Transformer basics: What is true of a fixed transformer with respect to its turns ratio and adjustability?

Introduction / Context:
Power supplies and signal interfaces frequently use transformers. Understanding the distinction between fixed and adjustable (tapped/variable) transformers ensures correct selection for voltage conversion and isolation tasks.

Given Data / Assumptions:

• “Fixed transformer” means no user-adjustable taps or movable core/brush arrangements.
• Windings are predetermined at manufacture.
• Turns ratio sets the nominal voltage conversion.

Concept / Approach:
The output-to-input voltage ratio is determined by the turns ratio Ns/Np. In a fixed transformer, this ratio is set by the design and cannot be changed in service. Adjustable devices use multiple taps or autotransformer sliders; fixed ones do not. A center tap or multiple windings may or may not be present, but those features alone do not imply adjustability.

Step-by-Step Solution:

Verification / Alternative check:
Datasheets label transformers as “fixed” with a single secondary rating. Adjustable or “tapped” units explicitly list multiple ratios/terminals.

Why Other Options Are Wrong:

• Center-tapped secondary / multiple windings / multiple-tapped secondary: These can exist, but do not define “fixed” unless taps are brought out for selection. The defining feature is the unchangeable ratio.

Common Pitfalls:
Equating “center-tapped” with “adjustable.” A center tap provides two equal halves but does not give arbitrary ratio control.

Final Answer:
turns ratios that cannot be changed