Transformers are used to change a source's:

• A. Frequency
• B. Power
• C. Voltage
• D. Resistance

Answer: (C)

Transformers change the voltage level of an AC signal using the turns ratio between the primary and secondary windings. The voltage on the secondary is determined by how many turns are on that winding relative to the primary, via mutual induction. Because the magnetic flux in the core follows the frequency of the input, the output frequency stays essentially the same as the input—transformers don’t alter the signal’s frequency. Power roughly stays the same (minus core and copper losses), so increasing the voltage on the secondary corresponds to a decrease in current there, and vice versa. They don’t inherently change resistance; a transformer can make an impedance look different on the other side based on the turns ratio, but its primary function is to change voltage levels.