The growing number of consumers representing non-linear loads has led to an increase in the level of harmonics and poor power quality. It is not news to apply active power filtering (APF), but the drawback is the lower efficiency and complexity. The described here active power filter control system is simpler and more efficient. It is based on a relay current regulator and its converter uses the existing DC bus of the rectifier. The theoretical basis of the APF is described. A model of the proposed APF (with its suggested control system and the common DC link) is simulated in Matlab. The result of this simulation demonstrates that the level of the Total Harmonic Distortion (THD) of the input current is less than 3 %. The input current matches the phase of the input (mains) voltage: i.e. the consumption of reactive power from the grid is minimized. The simulation is confirmed by experimental study of the relay current controller. The corresponding results are provided.