What is the purpose of a transformer?

• A. To change voltage and current levels between circuits
• B. To store energy
• C. To rectify AC to DC
• D. To filter signals

Answer: (A)

Transformers operate by electromagnetic induction to transfer power between two circuits while changing voltage and current levels. The primary and secondary windings share a magnetic core, and when AC is applied to the primary, a changing magnetic flux induces a voltage in the secondary. In an ideal transformer, voltages are proportional to the turns on each winding: Vp/Vs = Np/Ns. Because power is roughly conserved, the currents are inversely related: Ip/Is = Ns/Np. So increasing voltage on the secondary lowers current there, and decreasing voltage raises current, allowing the voltage and current to be transformed without changing the frequency.

A transformer does not store energy like a battery or capacitor, nor does it rectify AC to DC or filter signals. Those functions are performed by other components. The transformer’s purpose is to change voltage and current levels (and can aid in impedance matching and isolation) with real devices showing losses and frequency dependence.