How a transformer works
A transformer has two coils of wire wound around a shared iron core. Alternating current in the primary coil creates a changing magnetic field in the core, which induces a voltage in the secondary coil. There is no electrical connection between the two sides, only magnetic coupling.
The turns ratio
The voltage on each side is proportional to its number of turns:
Vs ÷ Vp = Ns ÷ Np
A transformer with fewer turns on the secondary steps voltage down; with more turns, it steps voltage up. Because power in equals power out apart from losses, current changes the other way:
Ip ≈ Vs × Is ÷ (Vp × efficiency)
Worked example
A transformer with 500 primary turns and 50 secondary turns on a 120 V supply, feeding a 2 A load at 95% efficiency:
| Result | Value |
|---|---|
| Secondary voltage | 12.00 V |
| Turns ratio | 10:1, step-down |
| Power delivered | 24 VA |
| Primary current | 0.211 A |
| Power lost as heat | 1.26 W |
| Turns per volt | 4.17 |
Ratings and losses
Transformers are rated in volt-amperes (VA) or kVA rather than watts, because the heating depends on current whatever the load’s power factor. Choose a rating with some margin above the load: a 24 VA load is comfortably served by a 30 VA or larger transformer.
Losses come from:
- copper losses, the resistance of the windings, which grow with the square of the current;
- core losses, from magnetizing the iron, present even with no load.
Small transformers are typically 80% to 95% efficient, large power transformers above 98%.
Where transformers are used
- Power grids step voltage up to hundreds of kilovolts for long-distance transmission, then down for homes, because high voltage means low current and smaller losses.
- Doorbells, thermostats and irrigation controllers run on 16 to 24 V from small transformers.
- Isolation transformers with a 1:1 ratio separate equipment from the mains for safety.
Tips
- Transformers only work on AC. Most phone chargers today use switch-mode supplies that contain a small high-frequency transformer.
- The secondary voltage of a small transformer is often a little higher than rated with no load and drops under full load.
- Working on mains wiring is dangerous: use a qualified electrician.