Transformer
A transformer transfers AC energy from one coil to another through a shared magnetic field. The turns ratio sets the voltage ratio, and the two windings stay electrically separate.
Primary current creates a changing magnetic field inside a shared core. That field induces voltage in the secondary winding. More turns on the secondary mean a higher output voltage at lower current, and vice versa.
In plain terms
Two waterwheels sharing the same belt. Spin one and the other turns; choose belt-pulley sizes and you change how fast.
Also called: xfmr, magnetic.
Why designers use it
- Step mains AC down to a safer level for rectification.
- Isolate a circuit galvanically from the wall or a noisy ground.
- Match impedances between a tube amp and a speaker.
- Couple Ethernet pairs while blocking common-mode noise.
- Drive flyback converters in tiny phone chargers.
Best for
- Mains step-down
- Galvanic isolation
- Ethernet magnetics
- Flyback / forward converters
Key specifications
- Power handling: Use the selected transformer's rating and conditions
- Turns and voltage ratio: Design-specific (Loading and winding losses affect the actual output voltage.)
- Frequency and waveform: Use the specified excitation conditions (Do not apply steady DC to a winding intended for AC.)
- Efficiency: Depends on design, frequency and load
- Insulation: Use the exact safety and working-voltage ratings (A dielectric test voltage alone does not establish suitability for medical or mains isolation.)
When not to use it
- Pure DC — transformers need a changing magnetic field to work.
- Tight, low-profile spaces — they're heavy and tall compared to inductors.
- When a buck converter would do the same job at a fraction of the size.
Common mistakes
- Applying steady DC to an AC winding, risking excessive current and heating.
- Reversing windings without checking voltage, insulation, frequency and current ratings.
- Treating a low output voltage or a dielectric test rating as proof that the entire circuit is safe.
Where you will find it
- A suitably selected transformer can change an AC voltage ratio or form part of an isolated power supply.
- An isolation barrier has finite working-voltage and transient ratings. Do not assume that a transformer makes exposed terminals safe or prevents damage from every lightning event.
A short history
Transformer development helped make AC electricity distribution practical. Gaulard and Gibbs demonstrated a commercially successful transformer in Paris in 1881. William Stanley later built on their work for the 1886 electrification of businesses in Great Barrington, Massachusetts. Raising and lowering AC voltage became a central tool in power distribution.
Good to know
- A transformer changes voltage through its turns ratio. The same basic principle supports both stepping voltage up and stepping it down.
- Great Barrington's 1886 AC demonstration supplied offices and stores, showing how transformer technology could serve a town.