A transformer is based on a very simple fact about electricity: electromagnetic induction.
When an electric current flows through a wire, it generates a magnetic field or magnetic flux all around it. A magnetic ﬁeld may be considered as being the medium by which forces are transmitted between magnetized materials. In everyday life, magnetic fields are most often encountered as an invisible force created by permanent magnets which pull on ferromagnetic materials such as iron, cobalt or nickel and attract or repel other magnets. The strength of this ﬁeld is directly proportional to the value of the current. Thus a magnetic ﬁeld produced in this way may be turned on and off, reversed, and varied in strength very simply. The magnetic field can be visualized as lines of magnetic flux that form closed paths. The figure below represents a magnetic field (flux lines) created around a wire that carries current.
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. We can generate a fluctuating magnetic field by allowing a current in a wire which is also fluctuating.
Next we try to combine the above two phenomenon. 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. This is called electromagnetic induction because the alternating current in the first coil causes (or “induces”) a current in the second coil. All transformers work according to this principle. If the first wire carries sine-wave ac of a certain frequency, then the induced current will be sine-wave ac of the same frequency in the second wire.
The closer the two wires are to each other, the greater the induced current will be, for a given current in the first wire.
If the wires are wound into coils and placed along a common axis ( as in the picture above ), the induced current will be greater than if the wires are straight and parallel.
The first coil, which takes electrical power from the source, is called the primary winding, and the second coil, which gives the desired output voltage, is known as the secondary winding.
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).
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 a steady current constantly flows in the same direction.
Secondary voltage ÷ Primary voltage = Number of turns in secondary ÷ Number of turns in primary
Secondary current ÷ Primary current = Number of turns in primary ÷ Number of turns in secondary
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.)
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. The current is transformed the opposite way—increased in size—in a step-down transformer.
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. 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.