Leading-edge and trailing-edge phase cutting

Figure 2.82: Schematic representation: Trailing-edge phase cutting dimmer, e.g. as a rotary dimmer for recessing in flush sockets.


  • For dimmed operation of thermal radiators.

  • Some electronic control gear units, e.g. for LED light sources, can also be operated with leading-edge or trailing-edge phase cutting (to a limited extent).

Leading edge/trailing edge cutting of a phase means that the periodically progressing AC voltage is interrupted during each half-wave. This method is used e.g. to dim thermal radiators (incandescent lamps of various types). According to the desired dimming level, the lamp is always operated for a smaller or larger part of a hundredth of a second and then switched back off.

Due to the frequency of 100 Hz and the lamp’s inertia in heating up and cooling off the filament, the eye does not perceive light flicker. In the present context, this method cannot be called an interface in a narrower sense, since there is no transmission of information, but rather a manipulation of energy supply (see figure).

Many power supplies for LED bulbs, however, today offer besides dimming with e.g. the DALI interface also a dimming operation with phase control or phase section. In such cases, the control gear unit evaluates the incoming signal to in turn operate the light source in a dimmed state (see chapter 2.1.9 "LED luminaire operation"). Based on leading or trailing edge phase cutting, the power supply to the control gear unit is intermittent.

Practice shows that this type of dimming operation is only possible to a limited extent with LED light sources in general. Particularly in the lower dimming range, it can cause flicker. When operating light sources with uninterrupted power supply phases and an additional control signal, however, it is possible to dim the lumen output down to one per cent with many control gear units.