Which device may share winding and potentially have no isolation between circuits?

• A. Two-winding transformer
• B. Isolation transformer
• C. Autotransformer
• D. Single-winding transformer

Answer: (C)

An autotransformer shares a winding between the input and output. It uses a single continuous winding with taps, so the primary and secondary circuits are connected through that common winding. Because they aren’t galvanically isolated, the same winding carries current for both sides, which is why there can be no isolation between circuits. This is exactly what makes an autotransformer compact and efficient for stepping voltage up or down when isolation isn’t required. The other options rely on separate windings to provide isolation or are defined by having distinct primary and secondary windings, so they do not share a winding in the same way.