FLUORESCENT TUBE

Principal – Fluorescent lamps work by ionizing mercury vapour in a glass tube. This causes electrons in the gas to emit photons at UV frequencies. The UV light is converted into standard visible light using a phosphor coating on the inside of the tube.

Working –A tube light/rod if made up of 4 ft long glass tube whose inner side is coated with phosphor & it is filled with mercury vapour. It has two metal electrodes/filaments at both ends. To complete its operation two more items are required – 1) Choke & 2)Starter.

When we switch on light, the current passes through the choke then one of electrodes & reaches up to starter. Initially no current flows through the starter as starter’s contacts are open,these contacts are surrounded by a gas. This gas starts heating up and makes the gas ionized & current flows through it. When current starts flowing through the starter, gas starts cooling down & which stops the current to flow as it de-ionize of gas in the starter. This process is repeated. During this process of ionization/deionization of gas in the starter, a high voltage is generated by the choke which causes ionization of inert gas in the tube.

In few attempts, the gas inside tube gets fully ionized (an arc is set up) between two electrodes & current starts flowing from one electrode to another electrode & tube light stats glowing (or light is emitted). Once the gas is ionized the high voltage is reduced to a great extent for normal operation of tube light no current flows through starter.

The ionization of mercury gas causes electrons in the gas to emit photons at Ultra Violet light (or frequencies or radiation). The UV light hits the phosphor coating inside it & coating glows to produce visible light.

FUNCTION OF CHOKE

There are two types of chokes used in fluorescent – electromagnetic & electronic.

Choke (or Magnetic choke/ballast) – The function of choke is to produce very high voltage between the two electrodes (across two ends of tube). Once the gas is ionized, a path is created (an arc is set up) between two electrodes & current starts flowing through it, then there will be no current through the starter. The starter stops functioning & choke provides low voltage.

Choke limits the current also when arc takes place between two electrodes in order to avoid lamp to burn out or cause the power supply to fail.

Choke is able to produce high voltage with help of starter. The starter switches on & off the current (passing through choke) very frequently (causes flickering of light) which produces very high voltage across the choke & hence between the ends of tube.

The starter will continue to function till the time the gas is ionized inside the tube.
Since the starts stops functioning, no more high voltage is across the choke. And the voltage across the choke (and between the filaments) is very much reduced.

Electronic Ballast – Electronic ballast performs the function of both Choke & Starter. When electronic choke is used with light then starter is not required. Electronic ballast converts AC to DC and then back to AC at a higher frequency to operate the lamp. There are two primary functions of the electronic ballast:

  1. To provide the initial high voltage required to ionize the gas, thereby setting up an arc between the two electrodes.
  2. To limit the current through the tube once it has started up. If the current is not controlled it may result in voltage spike thereby damaging the lamp.

The other functions of electronic ballast include ignition, warm-up, constant power control, power factor correction, and protection against all lamp and ballast fault conditions.Electronic ballast operates at 20 KHz-80KHz unlike the magnetic ballast which operates at 50 -60 Hz. At high frequency the lamp requires less input power, thereby increasing the efficiency.Electronic ballast is used to operate Fluorescent Lamp, neon lamp or High Intensity Discharge (HID) lamp.

Wiring diagram of fluorescent lamp with electronic ballast –

FUNCTION OF STARTER

The starter consists of a small bulb containing a gas (usually argon) and a bi-metal contact (normally not touching with each other). When power is applied to the fixture and current is unable to flow through florescent lamp then an arc is produced (through argon gas) in the starter & current flows in it and gas in the starter heats up & one of the metal contact starts bending. When the gas is sufficiently heated up, the bi metal moves out of the position & creates a direct path for the current in the starter. Now the maximum amount of current flows through the filaments /electrodes of the fluorescent lamp which heats up the gas in the lamp.

Meanwhile, in this situation, no arcing takes place in the starter & starter starts cooling &bi-metal contact will begin bending back to its original position. This process repeats till the mercury vapour is ionized in the lamp& current starts flowing through it. Once fluorescent bulb successfully starts, the starter bulb continues to cool and eventually the bi-metal contact returns to its “rest” position. The starter is deliberately designed to have a higher break-down voltage than the much-longer fluorescent lamp. The starter also has the resistance of the two filaments in the fluorescent lamp as part its electrical circuit.

Some starters also contain a capacitor (also known as a condenser) that can reduce electrical noise and assist in the starting process.

RANGE OF FLUORESCENT LAMPS

Fluorescent lamps are available in different shapes & sizes – T5, T8 & T12. Where T stands for tubular shape & number indicates the diameter of tube. For ex. – T5 where 5 means 5/8”. T12 tube lights were the first tube light which had diameter of 38mm (=12/8”). T8 lights have diameter (25mm= 8/8”) less than T12 while T5 (16mm) tubes have less diameter than T8. These days T12 are no longer being manufactured due to inefficiency.Following types of lamps are available in market –

  • According to length – 4 ft& 2 ft
  • According to diameter – T12, T8 & T5,

These are available in following wattage – 36W in 4ft length, 18W in 2ft length etc.