In order to calculate the output voltage of a transformer, divide the input voltage by the __________.

• A. Turns ratio
• B. Voltage ratio
• C. Current ratio
• D. Resistance ratio

Answer: (B)

The main idea is how voltages scale with the number of turns in a transformer. In an ideal transformer, the voltage ratio between the primary and secondary is equal to the turns ratio: Vp/Vs = Np/Ns. Rewriting this gives Vs = Vp / (Np/Ns) = Vp × (Ns/Np). So, to get the output voltage from the input, you divide the input voltage by the turns ratio (the primary-to-secondary turns, Np/Ns). The turns ratio is the precise quantity that governs how much the voltage is stepped up or down. The voltage ratio concept is essentially the same idea, but the standard, physically meaningful term to reference is turns ratio. The other options—current ratio and resistance ratio—do not determine the voltage transformation in an ideal transformer.