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There are two types of transformer:

• a step-up transformer which increases or steps-up voltage.
• a step-down transformer which reduces or steps-down voltage.

A basic transformer is made from two coils of wire:

• a primary coil connected to an alternating current (a.c) supply;
• a secondary coil leading to the ac output.

The coils are wound around an iron core.

They are magnetically connected but not electrically connected.

When a transformer is working:

1. A primary voltage drives an alternating current through the primary coil.
2. The primary coil current produces a changing magnetic field. This magnetic field changes as the current changes.
3. The iron core is easily magnetised and carries the changing magnetic field from the primary coil to link the secondary coil.
4. Alternating voltage is induced in the secondary coil.
5. The induced voltage causes an alternating current to flow.

Key points

• Transformers can only work with alternating current.
• Input and output voltages and currents are both alternating (a.c.)
• Mains voltage is a.c. and not d.c. so that transformers can be used.

The size of the output voltage depends on the number of turns on the secondary compared to the primary coil.

• a step-up transformer, which steps-up voltage, has more turns in the secondary than in the primary;
• a step-down transformer, which steps-down voltage, has fewer turns in the secondary than in the primary

The transformer in the diagram above is a step-down transformer – there are fewer turns on the secondary coil than on the primary coil.