Definition
A transformer in which a single winding serves as both the primary and the secondary. Part of the winding is shared by the input and output circuits, allowing the device to step voltage up or down through tap points on that single coil rather than through two electrically separate windings.
Plain English
An electrical device that raises or lowers voltage using just one coil of wire instead of two. The input and output share part of the same winding, with connection points along the coil deciding how much the voltage changes.
Context Anchor
Seen in aircraft electrical system discussions, especially where AC voltage must be stepped up or stepped down for equipment.
Derivation
From the Greek 'autos' meaning 'self,' joined with 'transformer.' The 'auto' part signals that the device transforms voltage using its own single winding, rather than relying on two separate ones like a standard transformer.
Why Pilots Care
Autotransformers provide necessary voltage conversion in aircraft electrical systems while adding minimal weight.
Analogy
Think of it like taking water from different points along the same pipe instead of using two separate pipes. You can get a different pressure point, but it is still the same connected path.
Intuition Check
Do not read “auto” as meaning automatic. In autotransformer, “auto” means self: the transformer uses one shared winding to do the voltage change.
Example Sentence 1
The aircraft's power supply uses an autotransformer to step the input voltage down to the level required by the avionics.
Example Sentence 2
Older aircraft often use an autotransformer to step battery voltage up for certain instrument circuits.