Isolation Transformers and Autotransformers
Isolation transformer
Separate primary and secondary windings provide galvanic isolation. Energy crosses the barrier magnetically, not through a conductor.
Isolation can break ground loops, create a separately derived secondary, and make service work on line-connected equipment easier to reference safely.
Service bench
For servicing older line-connected equipment, the device under test is normally powered from the isolation transformer. Grounded oscilloscopes and other test instruments remain grounded.
Autotransformer
An autotransformer uses one continuous winding with taps, so part of the winding is shared by input and output. It can be smaller and more efficient when the voltage change is modest.
Variac
A Variac-style variable autotransformer uses a movable brush on exposed turns to provide smoothly adjustable AC voltage. It is a voltage-control device, not automatically an isolation device.
Buck-boost
Small transformers can be connected as autotransformers to add or subtract a modest voltage from an existing supply.
Electrostatic shields
Some isolation transformers include a shield between primary and secondary to reduce capacitive coupling of high-frequency common-mode noise.
Secondary grounding
An isolated secondary may be intentionally grounded at one point or handled differently where code permits. The correct arrangement depends on the system and NEC grounding rules.