How Does a Toroidal Transformer Work?


A transformer changes one alternating current (AC) voltage from one level to another without using any moving parts. Probably the simplest of all electrical devices, the transformer can be found in tiny battery chargers or massive power generating stations. The toroidal transformer, shaped somewhat like a donut, has specific advantages over other shaped transformers.

Alternating Current

  • Alternating current electricity changes directions in what is called a sine wave pattern. Standard AC voltage starts at zero volts and rises to its positive peak going in one direction along the wire, then falls to zero volts in the first half of a cycle. It then rises to its peak in the opposite direction and falls back to zero again. It does this 60 times a second creating what known as 60 cycles or 60 Hertz (Hz).

The Primary Coil

  • The input of a toroidal transformer connects to the primary, a coil of wire wrapped around a donut shaped ferromagnetic core. As the electricity passes through the coil, it builds a positive magnetic field increasing to a peak. The magnetic field collapses as the voltage decreases to zero, in the first half of the cycle. The current then builds a negative magnetic field as the electricity passes through the coil in the opposite direction. The negative magnetic field collapses as the voltage returns again to zero.

The Secondary Coil

  • As the positive and negative magnetic fields build and collapse, they pass through a second coil of wire wrapped around the same core, called the secondary. As these magnetic fields pass through the secondary winding, they produce the output voltage. The amount of voltage they produce depends on the number of coils of wire in the secondary compared to the primary. A coil ratio of 2:1 will halve the voltage, and a ratio of 1:2 will double it.

The Core

  • The core or a toroidal transformer uses a tightly wrapped strip of steel somewhat like a clock spring. This specially made coil improves the building and contraction of the magnetic fields and helps them induce the voltage into the secondary, making the transformer more efficient because more voltage actually gets induced from the primary to the secondary. A solid ferromagnetic core in the shape of an “E” or square, and in a traditional transformer, would have far less efficiency.

Toroidal Advantages

  • Because of its higher efficiency, for equivalent applications, the toroidal transformer can be up to 50 percent smaller, lighter. It also operates more quietly, producing less humming noise. As a result of its round shape, the magnetic fields its creates remains inside the transformer, causing less interference with adjacent circuits. This makes it the ideal choice for transformers in highly concentrated environments where interference can be critical. Also it mounts more easily to printed circuit boards.


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