A controller for a thermo-electric cooler is disclosed. The controller comprises a current source for providing current for driving the thermo-electric cooler and a plurality of voltage supply connections for providing a plurality of different voltages for driving current controlled by the current source through the thermo-electric cooler. Voltage selection circuitry is provided for selecting a voltage from the plurality of different voltages, when connected, and for applying the voltage selected to the thermo-electric cooler. When selecting the voltage from the plurality of different voltages, the voltage selection circuitry is configured to select the voltage that, when compared to the other voltages of the plurality of voltages, minimises a potential difference across the current source.