Key idea about transformers

A transformer changes alternating voltage according to the turns ratio: $\frac{V_s}{V_p}=\frac{N_s}{N_p}$ where:

• $V_p$ = primary voltage
• $V_s$ = secondary voltage
• $N_p$ = number of turns in primary
• $N_s$ = number of turns in secondary

Identify transformer type from turns/voltage relation

• If $N_s > N_p$, then $V_s > V_p$ and the transformer is a step-up transformer.
• If $N_s < N_p$, then $V_s < V_p$ and the transformer is a step-down transformer.
• An auto-transformer has a single continuous winding with tapping.
• “Auxiliary transformer” is not the standard classification here.

From the given circuit

The circuit corresponds to a transformer in which the secondary side has higher voltage / more turns than the primary side. Therefore, $V_s > V_p$ so it is a step-up transformer.

Check options

• A: Auxiliary transformer — Incorrect
• B: Step down transformer — Incorrect
• C: Step-up transformer — Correct
• D: Auto transformer — Incorrect

Final answer

The transformer used is: $\boxed{\text{Step-up transformer}}$