20.7 - How Transformers Work

What a transformer is made of

• A transformer is a device used to change the size of an alternating voltage.
• Every transformer has three main parts:
  1. A primary coil, which the input voltage is connected across.
  2. A secondary coil, which provides the output voltage.
  3. An iron core, which both coils are wound around.
• The two coils are not joined electrically - there's no wire connecting them.
• The iron core links the two coils magnetically.

How a transformer changes the voltage

A transformer works by electromagnetic induction, and only works with alternating current.

Inducing a voltage in the secondary coil

1. An alternating voltage across the primary coil drives an alternating current through it.
2. This sets up a changing magnetic field in the iron core.
3. The secondary coil is on the same core, so this changing magnetic field passes through it.
4. A changing magnetic field induces an alternating voltage across the secondary coil (and an alternating current if it's part of a complete circuit).

Why iron and alternating current are used

• Iron is used because it's a magnetic material that's easily magnetised and demagnetised, so the magnetic field closely follows the alternating current.
• The input must be alternating current. A steady direct current gives a constant, unchanging magnetic field, which induces no voltage in the secondary coil.

Step-up and step-down transformers

• The number of turns on each coil decides whether the voltage increases or decreases:
  • Step-up transformer - Increases the voltage. It has more turns on the secondary coil than the primary, so output voltage is greater than input voltage.
  • Step-down transformer - Decreases the voltage. It has more turns on the primary coil than the secondary, so output voltage is smaller than input voltage.
• The coil with more turns has the larger voltage across it.