About this calculator
A transformer trades voltage for current at constant power. The ratio of turns on each winding sets the voltage ratio, and the current ratio is its exact inverse — take half the voltage out and you can take twice the current.
This calculator covers the three questions that come up: what turns ratio gives a voltage you want, what secondary voltage a given ratio produces, and what VA rating to specify once you know the load.
How it works
Voltage per turn is identical on both windings, because both see the same changing magnetic flux in the core. So the voltage ratio equals the turns ratio: Vp/Vs = Np/Ns.
Power has to be conserved — a transformer has no energy source of its own. Whatever the voltage ratio does, the current ratio does in reverse, so Is/Ip = Np/Ns. A 10:1 step-down transformer delivering 2 A on the secondary draws only 0.2 A from the primary.
Impedance transforms by the square of the turns ratio, which is why transformers are used for impedance matching in audio and RF as well as for changing voltage.
Transformers are rated in VA rather than watts because their limit is heating from current and flux, regardless of the load's power factor. For a resistive load VA and watts are the same number; for a reactive or rectifier load they are not.
That distinction matters enormously with a capacitor-input filter. The rectifier draws current in short spikes near the voltage peaks, and the RMS value of those spikes is far above the DC average. The usual rule is to specify a transformer at around 1.8× the DC watts you actually need.
Worked example
Specifying a transformer for a 12 V, 1 A regulated bench supply fed through a bridge rectifier.
- A 7812 needs about 14.5 V minimum at the input, allowing for ripple
- Add two diode drops: peak needed ≈ 15.9 V
- Required RMS secondary: 15.9 / 1.414 ≈ 11.2 V → a 12 V transformer
- DC power out: 12 V × 1 A = 12 W
- Capacitor-input derate: 12 × 1.8 = 21.6 VA
- Primary current at 120 V: 21.6 / 120 = 0.18 A
A 12 V secondary rated at 25 VA or more. Specifying 12 VA — matching the DC watts — would overheat, because the rectifier draws far more RMS current than the DC average suggests.
Practical notes
- Small transformers have poor regulation. A nameplate 12 V secondary often measures 14–15 V unloaded and only hits 12 V at full rated current. Design around the loaded figure but check the unloaded one does not overstress anything downstream.
- Never apply DC to a transformer primary. Without a changing flux there is no back-EMF, and the winding is just a low-resistance short.
- Toroidal transformers are more efficient and quieter than laminated EI types, but have much higher inrush current at switch-on.
- A centre-tapped secondary can give you a dual rail, or a full-wave rectifier with only two diodes at the cost of using half the winding at a time.
- Isolation is often the real reason a transformer is there. A 1:1 transformer changes nothing electrically but breaks the galvanic connection to mains, which is a safety feature, not a waste.
- Mains wiring carries real risk of electrocution and fire. Fuse the primary, sleeve every connection, and if you are unsure, use a commercial supply.
Frequently asked questions
How do I calculate transformer turns ratio?
Divide primary voltage by secondary voltage. 120 V in, 12 V out gives a 10:1 ratio. The current ratio is the inverse, so the secondary can supply ten times the primary current.
What does VA mean on a transformer?
Volt-amperes — secondary voltage multiplied by secondary current. It is used instead of watts because the transformer's heating depends on current regardless of the load's power factor.
Why is my transformer output voltage higher than the rating?
Transformers are rated at full load. With little or nothing drawn, winding resistance drops no voltage and the output reads 10–25% high on a small transformer. It falls to the nameplate figure as you load it.
What VA rating do I need for a DC supply?
Roughly 1.8× the DC watts you are drawing, if there is a capacitor-input filter. The rectifier draws short high current pulses whose RMS value greatly exceeds the DC average, and it is RMS current that heats the windings.
Can I run a 120 V transformer on 240 V?
No — it will saturate, draw enormous magnetising current and burn out. Some transformers have dual primaries that can be wired in series for 240 V or parallel for 120 V; check the datasheet before assuming.