Being able to transform a voltage is not possible with which circuits?

• A. AC
• B. DC
• C. Pulsed
• D. Mixed

Answer: (B)

Voltage transformation relies on a transformer, which works through changing magnetic flux in the core. A steady direct current, once the initial energizing transient passes, creates a constant flux, so there’s no changing flux to induce a different voltage in the secondary. Without that changing flux, a pure DC signal cannot be stepped up or down by a simple transformer. If the input includes time-varying components—like alternating current, or pulses with rising and falling edges—the flux changes and the transformer can transfer energy to produce a different voltage level. Mixed signals also contain changing parts that allow transformation. In practice, to change a DC level, people use DC-DC converters that switch to create an effective AC-like waveform and then reassemble it at the desired DC level. So the circuit type where voltage transformation isn’t possible in the usual transformer sense is direct current.