What is a transformer?

A transformer uses mutual induction to change values of AC voltages and currents. They are used in many electrical pieces of equipment as well as in the electrical distribution network.

Low power step-down transformers

11KV Distribution transformer

Transformer symbol

Step-down transformer

In a step-down transformer the secondary windings have less turns than the primary. This means the secondary voltage will be less than primary voltage.

Step-up transformer

In a step-up transformer the secondary windings have more turns than the primary. This means the secondary voltage will be more than the primary.


Transformer cores

The cores of electrical transformers are usually made of laminated iron. There are different types of core including: core type, shell type and toroidal. A core transformer consists of windings on either side of a rectangular core. A shell type transformer has all of the windings mounted on a central limb. In a toroidal transformer the windings overlap each other on a doughnut shaped core material. A toroidal transformer is the most efficient and has the least magnetic leakage which can interfere with sensitive electronic circuits.

Shell type

Core type


Transformers can be designed to have specific input and output voltages. This is a achieved by selecting the correct ratio of windings on the core. The formula below shows the relationship between the number of windings, voltages and currents. The light blue boxes show how the formula can be re-arranged to calculate specific properties.

Calculating the secondary current

Calculating the secondary voltage

Calculating the primary voltage

Calculating the primary current

Types of transformer

Isolating transformer symbol

Isolation transformer

An isolation transformer is often used in environments where using a supply of 230V may be unsafe such as on construction sites. The transformer steps the voltage down to 110V which decreases the risk of a fatal shock. In addition the secondary winding of the transformer can have a central “tap” which is earthed, this means that the secondary voltage on either terminal can only be at 55V with respect to earth. Although a person can still receive a shock it is highly unlikely that sufficient current will flow to cause injury.

Transformer construction
Transformer Calculations