The Transformer

What is a transformer?

A transformer is a device that changes the form electricity it takes.

Electrical power is made up of current and voltage

P = IV

A transformer allows you to change the current and voltage values within that power equation.

Why do we need transformers?

Most of the electronic devices that we use need a transformer if they are to run or be chargedup by being plugged into the mains.

Microchip circuitry does not need a big voltage to operate, in fact a big voltage will simply cause the chip circuitry to burn out.

Such circuits run on voltages of between 5V and 12V. Therefore a transformer is necessary to 'step-down' mains voltage (230V) to this level.

Any electronic device you have that can run off batteries or via a mains connector will have a transformer incorporated into that connector.

Sometimes they 'hum' and they always get warm after being switched on for a while.

Transformer Construction

A transformer is a laminated, soft iron core with two insulated coils of wire wrapped around it.

Q1. Why must is be made of a soft magnetic material?

(Click here for the answer).

Q2. Laminated means 'made up in layers' - see the photo below. Why must the core be laminated?

(Click here to see the answer).

Click here for Faraday's early experiment

The windings of the coils are made of low resistance copper.

Q3. Why do the windings have to be of low resistance?

(Click here for the answer)

Naming of the coils and drawing a diagram

The first coil is called the primary coil. This is the coil that has the supply voltage applied across its ends.

The second coil is called the secondary coil. This is the coil that has the output voltage across its ends. This is the coil that is connected to your appliance.

The diagram should be simple - do not draw too many windings - the diagram is only representative of the real thing. Make sure you label it.

How a transformer works

An alternating voltage (VP) is applied across the primary coil. This causes a changing magnetic field to be formed around the primary coil.

The magnetic domains inside the soft iron core line up in response to the magnetic field from the coil.

The secondary coil experiences the changing magnetic field produced by the primary and the core.

It responds to this changing magnetic field by producing a voltage (VS) across its ends (an induced EMF) by electromagnetic induction.

(The key words you must mention when explaining this are in bold type)

Why must the source voltage be an alternating voltage?

Electromagnetic induction only happens when a wire loop experiences a changing magnetic field . The wire and field lines must move with respect to each other so that the wire can 'cut the lines of flux'.

How can we get a different voltage out of a transformer from the one we put in?

This is achieved by varying the ratio of the number of turns of wire on the primary and secondary coils. If there are more turns on the secondary then the output voltage will be bigger by the same ratio and vice versa.

A step-up transformer has more turns on the secondary than on the primary. It therefore produces a bigger output voltage than input voltage.

A step-down transformer has less turns on the secondary than on the primary. It therefore produces a smaller output voltage than input voltage.

Although transformers lose some of the energy input as heat (that is why the transformer gets warm), they are pretty efficient and about 99.9% of the input electrical energy is converted to output electrical energy. Therefore in examination questions at this level you can assume that the transformer is 100% efficient.

In reality they are not 100% efficient and when we are dealing with large energy transfers (GW or TW in the case of power stations) even a small percentage is a large energy loss.

Q4. How can the design of a transformer cut energy losses? (Click here for the answer)

This means that:

power in = power out

power of primary = power of secondary



As the voltage is stepped up, so the current is stepped down!

In the above photograph the school transformer was used to demonstrate how voltage can be 'stepped-up' by a transformer..

10.05 V (rounded by the meter to 10.1 V) across the primary coil of 2,000 turns resulted in an output of 100.5 V across the secondary coil of 20,000 turns.

The Symbol

The symbol consists of two curled lines, representing the coils.

These are unconnected, as there is no connection between the coils on a real transformer.

The line (sometimes two or three are drawn) in the middle represent the magnetic link between the coils that is made by the soft iron core.

The Equation

There is a mathematical link between the number of turns and the voltages on each coil.

This ratio equation must be known for examination purposes. It doesn't matter which way up you write it, or which way round. Therefore always start with the unknown quantity in the top left position.

Click here to see some example questions