Inside the transformer
Two coils sharing one magnetic core. A changing current in one makes a changing flux, and the flux makes a voltage in the other: no wire between them. It is how the grid carries power across countries and how a charger turns 230 V into 5 V safely. Every picture below is computed while you watch.
What it is
A transformer is two (or more) inductors wound on the same core. The alternating current in the primary drives an alternating flux through the core (Faraday’s law), and that same flux passes through every turn of the secondary, so each turn of either winding sees the same voltage:
V2 / V1 = N2 / N1 I2 / I1 ≈ N1 / N2 V = 4.44 · f · N · B · A
Step the voltage down and the current steps up by the same ratio: the power in equals the power out, less a few percent of loss. The last equation is the designer’s: for a given voltage and frequency, the turns and the core area set the flux density B, which must stay below the core’s saturation.
How it’s made
How it works
An AC source on the primary, a load on the secondary. The arrows are the flux, reversing with the supply. Change the turns, the frequency and the load and watch the voltages, the currents, the flux density and where the power goes. Push the flux past saturation (too few primary turns, or too low a frequency) and the magnetising current takes over.
A small mains-type core: area 6 cm², silicon steel (saturates near 1.6 T). The winding resistance grows with the turns. The waveforms always show two cycles.
Losses, laminations and frequency
The changing flux induces voltages in the core itself, and since steel conducts, currents circulate inside it: eddy currents, pure heat. They grow with the square of the frequency and of the thickness of the conducting path, so mains cores are stacks of thin insulated sheets. Each cycle also costs a little energy to swing the core’s magnetisation back and forth: hysteresis loss. At high frequency, steel is hopeless and ferrite, a ceramic that hardly conducts, takes over. In exchange, a higher frequency needs far less core for the same power: that is the whole trick of switch-mode supplies.
Typical figures per kilogram of core, at the flux a designer would use (about 1.2 T at mains frequency, 0.15 T at high frequency). On the right: the core needed for the same power, against a 50 Hz one.
What goes wrong
- DC or too low a frequency: the flux runs away into saturation and the primary current is limited only by its copper. Never put DC on a mains transformer.
- Overload: copper loss grows as the current squared; the windings’ insulation (class B, 130 °C; class F, 155 °C) cooks. Many small transformers have a thermal fuse buried in the winding.
- Inrush: switched on at the wrong moment of the cycle, a big transformer’s core saturates for the first few cycles and draws many times its rated current. Slow-blow fuses exist for this.
- Insulation failure: in a charger this is a safety failure, because mains can reach the USB side. That is why the barrier is tested at several kilovolts and why cheap chargers that skimp on it are dangerous.
- Hum: the laminations flex with the flux (magnetostriction) at twice the mains frequency: the 100 Hz buzz. Loose clamping or varnish makes it louder.
Use it on the bench
- Basic transformer — a transformer stepping mains down: turns ratio and current (about it)
- Linear supply: transformer, bridge, capacitor, regulator — a whole linear power supply: transformer, bridge, capacitor, regulator (about it)
- Bridge rectifier and filter — the rectifier and filter after the transformer (about it)
The lab’s parts include a mains transformer, an isolation transformer and a flyback transformer; the inductor page covers the core and its saturation in more depth.
Check yourself
Exam-style questions (ECE, EE and NC-II reviewers ask these). Each answer links to the scene that shows it.
1. A transformer has 1 150 primary turns and 60 secondary turns on 230 V. The secondary gives about:
V2 = V1 · N2/N1 = 230 × 60/1 150 = 12 V. The volts per turn (0.2 V) is the same in both windings. See the scene ▸
2. That transformer’s secondary delivers 2 A. Ignoring losses, the primary draws about:
The power in equals the power out: I1 = I2 · N2/N1 = 2 × 60/1 150 ≈ 0.104 A. Step the voltage down and the current steps up. See the scene ▸
3. Why won’t a mains transformer work on DC?
Only a changing flux induces a voltage. On DC the flux keeps growing, the core saturates, and the primary becomes a few ohms of copper across the supply: it burns. Lower the frequency in the working scene and watch the flux. See the scene ▸
4. Why are mains transformer cores made of thin, insulated laminations?
The changing flux induces circulating currents in the core itself. Thin insulated sheets cut their paths: the loss falls with the square of the sheet thickness. Compare the solid and laminated cores in the losses scene. See the scene ▸
5. Why is a 65 W phone or laptop charger’s transformer so much smaller than a 65 W mains transformer?
From V = 4.44·f·N·B·A, the core area needed falls as the frequency rises. A switch-mode supply chops the mains at tens to hundreds of kHz and uses a small ferrite core. See the scene ▸
6. At 100 kHz, why use ferrite instead of silicon-steel laminations?
Eddy loss grows with f²; steel’s low resistivity makes it useless at 100 kHz, even laminated. Ferrite saturates at a lower flux (about 0.4 T) but barely conducts. Try both in the losses scene. See the scene ▸
7. With no load on the secondary, a mains transformer’s primary still draws a small current. That is:
The primary is an inductor on its own: it draws V/(2πf·Lm) to magnetise the core. It is mostly reactive, so little power, but it warms an idle transformer slightly. See the scene ▸
8. A 230 V 50 Hz transformer is used on 230 V 60 Hz. Its core runs:
B ∝ V/(f·N): the same voltage at a higher frequency means less flux. The reverse (a 60 Hz design on 50 Hz) runs 20 % closer to saturation and hotter. See the scene ▸
9. What is the main safety job of the transformer in a phone charger?
The windings are separated by insulation tested at several kilovolts. The energy crosses the gap as magnetic flux; the mains does not. See the scene ▸
10. A transformer’s efficiency is reduced by:
Copper loss grows with the load current squared; core loss is almost constant with load (it depends on the flux and frequency). A transformer is most efficient where the two are about equal. See the scene ▸
Inside the other parts
The inductor · the capacitor · the resistor · the diode and LED · the transistor · the battery · all the parts