Ideal transformer voltage, turns, and current ratios
For an ideal transformer, secondary-to-primary voltage ratio equals secondary-to-primary turns ratio. Conservation of power makes the current ratio inverse to the voltage ratio.
The ideal model is useful for instruction and first-pass checks. Real transformer selection also requires frequency, VA rating, regulation, insulation, loss, temperature, and safety information.
How to use the transformer calculator
- Choose the unknown: Select one primary or secondary voltage or turn count.
- Enter three ratio values: Use positive voltages and turn counts.
- Enter one current: Choose its winding side and provide a positive current.
- Calculate: Review the solved quantity, ratios, opposite-side current, and ideal VA.
Formula and variables
Voltage follows the turns ratio while current changes inversely in an ideal lossless transformer.
Vs/Vp = Ns/Np = Ip/Is; VpIp = VsIs- Vp — Primary voltage
- RMS or consistently defined primary voltage (V)
- Vs — Secondary voltage
- Corresponding secondary voltage (V)
- Np — Primary turns
- Primary winding turn count
- Ns — Secondary turns
- Secondary winding turn count
- Ip, Is — Currents
- Primary and secondary current (A)
Ten-to-one step-down transformer
A 240 V primary has 1,000 turns and the secondary has 100 turns while supplying 5 A.
- Vp
- 240 V
- Np
- 1,000 turns
- Ns
- 100 turns
- Is
- 5 A
- Vs = 240 × 100/1000 = 24 V
- Ip = 5 × 100/1000 = 0.5 A
Result: The ideal secondary is 24 V, primary current is 0.5 A, and apparent power is 120 VA.
Voltage is stepped down by ten while current capability is stepped up by ten in the lossless model.
Understanding your results
Step-up voltage means step-down current
Ideal input and output power are equal.
- Ns/Np below 1 gives a step-down voltage ratio.
- Ns/Np above 1 gives a step-up voltage ratio.
- Current ratio is the inverse of the turns ratio.
- Real loaded secondary voltage differs because of regulation and losses.
Assumptions
- The transformer is ideal, lossless, and operating with suitable alternating excitation.
- Compared voltages use the same convention, such as RMS values.
- All winding turns link the same core flux.
Limitations
- Does not model winding resistance, leakage flux, magnetizing current, core loss, saturation, regulation, harmonics, inrush, or temperature rise.
- Does not establish a safe VA rating, insulation class, creepage, fuse, or protective-device requirement.
- Not valid as a complete transformer design or mains-safety tool.
Common mistakes
- Reversing primary and secondary turns.
- Assuming current increases with voltage.
- Using the ideal result as a loaded regulation guarantee.
- Ignoring frequency, saturation, insulation, and VA limits.
Practical use cases
Transformer coursework
Solve ideal winding, voltage, and current ratio exercises.
First-pass checks
Estimate ideal secondary voltage and current before consulting verified component data.
Frequently asked questions
Why is current ratio inverted?
In the ideal model input and output power are equal, so raising voltage requires lowering current.
Does this calculate real output voltage?
No. Real output depends on regulation, losses, load, frequency, winding design, and core behavior.
Sources and review
- Transformers — OpenStax University Physics Volume 2. Accessed 2026-07-14.
Reviewed 2026-07-14.