Figure 22.5 -Transformer Quick-start Circuit

The lamp discharge begins as soon as the cathodes reach their operating temperature. A capacitive effect between the cathodes and the earthed metalwork of the fitting ionises the gas and the tube 'strikes' very quickly. Most tubes have a conducting path through the phosphor coating or, alternatively, a special metal earth strip running between the end-caps assists the starting process. The transformer ballast gives an immediate start but some difficulty can occur with low ambient temperatures, low supply voltage and poor earthing. Many other variations of the quick-start circuit using transformers and resonant effects are used. Capacitors are used with a discharge tube for:

(i) Power factor correction (PFC)

(ii) Radio interference suppression (RIS)

The PFC capacitor is used to raise the power factor of the power supply to around 0.9 lagging and is connected in parallel with the power supply. Without this capacitor, the power factor may be as low as 0.2 lagging due to the choke's inductance. For a 125 W tube, a PFC value of about 7.2 µF is typical.

The ionisation process of the discharge causes radio interference from discharge tubes. This is suppressed by a capacitor fitted across the tube's ends. In glow-switch circuits, the RIS capacitor is actually fitted within the starter. RIS capacitor values are approximately 0.0005 (XF.

If the RIS capacitor in a glow-switch fails due to a short-circuit condition, the tube would not strike but would glow at its ends while the choke may overheat and eventually fail. A similar result would occur if the bi-metallic strips of the starter are welded together.