Transformer fundamentals — identifying input and output windings: In a conventional transformer used for power or signal isolation, which statement correctly identifies the typical roles of the windings?

Introduction / Context:
Transformers are passive electromagnetic devices that transfer energy between circuits through magnetic coupling. Understanding which winding is driven by a source and which delivers energy to a load is essential for interpreting datasheets, wiring equipment correctly, and predicting voltage/current transformation and impedance matching behavior.

Given Data / Assumptions:

• Two distinct windings on a magnetic core (primary and secondary).
• Operation in linear region with sinusoidal or periodic excitation.
• No direct electrical connection between windings (for an isolation transformer).

Concept / Approach:

The primary winding is connected to the input source. The alternating current in the primary creates time-varying magnetic flux in the core. This flux links the secondary turns, inducing a voltage that drives the load connected to the secondary. The turns ratio determines the voltage ratio, while power balance (neglecting losses) dictates that increasing voltage conversion reduces current and vice versa. Therefore, primary is the input side; secondary is the output side.

Step-by-Step Solution:

Verification / Alternative check:

Check nameplate/datasheet notation: primary terminals are labeled for rated input (e.g., 230 V), secondary terminals are labeled for rated outputs (e.g., 12 V). Reversing roles is possible in some transformers if insulation and ratings allow, but the conventional designation remains primary as input and secondary as output.

Why Other Options Are Wrong:

• Both input or both output: violates energy flow definition; one side must be driven.
• Primary as output and secondary as input: reversed from standard usage.
• Single winding serving both simultaneously: that describes an autotransformer, not a two-winding isolation transformer.

Common Pitfalls:

• Confusing two-winding transformers with autotransformers where windings are shared.
• Ignoring insulation ratings when using windings in reverse.

Final Answer:

a primary winding used as an input and a secondary winding used as an output