What Is a Toroidal Transformer?

A toroidal transformer is a transformer whose magnetic core is shaped like a doughnut (a torus). The primary and secondary windings are wound through the centre hole and around the ring, covering the whole core. You will find them in audio amplifiers, studio equipment, medical devices, UPS systems, lighting and industrial controls: anywhere that needs a compact, quiet and efficient mains transformer.

How it is built

The core is made from a long, thin strip of cold-rolled grain-oriented (CRGO) silicon steel, wound tightly on itself like a roll of tape. It is then annealed (heat-treated) to relieve the stress from winding and restore the steel's magnetic properties. Because the strip is continuous, the core has no joints and no air gap. The grain of the steel also runs in the same direction as the magnetic flux all the way round the ring.

The core is insulated, and then a toroidal winding machine threads copper wire through the centre hole turn by turn. The primary goes on first, then a layer of insulation, then one or more secondaries. The finished part is usually wrapped in tape and supplied with flying leads and a mounting kit.

How it works

A toroid works exactly like any other transformer. An alternating current in the primary sets up an alternating magnetic flux in the core, and that changing flux induces a voltage in every other winding on the same core. The voltage ratio equals the turns ratio:

The real difference lies in the shape of the magnetic path, not in the physics.

Why the ring shape matters

Smaller and lighter

Grain-oriented steel carries the most flux when the flux runs along the grain. In a toroid the flux follows the grain for the entire loop, so the core can run at a higher flux density with lower losses. That means less steel and less copper for the same power. A toroid typically weighs about half as much as an EI transformer of the same rating and takes up less volume.

Low stray magnetic field

The winding covers the whole core evenly, so almost all of the magnetic field stays inside the ring. The leakage (stray) field is often quoted at around one-tenth of that of an equivalent EI transformer. This is why audio designers like toroids: less field reaches nearby circuits, so there is less hum induced into sensitive stages.

Quiet

There are no loose laminations and no E–I joints to vibrate at 100 Hz, so a correctly used toroid is mechanically very quiet.

Efficient

With no air gap, the magnetising current and no-load losses are low. Efficiency is typically above 90% for small units and 95% or more for larger ones. Low no-load loss matters for equipment that is left switched on all day.

The trade-offs

• High inrush current. The same gap-free core that makes toroids efficient means a very large current surge can flow for the first few cycles after switch-on. It is manageable, and our inrush guide explains how.
• Sensitivity to DC on the mains. A small DC offset on the supply can make a toroid buzz mechanically. See troubleshooting hum.
• Winding cost. Every turn has to pass through the centre hole, which makes winding slower than on an EI bobbin. The savings in steel, copper and weight usually offset this, especially above about 100 VA.

Where toroids are used

• Audio: power amplifiers, preamps, DJ and PA equipment, and studio monitors, where low hum and a compact size matter.
• Power electronics: inverters, UPS systems, stabilisers and battery chargers.
• Medical and laboratory equipment: where low stray field and low noise are required.
• Lighting and control panels: low-voltage supplies where space in the enclosure is tight.