Electricity transformers

Last updated: February 8, 2009.

The mighty power lines that criss-cross our countryside or wiggle unseen beneath city streets carry electricity at enormously high voltages from power plants to our homes. It's not unusual for a power line to be rated at 400,000 to 750,000 volts! But the appliances in our homes use voltages thousands of times smaller—typically just 110 to 250 volts. If you tried to power a toaster or a TV set from an electricity pylon, it would instantly explode! (Don't even think about trying, because the electricity in overhead lines will kill you instantly.) So there has to some way of reducing the high voltage electricity from power plants to the lower voltage electricity used by factories, offices, and homes. The piece of equipment that does this, humming with electromagnetic energy as it goes, is called a transformer. Let's take a closer look at how it works.

Photo: A typical electricity transformer supplying homes in a small English village.

Why do we use high voltages?

Your first question is probably this: if our homes and offices are using photocopiers, computers, washing machines, and electric shavers rated at 110-250 volts, why don't power stations simply transmit electricity at that voltage? Why do they use such high voltages? To explain that, we need to know a little about how electricity travels.

As electricity flows down a metal wire, the electrons that carry its energy jiggle through the metal structure, bashing and crashing about and generally wasting energy like unruly schoolchildren running down a corridor. That's why wires get hot when electricity flows through them (something that's very useful in electric toasters). It turns out that the higher the voltage electricity you use, and the lower the current, the less energy is wasted in this way. So the electricity that comes from power plants is sent down the wires at extremely high voltages to save energy.

But there's another reason too. Industrial plants have huge factory machines that are much bigger and more energy-hungry than anything you have at home. Instead of running on 110-250 volts, they might use 10,000 to 30,000 volts. Smaller factories and machine shops may need supplies of 415 volts or so. In other words, different electricity users need different voltages. It makes sense to ship high-voltage electricity from the power station and then transform it to lower voltages when it reaches its various destinations. Even so, about two thirds of the energy that leaves a power plant is still wasted on the journey!

How does a transformer work?

A transformer is based on a very simple fact about electricity: when a fluctuating electric current flows through a wire, it generates a magnetic field (an invisible pattern of magnetism) or "magnetic flux" all around it. The strength of the magnetism (which has the rather technical name of magnetic flux density) is directly related to the size of the electric current. So the bigger the current, the stronger the magnetic field. Now there's another interesting fact about electricity too. When a magnetic field fluctuates around a piece of wire, it generates an electric current in the wire. So if we put a second coil of wire next to the first one, and send a fluctuating electric current into the first coil, we will create an electric current in the second wire. The current in the first coil is usually called the primary current and the current in the second wire is (surprise, surprise) the secondary current. What we've done here is pass an electric current through empty space from one coil of wire to another. This is called electromagnetic induction because the current in the first coil causes (or "induces") a current in the second coil. We can make electrical energy pass more efficiently from one coil to the other by wrapping them around a soft iron bar (sometimes called a core):

To make a coil of wire, we simply curl the wire round into loops or ("turns" as physicists like to call them). If the second coil has the same number of turns as the first coil, the electric current in the second coil will be virtually the same size as the one in the first coil. But (and here's the clever part) if we have more or fewer turns in the second coil, we can make the secondary current and voltage bigger or smaller than the primary current and voltage.

One important thing to note is that this trick works only if the electric current is fluctuating in some way. In other words, you have to use a type of constantly reversing electricity called alternating current (AC) with a transformer. Transformers do not work with direct current (DC), where the current constantly flows in the same direction.

Step-down transformers

If the first coil has more turns that the second coil, the secondary voltage is smaller than the primary voltage:

This is called a step-down transformer. If the second coil has half as many turns as the first coil, the secondary voltage will be half the size of the primary voltage; if the second coil has one tenth as many turns, it has one tenth the voltage. In general:

Secondary voltage ÷ Primary voltage = Number of turns in secondary ÷ Number of turns in primary

The current is transformed the opposite way—increased in size—in a step-down transformer:

Secondary current ÷ Primary current = Number of turns in primary ÷ Number of turns in secondary

So a step-down transformer with 100 coils in the primary and 10 coils in the secondary will reduce the voltage by a factor of 10 but multiply the current by a factor of 10 at the same time. The power in an electric current is equal to the current times the voltage (watts = volts x amps is one way to remember this), so you can see the power in the secondary coil is theoretically the same as the power in the primary coil. (In reality, there is some loss of power between the primary and the secondary because some of the "magnetic flux" leaks out of the core, some energy is lost because the core heats up, and so on.)

Step-up transformers

Reversing the situation, we can make a step-up transformer that boosts a low voltage into a high one:

This time, we have more turns on the secondary coil than the primary. It's still true that:

Secondary voltage ÷ Primary voltage = Number of turns in secondary ÷ Number of turns in primary

and

Secondary current ÷ Primary current = Number of turns in primary ÷ Number of turns in secondary

In a step-up transformer, we use more turns in the secondary than in the primary to get a bigger secondary voltage and a smaller secondary current.

Transformers in the home

As we've already seen, there are lots of huge transformers in towns and cities where the high-voltage electricity from incoming power lines is converted into lower-voltages. But there are lots of transformers in your home also. Big electric appliances such as washing machines and dishwashers use relatively high voltages of 110-240 volts, but electronic devices such as laptop computers and chargers for MP3 players and mobile cellphones use relatively tiny voltages: a laptop needs about 15 volts, an iPod charger needs 12 volts, and a cellphone typically needs less than 6 volts when you charge up its battery. So electronic appliances like these have small transformers built into them (often mounted at the end of the power lead) to convert the 110-240 volt domestic supply into a smaller voltage they can use. If you've ever wondered why things like cellphones have those big fat chunky plugs, it's because they contain transformers!

Photo: Typical home transformers. Anticlockwise from top left: A modem transformer, the white transformer in an iPod charger, and a cellphone charger.

Induction chargers

Many home transformers (like the ones used by iPods and cellphones) are designed to charge up rechargeable batteries. You can see exactly how they work: electricity flows into the transformer from the electricity outlet on your wall, gets transformed down to a lower voltage, and flows into the battery in your iPod or phone. But what happens with something like an electric toothbrush, which has no power lead? It charges up with a slightly different type of transformer. You can read more in our article about induction chargers.

Photos: An electric toothbrush standing on its induction charger. The battery in the brush charges up by induction: there is no direct electrical contact between the plastic brush and the plastic charger unit in the base.