Transformer Calculator (Turns Ratio, Voltage & Power)

Transformer Guide

Transformer Equations

Voltage ratio equals turns ratio: Vp/Vs = Np/Ns. For an ideal transformer (100% efficiency): power in = power out. Vp × Ip = Vs × Is. Therefore: Ip/Is = Ns/Np. Voltage and current scale inversely with turns ratio — stepping up voltage steps down current by the same factor. Example: UK mains 230V to 12V. Ratio = 230/12 = 19.17:1. If primary current = 0.1A and efficiency 100%: secondary current = 0.1 × (230/12) = 1.92A. Secondary power = 12 × 1.92 = 23W = primary power.

Step-Up and Step-Down

Step-down transformer: Ns < Np — reduces voltage, increases current. Used in phone chargers, laptop adapters, doorbell transformers, welding equipment. Step-up transformer: Ns > Np — increases voltage, reduces current. Used in power transmission. National Grid transmits at 275-400kV — stepping up from power station output (~25kV) reduces current by 16× and therefore reduces power loss (P_loss = I²R) by 256× in the transmission lines. The electricity is then stepped back down through a series of

Transformer Losses

Real transformers are typically 95-99% efficient. Losses: copper losses (I²R in the windings) — reduce by using thick copper wire. Core losses: eddy current losses (using laminated cores reduces these) and hysteresis losses (energy lost magnetising and demagnetising the core each cycle). Ideal transformer assumption: assumes no flux leakage and perfect coupling between primary and secondary coils. Efficiency = (output power / input power) × 100. A 95% efficient transformer delivering 100W output

Applications

Power supplies: every mains-powered device with a 'brick' adapter contains a transformer (often a high-frequency switching transformer). Isolation transformers: 1:1 ratio — no voltage change but electrically isolates the secondary circuit from the primary (safety in medical equipment and lab work). Current transformers: sense high currents in power lines for metering and protection. Audio transformers: impedance matching between amplifier and speaker. The turns ratio affects impedance as Z ∝ (N)