Transformer Calculator

Transformers transfer AC power between voltage levels using magnetically coupled windings. Power adapters, distribution substations, audio output stages, and isolation barriers all rely on the turns ratio relating primary and secondary voltages. Ideal transformers (ignoring losses) follow a simple proportional law that lets you solve for unknown voltages or required winding counts during design and troubleshooting.

The fundamental relationships are:

V_secondary / V_primary = N_secondary / N_primary

N_secondary = N_primary × (V_secondary / V_primary)

Where V is RMS voltage and N is the number of turns on each winding. A step-down transformer has fewer secondary turns; step-up has more. Current transforms inversely — halving voltage doubles available secondary current at the same volt-amperes (ignoring losses).

Real transformers add copper and core losses, leakage inductance, and saturation limits. The calculator assumes ideal ratios — derate 5–15% for loaded secondary voltage sag on small wall-warts. Frequency matters too: a 60 Hz laminations core cannot simply run at 400 Hz without redesign.

Worked example: A 120 V primary transformer needs 12 V secondary for a halogen lighting circuit. Turns ratio = 12/120 = 0.1, written 1:10 (primary:secondary) or 10:1 step-down. If the primary has 600 turns, secondary needs 600 × 0.1 = 60 turns. For a 240 V to 24 V control transformer: ratio 24/240 = 0.1 again — same 10:1 step-down, double primary voltage compensated by proportionally higher secondary turns if the core is redesigned, or use a 120 V secondary tap wired differently.

Verify fuse and wire sizing on both sides using the power calculator and watts-to-amps conversion — secondary current can surprise on low-voltage, high-current outputs.