Technical note: a mains drop-out simulator

During an investigation into mains-borne and radiated interference in the Ultrasound Department of St James's University Hospital, the need arose to determine the susceptibility of some equipment to the mains supply failing for very brief periods. This is known as 'drop-out'. A circuit was built that can drop out the mains supply to a piece of equipment for up to 15 consecutive half-cycles at a selectable repetition rate between approximately 0.4/s to 6.5/s. Circuit design was simplified by using a solid state relay (SSR), which turns on and off only at the zero crossing points in the mains supply waveform.