Which statement about autotransformers is true?

• A. Autotransformers use two separate windings with full isolation
• B. Autotransformers provide full electrical isolation between windings
• C. Autotransformers use a single winding with a tap and are generally cheaper
• D. Autotransformers cannot change voltage ratio

Answer: (C)

Autotransformers use a single winding with taps. The input is applied to a portion of that winding and the output is taken from another portion, so the primary and secondary share a common conductor and magnetic path. This shared winding idea is why autotransformers are usually cheaper—less copper, fewer insulation requirements, and a smaller core compared to transformers with two completely separate windings.

Because the windings are not isolated from each other, there is no galvanic isolation between input and output. That’s the trade-off: you can still change the voltage up or down by choosing the appropriate tap, but you don’t get isolation safety between sides.

So the statement that matches reality is that autotransformers use a single winding with a tap and are generally cheaper. The other descriptions describe dual, isolated windings or claim that the voltage ratio cannot be changed, which isn’t true for autotransformers.