Economy and LED lamps have a ballast built into the base, which produces the voltage suitable for the illuminant from the mains voltage. When dimming, it may happen that this ballast prevents the detection of the voltage zero crossing of the mains voltage required for the dimmer. The dimmer can therefore "fall out of step", which manifests itself in a flickering of the lamps.

The Additional Circuit of the ZSL-UP regulates this and improves the dimming behaviour.