3.20 - 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:
- A primary coil, which the input voltage is connected across.
- A secondary coil, which provides the output voltage.
- 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
- An alternating voltage across the primary coil drives an alternating current through it.
- This sets up a changing magnetic field in the iron core.
- The secondary coil is on the same core, so this changing magnetic field passes through it.
- 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.
TermDefinition
TransformerA device that changes the size of an alternating potential difference (voltage).
Primary coilThe coil in a transformer that the input voltage is connected across.
Secondary coilThe coil in a transformer that provides the output voltage.
Electromagnetic inductionThe production of a potential difference (voltage) across a conductor when it moves relative to a magnetic field or when the magnetic field around it changes. It is also called the generator effect.
Step-up transformerA transformer that increases the potential difference (voltage) and decreases the current. It has more turns on its secondary coil than on its primary coil.
Step-down transformerA transformer that decreases the potential difference (voltage) and increases the current. It has fewer turns on its secondary coil than on its primary coil.
Key points to remember
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