An illumination device (1) comprises: input terminals (2) for coupling to AC mains; a LED string (10) connected in series with the input terminals; a rectifier (30), having input terminals connected in series with the LED string; a controllable voltage source (40), having input terminals coupled to the rectifier output terminals; a series arrangement of at least one auxiliary LED (51) and a second ballast resistor (52) connected to the output terminals of the controllable voltage source. The voltage source comprises: a series arrangement of an adjustable first resistor (46) and a second resistor (47) connected in parallel to the input terminals; a tuneable Zener diode (49) connected in parallel to the output terminals, having a control input terminal (48) connected to the node between the two resistors; wherein positive output terminal is connected to positive input terminal and negative output terminal is connected to negative input terminal.