A high quality autotransformer

An autotransformer is an electrical transformer that uses a single continuous winding (instead of separate primary and secondary coils like conventional two-winding transformers), with part of the winding serving as both input (primary) and output (secondary). Its operation relies on electromagnetic induction: the input AC induces magnetic flux in the iron core, and the voltage ratio between input and output follows the transformer equation (V₁/V₂ = N₁/N₂), enabling step-up or step-down functions—for example, a 200-turn winding can double voltage if input is applied to 100 turns and output tapped from all 200.

Its key advantages include high efficiency (often over 95%) due to less copper usage (reducing copper losses), smaller size, lighter weight, and lower manufacturing costs—especially for small voltage differences. Additionally, its apparent power rating exceeds actual transferred power, as part of the power is conducted directly through the shared winding.

A critical limitation is the lack of electrical isolation between primary and secondary circuits. A fault or insulation breakdown can connect high and low voltages, posing safety risks, so it’s unsuitable for scenarios needing isolation (e.g., medical equipment, sensitive electronics).

Common applications include voltage stabilization for appliances/industrial machinery, motor starters (reducing inrush current), portable travel transformers (converting 110V/220V for devices like laptops), and slight voltage adjustments in power grids. It excels in cases prioritizing efficiency, size, and cost, where isolation risks can be managed.