Transformer Calculator
Calculate secondary voltage, turns ratio and ideal secondary current for a transformer from its turns counts.
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What this tool does
Calculate secondary voltage, turns ratio and ideal secondary current for a transformer from its turns counts. Ideal transformer: no losses, no magnetising current, and the secondary current assumes the entered primary current is fully load current.
How to use the Transformer Calculator
- Enter or select primary voltage (v).
- Enter or select primary turns np.
- Enter or select secondary turns ns.
- Enter or select primary current (a).
- Read the calculated result; change any measurement to compare alternatives.
Formula
Vs = Vp × Ns ÷ Np; Is = Ip × Np ÷ Ns; apparent power = Vp × Ip
- vp
- Primary voltage (V)
- np
- Primary turns Np
- ns
- Secondary turns Ns
- ip
- Primary current (A)
Ideal transformer: no losses, no magnetising current, and the secondary current assumes the entered primary current is fully load current.
Worked example
For transformer calculator, the following measurements illustrate the exact method: Primary voltage (V): 120; Primary turns Np: 500; Secondary turns Ns: 50; Primary current (A): 2.
Inputs
- Primary voltage (V)120
- Primary turns Np500
- Secondary turns Ns50
- Primary current (A)2
Result
- Secondary voltage (V)12
- Turns ratio Np ÷ Ns10
- Ideal secondary current (A)20
- Apparent power (VA)240
Results explained
- Secondary voltage (V)
- Secondary voltage (V) from the formula above. Ideal transformer: no losses, no magnetising current, and the secondary current assumes the entered primary current is fully load current.
- Turns ratio Np ÷ Ns
- Turns ratio Np ÷ Ns from the formula above. Ideal transformer: no losses, no magnetising current, and the secondary current assumes the entered primary current is fully load current.
- Ideal secondary current (A)
- Ideal secondary current (A) from the formula above. Ideal transformer: no losses, no magnetising current, and the secondary current assumes the entered primary current is fully load current.
- Apparent power (VA)
- Apparent power (VA) from the formula above. Ideal transformer: no losses, no magnetising current, and the secondary current assumes the entered primary current is fully load current.
Frequently asked questions
Vs = Vp × Ns ÷ Np. 120 V on a 500-turn primary with a 50-turn secondary gives 120 × 50 ÷ 500 = 12 V — a 10:1 step-down.
Ideally in inverse proportion: Is = Ip × Np ÷ Ns, so the 10:1 example turns 2 A primary into 20 A secondary while power (VA) stays the same before losses.
An isolation transformer: same voltage in and out, used for safety isolation rather than voltage conversion.
No. Core and copper losses typically cost 2–10% of throughput, regulation drops the secondary voltage under load, and magnetising current flows even with no load. This tool shows the ideal ratios.
No. Transformers need changing flux, so they require AC; steady DC produces no secondary voltage and can saturate and overheat the primary.