Two or three 'strikes' may be required to get the lamp working normally as the starter contacts may open before the cathodes are sufficiently heated. Such 'cold-striking' reduces the lamp's life by eroding the cathode material, which causes irregular lamp flashing. Severe blackening at the tube ends is a sure sign that its useful life is finished. Each time the starter's thermal contact closes, there are high-current surges through the choke coil, which increases its temperature. Excessive lamp flickering must, therefore, be swiftly corrected by replacement of the lamp or starter. More often than not it is the starter that is defective when such symptoms are observed. Only old or 'weak' lamps flicker due to their inability to bring about ionisation within.
Most choke coils are 'potted' in a thermosetting polyester compound within a steel case. While an earth fault is unlikely to occur within a choke, an open circuit is possible and can be simply checked with an ohmmeter. No repair of a choke is feasible so it must be completely replaced with an identically rated unit. Similarly, glow starters should only be replaced with an equivalent, which matches the wattage of the tube that it is to be used with. The commonly available glow starters have a dual rating i.e. 20 / 40 watts. Electronic starter switches are now available to eliminate flicker while switching on. These circuits are also called starter-less circuits or referred to as rapid start or instant start, where a drop in potential between the electrode and an earth strip is sufficient to ionise the gas adjacent to the electrode and this ionisation then spreads across the whole tube. An example of a transformer quick-start circuit is shown in Figure 22.5.
