Fluorescent Tube :
Basically fluorescent tube is a discharge lamp in which phosphor substances when flooded with ultra violet light, radiate it in the form of visible light.
Construction :
It consists of a long glass tube which is internally coated with a fluorescent powder.
Each end of the tube consist of electrode made of tungsten filament which is in the coiled form.
Electrodes are coated with emissive material like barium oxide or calcium oxide so that they can emit electrons at faster rate.
The straight glass tube is filled with argon gas at low pressure and drop of mercury.
The control circuit or starting circuit of the tube consists of a choke and a starter.
Fig. shows a schematic diagram of fluorescent tube with starter and choke.
Working :
Initially the contacts of starter are open and the starter is at cold condition.
When switch S1 is closed, circuit gets completed and heats up the electrodes, which starts emitting electrons.
After few seconds, bimetallic strip of started opens automatically. Therefore choke coil induces high voltage about 800 V to 1000 V across the two electrodes.
This high voltage across two electrodes of fluorescent tube through the supply is sufficient to ionize the argon gas in the tube between the two electrodes.
The generated heat vaporizes the mercury and a current starts flowing between two electrodes in the tube.
Due to this discharge, the voltage between the two electrodes falls to about 110 V which is also the voltage across the starter contacts.
This voltage is not sufficient to restart the starter circuit. Thus it is only used to start the tube.
Fluorescent tubes are available at 40 W and 36 Watts. Generally p.f. of the circuit is 0.5 lagging
and can be raised to 0.9 by connecting a capacitor across supply terminals.