The invention relates to a control circuit (250) for a power supply unit (200) that has an input (207, 209) for receiving a mains supply (208), the control circuit (250) configured to: sample the input (207, 209) in order to obtain a first sample value; sample the input (207, 209) in order to obtain a second sample value subsequent to obtaining the first sample value; compare the first and second sample values to provide an outcome; set a delay interval in accordance with the outcome of the comparison of the first and second sample values; and sample the input (207, 209) in order to obtain a third sample value after the delay interval has elapsed.