Why do AC Adapters Usually Use Heavy Transformers?
The main reasons are safety and cost. Line isolation is essential for safety with respect to electrical shock – no part accessible to the user must be connected to either side of the power line. A regular transformer provides this automatically. While combinations of passive components can reduce the risk of shock, nothing quite matches the virtually fail-safe nature of a simple transformer between the power line and the low voltage circuitry. To achieve similar isolation without a line transformer generally requires a switchmode power supply which actually contains a small high frequency transformer to provide the isolation. Until recently, such systems were much more expensive than a simple iron transformer but that is changing and many modern devices do now use a wall adapter based on this approach. These can be recognized by their light weight, DC (probably regulated) output, and the required warnings NOT to cut them off and replace them with an ordinary plug! I wonder how many peo
