Transformer Calculator
Calculate transformer secondary voltage, secondary current, turns ratio, power and step-up or step-down type. See a complete solution with live input values on a labelled transformer diagram.
Transformer Diagram and Live Values
Primary and secondary turns, AC voltage, current, core and transformer type are shown together.
Step-by-Step Solution
Here's exactly how this answer was calculated, one step at a time.
Given: Vp = 240 V, Np = 1,000, Ns = 100
Step 1: Write the ideal transformer voltage ratio
Voltage ratio equals turns ratio for an ideal transformer.
Vs / Vp = Ns / NpStep 2: Rearrange for secondary voltage
Vs = Vp × Ns / NpStep 3: Substitute values
Vs = 240 × 100 / 1,000Step 4: Calculate secondary voltage
Vs = 24 V
The transformer result is:
24 V
Free Transformer Calculator
This Transformer Calculator quickly calculates secondary voltage, secondary current, or the number of secondary turns for an ideal AC transformer. Select the value you need, enter the known primary and secondary values, and receive a complete answer with formula substitution, transformer turns ratio, input and output apparent power, and transformer type. The live diagram puts volts, amps, turns, the iron core and the step-up or step-down result in one easy-to-understand chart.
It is useful for school physics, electrical engineering revision, electronics projects, power-supply planning and checking transformer-ratio questions. The calculator uses the ideal transformer model. That gives a clear educational calculation, but real transformers lose energy in copper resistance, magnetic core losses and leakage flux. Always check datasheets, insulation, temperature and safety requirements for real hardware.
What Is a Transformer?
A transformer transfers alternating-current electrical energy between circuits using electromagnetic induction. It has a primary winding connected to the AC source, a magnetic core, and a secondary winding connected to a load. Alternating current in the primary creates changing magnetic flux in the core. That changing flux induces an alternating voltage in the secondary winding.
Because a transformer requires changing flux, an ordinary transformer does not operate correctly on steady DC. The core may saturate and the primary can draw damaging current. Transformers are used to step voltage up for transmission, step voltage down for appliances and adapters, provide isolation, match impedances and measure high voltage or current.
Transformer Turns Ratio Formula
The fundamental ideal transformer voltage formula is Vp / Vs = Np / Ns, or equivalently Vs / Vp = Ns / Np. Vp and Vs are primary and secondary voltage; Np and Ns are number of turns. The voltage ratio equals the turns ratio. More secondary turns gives a higher secondary voltage; fewer turns gives a lower voltage.
To find secondary voltage, rearrange to Vs = Vp × Ns / Np. Example: a 240 V primary with 1,000 turns and a 100-turn secondary gives Vs = 240 × 100 / 1000 = 24 V. The diagram will therefore identify this as a step-down transformer with a 0.1:1 secondary-to-primary turns ratio.
How to Calculate Transformer Current
In an ideal transformer, power is conserved: VpIp = VsIs. Since voltage and turns may change, current changes in the opposite direction. The current-ratio formula is Is / Ip = Np / Ns, so Is = Ip × Np / Ns. A step-down transformer lowers voltage but raises available current by the same ratio, before losses.
For the 240 V to 24 V example, if primary current is 0.5 A, secondary current is 0.5 × 1000 / 100 = 5 A. Input power is 240 × 0.5 = 120 VA and ideal output is 24 × 5 = 120 VA. Actual output will be lower because no practical transformer is 100% efficient.
Step-Up, Step-Down and Isolation Transformers
A step-up transformer has more turns on the secondary than the primary, so Ns/Np is greater than one and output voltage rises. It is used in high-voltage applications such as transmission systems. Current decreases correspondingly. A step-down transformer has fewer secondary turns, so output voltage falls and available secondary current rises; it is common in chargers, control circuits and power supplies.
An isolation transformer has approximately equal primary and secondary turns, so voltage remains similar while the windings are electrically separated. Isolation can reduce shock and ground-loop risks in appropriately designed systems, but it does not make exposed conductors safe. Follow all electrical standards and use correctly rated protective devices.
Transformer Efficiency and Real-World Losses
The ideal equations on this page assume 100% efficiency. A real transformer's efficiency is output power divided by input power, multiplied by 100%. Good power transformers can be highly efficient, but every unit loses some energy as heat. Copper loss comes from winding resistance; core loss comes from hysteresis and eddy currents; leakage flux and load regulation also affect output voltage.
Transformer ratings are commonly written in VA or kVA because heating depends on voltage and current. Select a transformer with sufficient VA rating, allowable temperature rise and insulation class. A 24 V transformer may not deliver exactly 24 V under every load; its no-load voltage can be higher and the loaded voltage can drop.
AC Frequency, Cores and Safety
Transformer design depends on AC frequency. A transformer designed for 50 Hz may overheat or saturate at a lower frequency because flux rises. High-frequency transformers can use smaller ferrite cores but must be designed for switching waveforms. Never connect a transformer to a frequency or voltage outside its approved rating.
Mains voltage can injure or kill and can start fires. Use enclosed, fused, correctly grounded equipment and never touch exposed primary wiring. A formula calculator cannot determine safe clearances, insulation coordination, inrush current, fusing, thermal performance or legal compliance. For practical mains work, use qualified design and testing procedures.
Transformer Calculation Tips
Write every value with units. Keep voltages in volts, current in amps, turns as a count and power in VA. Check the answer logically: if Ns is one tenth of Np, Vs should be one tenth of Vp and Is should be ten times Ip for the ideal model. If the expected relationship is reversed, verify which winding is primary and which is secondary.
Use RMS values for ordinary AC ratings. Waveform peak values, rectifier losses, capacitor-input supplies and regulated converters need additional calculations. For a DC output power supply, transformer secondary AC voltage is only one stage of the design; bridge-rectifier voltage drop and capacitor ripple are also relevant.
Frequently Asked Questions
What is the transformer formula?
For an ideal transformer, Vp/Vs = Np/Ns and VpIp = VsIs. Voltage follows turns ratio while current changes inversely.
How do I calculate secondary voltage?
Use Vs = Vp × Ns/Np. Enter voltage in volts and winding turns as counts.
How do I calculate secondary current?
Use Is = Ip × Np/Ns for an ideal transformer, or use output current = output VA / output voltage when rating information is known.
What makes a transformer step-up or step-down?
More secondary turns than primary turns makes it step-up. Fewer secondary turns makes it step-down.
Why does current increase in a step-down transformer?
Ideal power is conserved, so lower output voltage is balanced by higher output current.
Can a transformer run on DC?
No. A conventional transformer needs changing AC flux. DC can saturate the core and overheat the primary winding.
What is transformer efficiency?
It is output power divided by input power times 100%. Real efficiency is below 100% because of winding and core losses.
What does VA mean on a transformer?
VA means volt-amperes, the apparent-power rating. It indicates the voltage-current load capacity of the transformer.
Are transformer output values exact?
No. Actual voltage varies with load, supply voltage, regulation, frequency, temperature and losses.