Power supply unit, having: - an output (LED+, LED-) for outputting an operating current depending on an internal measurement signal (Vout), - a communications line, which is referred to the negative output (LED-), - a current-measuring device, which is connected to the communications line and has a current mirror, wherein - the current mirror is formed by a first transistor and a second transistor and furthermore has a third transistor, the working electrode of which is connected to the control electrode of the first transistor, the reference electrode of which is connected to the working electrode of the first transistor, and the control electrode of which is connected either to a microcontroller or to the centre point of a series circuit of a resistance (R Ref) and a Zener diode (D Ref), wherein the series connection is connected between the supply voltage and an output connection (LED-), wherein - at least one light source module is connectable to the output (LED+, LED-), wherein the at least one light source module has a current-setting resistance (Rset), which is connectable to the communications line and to the negative output (LED-), - the current-measuring device is designed to generate a current through the current-setting resistance (Rset) by applying a measurement voltage to the communications line, which current flows through the communications line and is proportional to the conductance of a current-setting resistance (Rset), - the current-measuring device is designed to mirror the generated current on the communications line using the current mirror, and - the current-measuring device is designed to convert the mirrored current into an internal measurement signal (Vout) with a reference potential which is different from the communications line, wherein the internal measurement signal (Vout) is referred to an internal circuit ground. - an output (LED+, LED-) for outputting an operating current depending on an internal measurement signal (Vout), - a communications line, which is referred to the negative output (LED-), - a current-measuring device, which is connected to the communications line and has a current mirror, wherein - the current mirror is formed by a first transistor and a second transistor and furthermore has a third transistor, the working electrode of which is connected to the control electrode of the first transistor, the reference electrode of which is connected to the working electrode of the first transistor, and the control electrode of which is connected either to a microcontroller or to the centre point of a series circuit of a resistance (R Ref) and a Zener diode (D Ref), wherein the series connection is connected between the supply voltage and an output connection (LED-), wherein - at least one light source module is connectable to the output (LED+, LED-), wherein the at least one light source module has a current-setting resistance (Rset), which is connectable to the communications line and to the negative output (LED-), - the current-measuring device is designed to generate a current through the current-setting resistance (Rset) by applying a measurement voltage to the communications line, which current flows through the communications line and is proportional to the conductance of a current-setting resistance (Rset), - the current-measuring device is designed to mirror the generated current on the communications line using the current mirror, and - the current-measuring device is designed to convert the mirrored current into an internal measurement signal (Vout) with a reference potential which is different from the communications line, wherein the internal measurement signal (Vout) is referred to an internal circuit ground.