A control unit for a printing apparatus has a plurality of heat sources, each heat source being operable at an individual power level. The control unit is configured to control the power supplied to the heat sources such that, at each instant, the sum of the delivered individual power levels is less than or equal to a maximum allowable power. The control unit is also configured to control the power delivered to the heat sources on the basis of sequential cycles and for each cycle, to receive for each heat source a requested power pulse duration, to schedule within the cycle instants at which power is to be delivered to the heat sources, based on the individual power levels and requested pulse durations and to deliver the power according to the scheduled instants within the cycle.