According to examples, an apparatus may include a processor and a nontransitory computer readable medium on which is stored machine readable instructions that are to cause the processor to generate a linear piecewise control signal based on an internal resistance of a heating lamp to control a rate of change of a phase angle of an applied voltage in the heating lamp until a peak current in the heating lamp is within a predefined threshold of a maximum current rating of an alternating current (AC) power source of a circuit.

There is disclosed print agent drying unit 10, 20, 30 to dry print agent 40 on a substrate 14, the drying unit comprising: a substrate support 12 to support a substrate 14; an exciter 18, 28, 38 to excite a boundary layer of print agent 40 applied on the substrate 14, to thereby dissociate the boundary layer from the print agent; and a radiation source 16 to direct radiation energy to the substrate to dry the print agent. Methods of drying print agent 40 on a substrate 14 are also disclosed, including a method comprising: causing the substrate 14 to vibrate to dissociate a boundary layer of print agent 40 on the substrate 14; and heating the print agent 40 to dry the print agent 40.