A charging unit for unipolar charging of cloud and fog droplets has been developed and calibrated regarding ion current, angular distribution of ions, mean droplet charge and droplet charge distribution. The charger is part of the droplet aerosol analyser, which is an instrument used for characterisation of droplets and interstitial particles in atmospheric clouds and fog. Three different calibration methods have been used for the charge calibration, whereof the third and newest method makes use of a differential mobility analyser, especially developed for large particles, up to at least 20 mum in diameter. The number of charges acquired is a strictly increasing function of the droplet diameter. Two charging models have been used for comparisons with the measurements and both models show good agreement with the data regarding the size dependency of the charge level. Both models can thus be used to describe the performance of the charger. (C) 2003 Elsevier Ltd. All rights reserved.