Mains step-down ratios
| Primary | Secondary | Ratio (p:s) |
|---|---|---|
| 120 V | 12 V | 10 : 1 |
| 120 V | 9 V | 13.3 : 1 |
| 120 V | 5 V | 24 : 1 |
| 240 V | 24 V | 10 : 1 |
| 240 V | 12 V | 20 : 1 |
| 240 V | 5 V | 48 : 1 |
Impedance-matching ratios (audio)
| Primary Z | Secondary Z | Turns ratio |
|---|---|---|
| 8 000 Ω | 8 Ω | ~31.6 : 1 |
| 4 000 Ω | 8 Ω | ~22.4 : 1 |
| 2 500 Ω | 8 Ω | ~17.7 : 1 |
| 600 Ω | 8 Ω | ~8.66 : 1 |
| 600 Ω | 600 Ω | 1 : 1 (isolation) |
Formulas
| Turns ratio n | = N_p / N_s = V_p / V_s |
|---|---|
| Impedance ratio | = n² = Z_p / Z_s |
| Required turns | Determined by core flux density: N = V · 10⁸ / (4.44 · f · B · A) |
Notes
- B is peak flux density (gauss); A is core cross-sectional area (cm²); f is line frequency.
- Real transformers include losses and leakage inductance — no-load voltage is higher than rated full-load voltage by ~5–10%.
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