OneTaskBy Tim King · Updated
Transformer Calculator
Turns Ratio, Secondary Turns and Primary Current
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How the Transformer Calculator works
This transformer calculator works out the turns ratio from the primary and secondary voltages and says whether it steps up or down. Add the primary turns to get the secondary turns, and a load in watts or VA with an efficiency to get the secondary current, primary current, input power and heat lost, plus the impedance ratio for audio and matching work.
A transformer changes AC voltage by winding two coils on one iron core. The ratio of turns sets the ratio of voltages, and because power in roughly equals power out, current changes the opposite way. That is why the grid sends power at hundreds of kilovolts with small currents, and why a doorbell transformer can be tiny while still drawing almost nothing from the mains.
How the transformer calculator works
The formula is Vp ÷ Vs = Np ÷ Ns, so the turns ratio equals the voltage ratio. Secondary turns are Ns = Np × Vs ÷ Vp. The secondary current is the load divided by Vs, and the primary current is the load divided by the efficiency and by Vp. The impedance ratio is the square of the turns ratio, which is how output transformers match speakers to valve amplifiers.
First example. A 230 V to 12 V transformer has a ratio of 19.17 : 1. With 1,150 primary turns the secondary needs 1,150 × 12 ÷ 230 = 60 turns, or 5 turns per volt.
Second example. A 60 W load at 12 V draws 5 A. At 90% efficiency the input is 66.7 VA, so the primary draws 66.7 ÷ 230 ≈ 0.29 A and 6.7 W is lost as heat.
How many amps per kVA at 230 V? 1,000 ÷ 230 ≈ 4.35 A on the primary side.
Turns ratio and current per kVA for common transformers
| Use | Primary (V) | Secondary (V) | Turns ratio | Primary A per kVA | Secondary A per kVA |
|---|---|---|---|---|---|
| Doorbell | 230 | 8 | 28.75 : 1 | 4.3478 | 125 |
| Halogen downlight | 230 | 12 | 19.167 : 1 | 4.3478 | 83.333 |
| Doorbell (US) | 120 | 16 | 7.5 : 1 | 8.3333 | 62.5 |
| Thermostat and HVAC control | 120 | 24 | 5 : 1 | 8.3333 | 41.667 |
| Garden lighting | 240 | 12 | 20 : 1 | 4.1667 | 83.333 |
| Irrigation controller | 230 | 24 | 9.5833 : 1 | 4.3478 | 41.667 |
| Site tools (UK) | 230 | 110 | 2.0909 : 1 | 4.3478 | 9.0909 |
| Travel step-down | 230 | 120 | 1.9167 : 1 | 4.3478 | 8.3333 |
| Travel step-up | 120 | 230 | 1 : 1.9167 | 8.3333 | 4.3478 |
| Valve amp output (8 Ω) | 4,000 | 89.4 | 44.743 : 1 | 0.25 | 11.186 |
| Microwave oven (HV) | 230 | 2,100 | 1 : 9.1304 | 4.3478 | 0.4762 |
| Neon sign | 230 | 10,000 | 1 : 43.478 | 4.3478 | 0.1 |
| US pole transformer | 7,200 | 240 | 30 : 1 | 0.1389 | 4.1667 |
| US pad-mount | 13,800 | 480 | 28.75 : 1 | 0.0725 | 2.0833 |
| UK street substation | 11,000 | 433 | 25.404 : 1 | 0.0909 | 2.3095 |
| Australian street substation | 11,000 | 433 | 25.404 : 1 | 0.0909 | 2.3095 |
| UK primary substation | 33,000 | 11,000 | 3 : 1 | 0.0303 | 0.0909 |
| Grid supply point (UK) | 132,000 | 33,000 | 4 : 1 | 0.0076 | 0.0303 |
| Grid (UK supergrid) | 400,000 | 132,000 | 3.0303 : 1 | 0.0025 | 0.0076 |
| US transmission | 345,000 | 138,000 | 2.5 : 1 | 0.0029 | 0.0072 |
| Power station generator step-up | 23,500 | 400,000 | 1 : 17.021 | 0.0426 | 0.0025 |
| Laptop charger (inside the brick) | 340 | 20 | 17 : 1 | 2.9412 | 50 |
Tips and pitfalls
- Transformers need AC. A steady DC voltage produces no output and can overheat the primary.
- Rate in VA, not watts. Motors and electronics draw extra current, so size by VA with headroom.
- Off-load voltage runs high. Small transformers can read 10–20% above their rating with no load.
- Fuse the primary. A shorted secondary pulls a large primary current that can start a fire.
FAQ
How do you calculate a transformer turns ratio?
Divide the primary voltage by the secondary voltage. A 240 V to 24 V transformer has a ratio of 10 : 1, so for every 10 turns on the primary there is one on the secondary. The same ratio applies to the number of turns, so a 500-turn primary needs a 50-turn secondary.
How do you calculate transformer primary current?
Divide the load power by the efficiency and then by the primary voltage. A 100 W load at 95% efficiency on a 230 V supply draws 100 ÷ 0.95 ÷ 230 ≈ 0.46 A. Ignoring efficiency gives a slightly low answer, which matters when sizing fuses and cables.
What is the difference between a step-up and step-down transformer?
A step-down transformer has fewer turns on the secondary, so the output voltage is lower and the current higher. A step-up has more secondary turns, raising the voltage and lowering the current. Power stations step up for transmission and local substations step back down.
Why is a transformer rated in VA instead of watts?
The heat in the windings depends on the current, whatever the power factor of the load. VA is volts times amps, so it captures that current directly. A load with a poor power factor can draw 1,000 VA while delivering only 700 W, and the transformer has to handle the full 1,000 VA.
What is the impedance ratio of a transformer?
The impedance ratio is the square of the turns ratio. An output transformer with a 22.4 : 1 ratio makes an 8 Ω speaker look like about 4,000 Ω to a valve amplifier. This is how transformers match a load to a source so that the most power is transferred.
See also: all Electronics tools, Physics, Chemistry.
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