An efficient and a versatile unipolar corona charger was developed. It has indirect charging characteristics, with corona and charging regions separated. Ions were generated by a needle electrode and then driven to the charging region by a sheath flow in order to reduce the electrostatic loss of nanoparticles. The distance of the electrode to the walls can be adjusted by a micrometer, modifying the onset voltage and N(i)t-product, and therefore the intrinsic and extrinsic charging efficiencies and loss of charged particles. Experimental results indicate that the generated ion current is practically the same independent of the variation in the applied voltage or corona current, but varying the electrode gap distance seems to be a much more efficient way to regulate the N(i)t-product. The charging efficiency can vary from high values, comparable to the higher efficiency chargers presented in the literature, to lower levels such as reached with bipolar chargers. (C) 2013 Elsevier Ltd. All rights reserved.