How this calculator works
A changing common magnetic flux induces voltage in each turn of an ideal transformer. Winding voltage is proportional to turn count, while current is inversely proportional so that input and output power agree. Real devices draw magnetizing current and lose power in winding resistance and the core. The turn-count results are continuous design ratios, which must be converted to practical whole turns.
Formula
Vs/Vp = Ns/Np; Is/Ip = Np/Ns; Vp Ip = Vs Is.
Worked example
A transformer with 100 primary turns and 20 secondary turns converts 120 V to 24 V ideally. If its primary carries 2 A, the corresponding secondary current is 10 A.
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Frequently asked questions
Can a transformer operate on steady DC?
Steady DC does not create changing flux after the transient. These voltage ratios describe AC or a properly switched and reset magnetic circuit.
Does voltage ratio determine the current rating?
No. Wire size, heating, core losses and insulation determine allowable current and power.
Does the calculation show phase reversal?
No. Dot convention determines polarity. This tool reports voltage and current magnitudes.