Variable transformer concept (Variac principle): Which transformer configuration is commonly constructed as a variable transformer for smoothly adjustable AC output voltage?

Introduction / Context:
Bench power control and many laboratory/test setups require a continuously adjustable AC output. Commercial “Variac” devices provide this functionality. Knowing the internal configuration helps with proper selection, safety, and application limits.

Given Data / Assumptions:

• Goal: variable, smooth AC output from a fixed AC input.
• Mechanical adjustment (slider/brush) is acceptable.
• Isolation from mains is not necessarily provided.

Concept / Approach:
A variable transformer is typically an autotransformer with a single continuous winding and a movable contact that varies the effective turns and hence the output voltage ratio. Because primary and output share a winding, these units are compact and efficient but do not provide isolation. Multiple taps give step changes only, not smooth variation, while center-tapped and multi-winding designs serve different purposes (e.g., dual polarities, multiple secondaries).

Step-by-Step Solution:

Verification / Alternative check:
Device datasheets and teardown photos of Variac units show a single toroidal/stacked core with one winding and a carbon brush wiper—the hallmark of an autotransformer.

Why Other Options Are Wrong:

• Multiple-tap transformer: Only provides discrete steps, not continuous adjustment.
• Center-tapped transformer: Supplies split voltages but not variable output.
• Multiple-winding transformer: Offers independent secondaries, not continuous adjust.
• Current transformer: Used for measurement, not variable-voltage supply.

Common Pitfalls:
Assuming a Variac gives isolation; it does not. Users must follow isolation and grounding practices when powering DUTs.

Final Answer:
autotransformer