Transformer Calculator
Secondary volts, amps, and watts from turns ratio.
What it does
Transformers trade volts for amps through turn counts: 120 V across 200 primary loops induces 15 V from 25 secondary loops, while 2 A in becomes 16 A out at 240 W both sides. Voltage follows turns up or down with current moving opposite.
The tool above takes primary volts with both turn counts and primary amps. It returns secondary volts and amps with power beside them. The default gives 15 V with 16 A at 240 W.
Equations match OpenStax with HyperPhysics beside them: Vs over Vp is Ns over Np from Faraday induction, and Is over Ip flips to Np over Ns with power conserved. Worked checks agree, from the 120 V to 100 kV x-ray step-up at 12.0 mA to the 200 to 25 loop charger at 2.00 A in.
Pure DC feeds nothing because induction needs changing flux; switching DC on and off only mimics rough AC. Real units run near 99% efficient with the rest lost to heat and hum.
How to use
- Type primary volts AC.
- Type primary and secondary turns.
- Type primary amps under load.
- Read secondary volts first, amps beside it.
- Check power both sides match.
- Step up means fewer primary loops.
Formula
- Np, Ns
- Primary and secondary loop counts.
- Vs/Vp
- Voltage ratio tracking turns ratio.
- P
- Conserved power in watts near ideal.
Ideal coupling assumed; resistance and leakage trim real outputs slightly. Rectifiers follow AC secondaries for DC loads.
Worked example
Example: 120 V, 200 to 25 turns, 2 A
Defaults: 120 V, 200 loops, 25 loops, 2 A.
- Volts: 120 x 25 / 200 = 15 V.
- Amps: 2 x 200 / 25 = 16 A.
- Power: 2 x 120 = 240 W in.
- Check: 16 x 15 = 240 W out.
- Type: step-down, fewer secondary loops.
- Tool shows 15 V, 16 A, 240 W.
Reference table
| Primary | Turns | Secondary |
|---|---|---|
| 120 V, 2 A | 200 to 25 | 15 V, 16 A, 240 W |
| 120 V, 10 A | 50 to 41700 | 100,080 V, 0.012 A, 1,200 W |
| Plug-in bricks | 120/240 in | 3 to 12 V out |
| Grid transmit | 10 kV+ gen | 200 to 700 kV lines |
| Home service | 5 to 13 kV | 120/240/480 V out |
Limits
- Ideal coupling; real losses excluded.
- AC sine assumed; distortion ignored.
- Saturation and inrush unmodeled.
- Safety isolation needs rated units.
Common mistakes
- Feeding DC and expecting output.
- Flipping Ns with Np in division.
- Expecting power gain from step-up.
- Sizing wire for volts instead of amps.
- Skipping rectifiers for DC loads.
Try it yourself
Quick checks. Answers sit tucked under each question.
X-ray 120 V, 50 to 41700 loops, 10 A?
100,080 V with 0.012 A at 1,200 W.
Zero turns?
Rejected: enter turns 1 to 1000000.
Step-up current?
Falls as volts rise: power conserved.
Questions
Default answer?
15 V with 16 A at 240 W.
Turns equation?
Vs over Vp equals Ns over Np.
Current equation?
Is over Ip equals Np over Ns.
DC input?
No output without switching; use AC.
Efficiency?
Near 99% in good units.
Ohm page versus this?
That solves V, I, R; this ratios windings.