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The fitting of an active power filter (APF) to mitigate the effects of a diode or thyristor bridge-rectifier is predicated on the assumption that the rating of the filter is reasonable (i.e. small) compared with the rating of the existing bridge rectifier or of a replacement active rectifier. The ratings of both shunt and series APFs are analysed in a variety of operating conditions. Ratings are assessed through peak voltage and mean current as appropriate for junction semiconductor devices. RMS ratings are also given because of their familiarity. The series APF, appropriate for the compensation of harmonic voltage sources, is of a generally higher rating than the shunt APF, appropriate for harmonic current sources. The use of a shunt APF where a series APF is appropriate results in poor device utilisation. The semiconductor ratings of an APF can be reduced by using hybrid schemes. Removing large-amplitude, low-order current harmonics with harmonic traps is effective for the shunt APF. Reducing peak voltages by attenuating high-frequency components is effective with the series APF. Arranging rectifiers into multipulse arrangements is seen to be effective for both shunt and series APFs. The ratings are approximately halved for the 12-pulse case.