Transformer Types

A transformer is a passive device that transfers electrical energy between two or more circuits through electromagnetic induction. An alternating current in one winding creates a changing magnetic field, usually guided by an iron or ferrite core, which induces a voltage in a second winding. The ratio of turns between the windings sets the ratio of voltages, so transformers can step voltage up or down while providing electrical isolation between circuits. They work only with alternating or changing currents, not steady direct current.

Transformers are built in many forms suited to different jobs. Power transformers handle the bulk transfer and conversion of mains electricity, ranging from large units in the grid down to the small mains transformers once common in equipment power supplies. Audio transformers are optimised to pass a wide band of audio frequencies with low distortion and are used for impedance matching, signal coupling and isolation in amplifiers and microphones. Isolation transformers have a one-to-one ratio and exist purely to separate circuits for safety. An autotransformer uses a single tapped winding for both input and output, which is compact but offers no isolation.

Other variations include instrument transformers, which scale large currents or voltages down to safe levels for metering, and the small high-frequency transformers used inside switch-mode power supplies, which can be far smaller than mains-frequency types because higher frequencies need less core material. Core construction also varies, from laminated steel for low frequencies to ferrite and powdered-iron cores for higher frequencies, each chosen to limit losses in its operating range.

Frequently asked questions

Why do transformers only work with AC?

Induction requires a changing magnetic field. A steady direct current produces a constant field and induces no voltage in the secondary, so transformers need alternating or otherwise varying current.

What is an autotransformer?

It is a transformer with a single winding tapped at one point, sharing turns between input and output. It is smaller and cheaper than a two-winding type but provides no electrical isolation.

What makes an audio transformer different from a power transformer?

Audio transformers are designed to pass a broad range of audio frequencies with minimal distortion, often for impedance matching, while power transformers handle larger power at a single fixed frequency.