What is a transformer?
A transformer is a device that changes one AC voltage into another. It has no moving parts: just two coils of wire, called windings, wound around a common iron core. Alternating current fed into the primary winding creates a magnetic flux in the core, and that flux induces a voltage in the secondary winding. The two windings are not connected by any wire — the energy is transferred entirely through the changing magnetic field in the core.
Why look inside?
From the outside, a transformer is often just a metal can with a few wires. Everything important happens out of sight. Opening it up shows the laminated core that carries the flux, the two windings that sit on its limbs, the insulation that keeps them apart, and how the voltage ratio is really just a ratio of the number of turns.
Reading the diagram
The grey frame is the iron core, drawn as stacked laminations. The coil on the left is the primary, fed by AC; the coil on the right is the secondary, which delivers the output. The dashed blue lines trace the magnetic flux as it circulates around the core, and the dashed arrows on the leads show the current arriving at the primary and leaving from the secondary. The faint outer box is the tank, cut away to reveal the parts inside.
1. AC enters the primary winding
An alternating supply is connected across the primary winding. The current rises, falls and reverses again and again — many times a second — so at every instant the current has a size and a direction that keep changing.
2. The current builds a magnetic flux in the core
Current in a coil creates a magnetic field, and the iron core guides that field into a strong magnetic flux that flows in a closed loop around the core — up one limb, across the top, down the other limb and back around the bottom.
3. The flux changes and reverses
Because the primary current keeps reversing, the flux it creates keeps changing too: it grows, shrinks, reverses, and grows again. It is this continuous change, not the flux itself, that matters.
4. The secondary winding picks up an induced voltage
The changing flux passes right through the secondary winding. A changing magnetic field induces a voltage in any coil it threads, so a voltage appears across the secondary — even though no wire connects it to the primary.
5. The turns ratio sets the voltage
The size of the induced voltage depends on how many turns the secondary has compared with the primary. More turns on the secondary than the primary step the voltage up (a step-up transformer); fewer turns step it down. Power is roughly conserved, so when the voltage is stepped down, the available current steps up.
How does a transformer change voltage with no moving parts?
It works entirely by induction. Alternating current in the primary winding sets up a magnetic flux in the shared iron core, and because the current keeps reversing, the flux keeps changing. That changing flux threads the secondary winding and induces a voltage in it.
Do the two coils touch each other?
No. The primary and secondary windings are separate circuits — there is no wire joining them. Energy crosses the gap only through the changing magnetic flux that flows around the iron core between them.
Why must a transformer use alternating current?
A steady current makes a steady magnetic flux, and a flux that never changes induces no voltage at all. Only a changing flux does, so a transformer needs the constantly reversing current of an AC supply to work.
Does a transformer create extra energy?
No. It trades voltage for current. Roughly speaking the power out equals the power in, so when the voltage is stepped down the available current steps up, and when the voltage is stepped up the current falls. Real transformers have small losses as heat.
Why is the core laminated?
The core is built from thin insulated sheets of silicon steel. Laminating it breaks up the paths that circulating eddy currents would take, which cuts the wasted heat and keeps the transformer efficient.