinc beryllium silicate 5,950 Yellow-white
Cadmium silicate 5,950 Yellow-pink
Cadmium borate 6,150 Pink

Standard Size of Fluorescent Lamp

Lamps of various size are available, particulars of the commonly used standard 230-V 80-watt size being given below:

Size ― 1.5 m long, 2.54 cm diameter (Industrial purpose).

Lumens output ― 2,400 (2,800 for the first 100 hours).

Luminous efficiency ― 30 lumens per watt.

Voltage across lamp ― 115 V.

Average life ― 2000 hours.

Power factor ― 0.5 without a condenser, 0.95 with a condenser.

Construction Details of Fluorescent Lamp

A fluorescent lamp light consists of:

  1. a lime glass tube
  2. drop of mercury
  3. argon gas
  4. phosphor coating
  5. electrode coils
  6. mounting assemblies
  7. aluminium cap

The first fluorescent lamps to be introduced employed the high-pressure discharge, and were similar in construction to the ordinary discharge lamp, except that the fluorescent powder was coated on the inside of the outer tube and this tube was made larger in order to keep it cool.

In these lamps (made in 400, 250, and 80 watt sizes), the light from the fluorescent powder supplemented that produced by the main discharge.

In the more recent and more common type of fluorescent lamp, a low-pressure discharge is used, and the lamps are made in the form of tubes 2.54cm or 3.8cm in diameter and 0.45m 1.5m long, with an electrode at each end.

The filaments of electrodes play both roles as anode and cathode, where small plates are attached to the filament to protect the electron bombardment and reduce the wattage loss at both ends.

The electrodes consist of coiled filaments (a mixture of barium, strontium and calcium carbonate) coated with an electron-emitting material.

The Phosphor powder is coated on the inside of the tube, and as the operating temperature is only about 50ยบ C no outer envelope is necessary as with the other types of the discharge lamp.

At a certain pressure argon gas is filled up inside the tube.

Two pins at each end are taken out of the lamp body through the Aluminium cap.

Fluorescent Lamp Circuit Diagram

Schematic of the fluorescent lamp circuit shown below.

As with other types of the discharge lamp, a stabilising choke must be used in series with it, and a capacitor is desirable to counteract the lagging power factor caused by this choke.

A starter is connected in parallel with the lamp, as shown in the diagram.

Inside the starter, a small capacitor (0.05uF) is connected in parallel across the bi-metallic contacts which placed within Neon Gas to suppress radio interference.

Working Principle of Fluorescent Lamp Circuit

At starting, a switch short-circuits the lamp and allows current to flow through the two electrodes and the choke in series.

After about two seconds the switch automatically opens, causing a voltage surge large enough to strike the arc between the electrodes, the arc being them maintained by the normal lamp voltage.

Bombardment of the electrode surfaces by the positive mercury ions maintains their temperature so that they continue to emit electrons.

The automatic switch maybe consists of a bimetallic strip heated by a small heating element or it may be a glow-starter switch consisting of two bimetallic electrodes in a argon-filled bulb.

A glow discharge takes place between the electrodes, which heats them and causes them to deflect and close two contacts in the starter circuit.

Closing the contacts extinguishes the glow and the contacts open again, causing the voltage surge to start the lamp discharge.

In this context, it will not be out of place to mention that fluorescent lamp some times produces a flickering effect called stroboscopic effect, which is due to periodic fluctuations in the light output of the lamp caused by cyclic variations of current on a.c. circuit.

This phenomenon creates multiple image appearance of moving objects and make the movement appear Jerky.

This flickering effect is more pronounced at lower frequencies. Frequency of such flickers is twice the supply frequency. The fluorescent power used in the tube is slightly phosphorescent, hence the stroboscopic effect is reduced to some extent due to afterglow.

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