Toroidal Transformer Glossary | Toroidal Transformer Guide

VA (volt-ampere): The power rating of a transformer: rated voltage × rated current. For a transformer with several secondaries, the VA is the sum of all of them.
Primary: The winding connected to the supply, usually 230 V AC mains in India.
Secondary: The output winding(s). A transformer can have several, each with its own voltage and current rating.
Centre tap: A connection at the midpoint of a secondary, written as 24-0-24 or 30-0-30. It is used to make split (±) supplies for amplifiers.
Turns ratio: Primary turns divided by secondary turns. The voltage ratio follows the turns ratio.
Regulation: How much the output voltage drops from no load to full load, as a percentage. Smaller transformers have higher (worse) regulation.
Inrush current: The large current surge that can flow for a few cycles when a transformer is switched on, caused by core saturation. More.
Remanence: The magnetism a core keeps after the supply is removed. High remanence contributes to inrush current.
Saturation: The point at which the core cannot carry more magnetic flux. Beyond it, the primary current rises very sharply.
Flux density: Magnetic flux per unit area of core, measured in tesla (T). The designer chooses it as a balance between size, losses and inrush.
CRGO steel: Cold-rolled grain-oriented silicon steel. It carries flux best along its grain, which suits the continuous strip of a toroidal core.
Stray (leakage) field: Magnetic field that escapes the core into the surroundings, where it can induce hum in nearby circuits. It is very low in toroids.
Electrostatic shield: A thin foil layer between primary and secondary, connected to earth, that reduces capacitive coupling of mains noise.
Shorted turn: Any closed conductive loop through the centre of a toroid, for example a mounting bolt touching the chassis at both ends. It causes heavy current and heating. More.
Soft-start: A circuit that limits inrush current at switch-on, typically an NTC thermistor or a resistor bypassed by a relay.
Slow-blow (T) fuse: A time-delay fuse that tolerates a short surge but still opens on a sustained overload. It is required on toroid primaries.
Insulation class: The maximum temperature the insulation system is rated for, e.g. Class B (130 °C) or Class F (155 °C).
Hi-pot test: A high-voltage test between windings, and between windings and core, to verify insulation strength.
Potted centre: A toroid whose centre hole is filled with resin so it can be mounted with a single screw, useful for vertical or high-vibration mounting.
DC offset (on mains): A small DC component on the AC supply, caused by half-wave loads. It can make toroidal transformers buzz audibly.