The present invention relates to controlling switches in a series connection of electrical devices, in particular to a circuit arrangement, and method of operating same, in which a transistor switch (44, 54) is used to control operation of the devices (12, 14). Because the transistor switches need a gate-source voltage difference, but on the other hand are connected with their source (s) and drain (d) to the main circuit branch, this voltage difference is built up by providing a control current (Ii , I2) over e.g. a resistor (42, 52). This control current (Ii, I2) enters the main current (I),which would influence the operation of the devices, e.g. LEDs (12, 14). In order to correct this, the control current (I2) is corrected for the values of one or more upstream control currents (I2), e.g. through adapting the pulse width in pulse width modulation.