Page 331 - Electrician - TT (Volume 1)

ELECTRICIAN - CITS

Basic principle:
• Gases are poor conductors at normal &higher pressure.
• But application of high ignition voltage b/w two electrodes in a sealed envelope containing gas at low pressure
ionises the gas and current pass through the gas medium.
• Once conduction starts due to ionisation the resistance in the circuit drops rapidly.
• To limit current due to low resistance a device must be provided.
• The current through the L.P gas is called discharge. This causes the gas/vapour to emit radiation in UV
region. The UV radiation cannot be perceived by human eye.
• Certain phosphors emit light when exposed to UV rays and the same coated over the tube to emit light
Electric gas discharge lamp s are of two types:
1 Cold cathode lamp

2 Hot cathode lamp
In the first type no filament is used to heat the electrode for starting but in the second type filament is provided for
heating the main electrode at the time of starting.

When a voltage is applied across two ends of filament contained in a gas filled glass tube electrons starts flowing
from one electrode to other and current is passed through the gas then it emits light rays.
The following are the different types of gas discharge lamps.
Cold cathode lamps

• Neon lamps
• Neon sign tubes
• Sodium vapour lamp
Hot cathode lamps
• Mercury vapour lamp

• Fluorescent tube
Neon lamp: A neon lamp (neon glow lamp) is a miniature gas discharge lamp. It is cold cathode lamp. The lamp
typically consists of a small glass capsule that contains a mixture of neon and other gases at a low pressure and
two flat or spiral electrodes (an anode and a cathode). When sufficient voltage is applied the gas become ionized
,and the lamp produces an orange glow discharge in ordinary neon lamp. The filled gas is generally a mixture,
of 99.5% neon and 0.5% argon In comparison with incandescent light bulbs, neon lamps have much higher
luminous efficiency.

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CITS : Power - Electrician & Wireman - Lesson 54-59