The rotary dimmer switches and dims light sources such as light bulbs, HV halogen, LV halogen with Gira Tronic transformers or LV halogen with conventional transformers. Auxiliary unit operation is possible with an auxiliary insert. The dimmer can be operated in combination with the mains decoupler. The operation of a mains decoupler is not possible when auxiliary units are connected. Capacitive loads (e.g. Gira Tronic transformers) and inductive loads (e.g. conventional transformers) cannot be connected to the universal dimming insert at the same time.