IN PRACTICE

摘要

A soft start circuit is a way of turning on the mains supply gradually, to avoid stress on components due'to a sudden voltage or current surge. In a typical transformer rectifier capacitor power supply (Fig 1a) at the moment of switch-on, the primary of the mains transformer looks like a very low resistance because the core is not magnetised. The larger the transformer, the lower the primary resistance, of course, and therefore the larger the switch-on surge. Mechanical movement inside the transformer is what's responsible for the loud 'thump' when you switch on, and obviously such stresses aren't doing the transformer any good - and likewise the mains switch.