Lighting Tutorial [E-BOOK]

6 The working principle with electronic ballasts

As already explained in the introduction, it is the working principle of electronic ballasts to generate a high frequency AC to feed the lamp. This technique is also applied in a steadily increasing number of so-called switch-mode power supplies, there to facilitate the use of a very much smaller transformer. This advantage comes more or less as a byproduct also to the electronic ballast because the principle of transforming at higher frequencies is the same. In most cases the complete ballast including the transformer and the conversion electronics has the same dimensions as an equivalent magnetic one but the weight is only one fifth (and thereby roughly reciprocal to the price).

As for CFLs, there is a wide span of final consumer prices. European high-price manufacturers claim that the cheap far-East products often do not match the European quality level, especially as cheap models mostly dispense with filament pre-heating. Pre-heating in principle excludes immediate start – this being a weak argument against pre-heating, since it takes barely one second. Dispensing with it cuts design and production costs, but it cuts lamp life heavily with increasing number of starts. Also the initial brightness reduction after cold start and the loss of luminous density at low temperatures and old age varies widely and may be more a problem of cheaper designs.

Fig. 6.1: Working principle of CFL or former electronic ballast

Fig. 6.2: Working principle of present electronic ballast above 25 W rating

The working principle that used to be the general one during the »stone age« of electronic ballasts, and that is still applied on all CFLs and on electronic ballasts with lamp ratings up to 25 W, was to rectify the incoming AC via a B2 bridge and to smooth the DC output with an electrolytic capacitor (Fig. 6.1). Somewhat later an upgraded electronic ballast technique was developed to enable at least an approximate restoration of the current sine wave. The incoming alternating voltage here is superimposed by a pulse width modulation or other chopping technique so that the current base line, the interconnection of the current peaks, represents an approximate sine wave (Fig. 6.2). The possible variances of design are multiple, so this generic description of the principle cannot go into detail. Quite an illustrative description of the various principles can be found in the internet.

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