Calculator KB-20
Power Transformer Calculator
Rewinding or winding a transformer on a steel core for 50 Hz mains.
Result
In brief: Calculate a 50 Hz mains transformer: core cross-sectional area, turns per volt, winding turns, currents, and winding wire diameter. Example: Primary / secondary turns: 1469 / 80.
Calculation example
| Primary winding | 230 V |
|---|---|
| Secondary winding | 12 V |
| Secondary current | 2 A |
| Frequency | 50 Hz |
| Flux density | 1.2 T |
| Primary / secondary turns | 1469 / 80 |
|---|---|
| Power | 24 W |
| Core cross-sectional area | 5.9 cm² (calculation) |
| Turns per volt | 6.39 t/V |
| Primary current | 115.9 mA |
| Primary wire diameter | 0.24 mm |
| Secondary wire diameter | 1.01 mm |
How it is calculated
S ≈ 1,2 · √P
d = 1,13 · √(I / J), J = 2,5 A/mm²
The number of turns per volt determines how strongly the core is magnetized. The secondary winding has 5% more turns to compensate for voltage drop under load. This is an engineering estimate: check that the wire will fit in the core window.
What to look for
- Mains voltage is lethal. Insulate the primary winding securely and install a fuse.
- If the flux density is set too high, the transformer gets very hot at no load. This shows up as no-load current.
Frequently asked questions
How do I calculate the number of turns in a transformer?
Turns per volt n = 10⁴ / (4.44 · f · B · S), where S is in cm². At 50 Hz, 1.2 T and a 5.9 cm² core, that’s about 6.4 turns per volt, or about 1470 turns for the 230 V primary winding.
How do I determine the core cross-section based on power?
S ≈ 1.2 · √P, where S is in cm² and P is in watts. For 24 W, you need about 5.9 cm². This is a rough formula suitable for 50 Hz mains transformers.
What wire diameter do I need for the winding?
d = 1.13 · √(I / J). For small transformers, use a current density J of 2–3 A/mm². At 2 A and 2.5 A/mm², the result is 1.01 mm.
