About

Designing a power transformer requires balancing magnetic flux density with physical space constraints. A common failure mode in custom transformer fabrication is the Window Fill Factor; the calculated copper turns physically exceed the available core window area, making assembly impossible. This tool calculates primary and secondary turn counts while simultaneously verifying physical feasibility against standard EI and Toroidal core geometries.

It cross-references the required wire gauge against a comprehensive copper standard database to ensure current density remains within thermal limits (typically 3 to 4 A/mm²). Accurate calculation of the Flux Density (B) prevents core saturation, which would otherwise lead to overheating and significant efficiency loss.

Formulas

The number of turns N is derived from Faraday's Law of Induction, adapted for sinusoidal waveforms:

N = V × 10⁸4.44 × f × B × A_c

Where V is RMS Voltage, f is frequency (Hz), B is magnetic flux density (Gauss), and A_c is the core cross-sectional area (cm²).

The Window Fill Factor K_u determines feasibility:

K_u = ∑ N_i × A_{wire_i}A_window × eff

Reference Data

Lamination	Tongue Width (mm)	Window Area (mm²)	Power Rating (60Hz)
EI-28	7.0	65	1.5 VA
EI-35	10.0	110	3.0 VA
EI-41	13.0	180	5.0 VA
EI-48	16.0	270	10.0 VA
EI-57	19.0	380	18.0 VA
EI-66	22.0	510	30.0 VA
EI-76	25.4	680	50.0 VA
EI-86	28.6	850	80.0 VA
EI-96	32.0	1100	120.0 VA
EI-114	38.0	1550	200.0 VA

Frequently Asked Questions

For professional machine winding, a fill factor of 0.8 (80%) is achievable. However, for hand-winding or custom prototyping, you should aim for a Fill Factor below 0.5 (50%) to accommodate for insulation tape, bobbin thickness, and human error in layering.

If the Flux Density (B) exceeds the saturation point of the core material (e.g., ~16,000 Gauss for Silicon Steel), the core ceases to amplify the magnetic field. This results in a sharp drop in inductance, massive current spikes, and rapid overheating.

EI cores are easier to wind and cheaper, but emit more electromagnetic interference (EMI). Toroidal cores offer higher efficiency and lower EMI/hum but are significantly more difficult to wind and mount.

Yes. As frequency increases, the required number of turns decreases for the same core area and voltage. This is why high-frequency switch-mode power supply transformers are much smaller than 50/60Hz mains transformers.