Lighting Tutorial [E-BOOK]

3 The working principle with magnetic ballasts

When commonplace line voltage, 230 V 50 Hz or something similar, is applied to a fluorescent light tube, normally nothing will happen. The withstand voltage of the gas inside, usually low pressure mercury vapour, 1.3 mg in a 58 W tube, is higher. When the filaments are being heated, they start to emit additional electrons, but this still does not suffice to reduce the breakdown voltage below the regular periodic peaks of the mains alternating voltage.

Fig. 3.1: Conventional glow starters

With a cold 8 W T5 type lamp (room temperature) self-ignition without any sort of firing was observed at 480 V TRMS (≈680 V peak). This value could be reduced to 380 V by pre-heating the filaments with a separate transformer. A 58 W tube was found to start off from the cold state at 1300 V sine wave, dropping to 550 V with pre-heated filaments.

Fig. 3.2: Electronic starters are available for all possible situations of application

A further reduction occurs when the voltage is applied abruptly, from 0 to full, instead of slowly increasing it by means of a variable transformer, but still self-start at 230 V 50 Hz does not occur.

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