Tungsten cathode Preheated Type Fluorescent lamp

Tungsten cathode Preheated Type Fluorescent lamp: In these types, the electrons are produced by thermionic emission. Lower starting and operating voltages are adequate. A transient voltage of 300-600 V, applied by the starter, initiates the arc stream. The cathodes, which are coated with emitting materials, lose a little bit of this material every time the lamp is started. The constant impact of electrons on the cathode also dislodges some of the emitting material. Finally so little of the materials remain that it is not possible to emit any electrons and the lamp becomes dead. Therefore this type of lamp is unsuitable for frequent starting.

 The efficiency is dependent on the mercury vapour pressure (and hence temperature) inside the tube. For efficient light production, these types of lamps are not used below a temperature of 10⁰C surrounding.

 Fluorescent lamps produce flicker or stroboscopic effect, since on 50 Hz supply; they are extinguished 100 times a second. Single lamps cannot be operated without flicker. Flicker correction can be applied to pairs of lamps. Radio interference is another effect produced by fluorescent lamps and has to be removed by suitable filter circuits.

 Starters of automatic starting switches care of two types i) thermal type, and ii) glow discharge type. The thermal starter has a heater coil which heats a bimetallic strip. The heater coil remains energized to keep the bimetallic switch open throughout the operation of lamp. It, therefore, consumes a small amount of power.

When the supply is switched on, the contacts of the bimetallic switch are closed and the current passes through the electrodes and heat them. But after an interval of few seconds, the heater coil heats up the bimetal strip and the bimetallic switch contacts open. This starts a high voltage transient across the electrode due to the presence of choke or ballast in the circuit. An arc is struck between the electrodes, due to the high voltage transient. The high frequency radio interference is bypassed through the filter circuit provided by the radio interference suppression condenser.

 The glow starter is enclosed in a glass bulb filled with neon or argon gas. One of the electrodes is a bimetallic strip.

 When the normal voltage is applied to the lamp, a glow discharge takes place across the glow switch and a small amount of current flows through the electrodes. The bimetallic strip expands due to the heating effect of current in the glow discharge. The expansion of bimetallic strip causes the electrodes touch each other (???) and the electrodes get pre-heated due to the flow of appreciable amount of current. Meanwhile the bimetal cools, the glow switch opens and the resultant high voltage transient starts the arc discharge through the tube. If the lamp does not strike, the foregoing cycle is repeated. The switch cannot glow after the lamp has started operating, as the available electrical potential