Transformers: Stepping Voltage Up and Down
See how a transformer steps a voltage up or down. Alternating current in the primary coil makes a changing magnetic field in the iron core, and that field induces a voltage in the secondary coil. The voltages follow the turns, so the secondary voltage is the primary voltage times the secondary turns divided by the primary turns, while the current changes the other way and the power stays almost the same. Watch the field in the core swing back and forth and the two voltages on one graph. A second view sends a fixed amount of power down a cable and compares the heat lost with and without stepping the voltage up, which is why power lines use high voltage. Set the voltage, the turns on each coil, the load, the efficiency and the mains frequency. For instance, 12 V across 100 turns into 500 turns gives 60 V.
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What you can adjust
- Primary voltage
- 1 – 240 V
- Primary turns
- 10 – 1000 turns
- Secondary turns
- 10 – 2000 turns
- Load resistance
- 1 – 1000 Ω
- Efficiency
- 80 – 100 %
- Mains frequency
- 50 – 60
- What to show
- 0 – 1
The alternating voltage (rms) put across the primary coil. It starts at a safe 12 V. Household supplies are 230 V or 240 V.
How many times the wire of the primary coil goes round the iron core. The coils are drawn with fewer loops than the real count, but more turns always show as more loops.
How many times the wire of the secondary coil goes round the core. More secondary turns than primary turns steps the voltage up, fewer steps it down.
The resistance connected to the secondary coil, such as a lamp or a heater. A smaller resistance draws more current. It is used by the single transformer view; the cable view sends a set amount of power instead.
The share of the power taken from the supply that reaches the secondary. The rest warms the core and the wires. A good transformer is above 95%, and 100% is the ideal case with nothing lost.
How many times a second the supply reverses direction: 50 Hz in most countries, 60 Hz in others. The picture is slowed down so you can see each swing.
Single transformer shows the voltages, currents and power at the secondary. Sending power down a cable sends 24 W along a cable of 1 Ω (a small power line; if the source voltage is very low the power is reduced so that no more than a quarter is lost) and compares the heat lost in the cable with and without the transformer.
About this simulation
What it shows
An alternating supply drives the primary coil of an iron-core transformer, and a second coil feeds a load. The field in the core swings back and forth, and a graph shows both voltages on one scale. A second view sends power down a cable and compares the heat lost with and without stepping up.
The key idea
A changing current in one coil makes a changing magnetic field, which induces a voltage in the other. The voltage follows the turns: secondary voltage = primary voltage × secondary turns ÷ primary turns. High voltage means small current, so less heat is lost in a cable.
Try this
Run the defaults: 12 V across 100 turns into 500 turns gives 60 V. Set Secondary turns to 50 for 6 V. Set What to show to the cable: 4 W is lost at 12 V but 0.16 W at 60 V.
A transformer only works with a changing current, so direct current from a battery makes no steady voltage in the secondary. The coils are drawn with fewer loops than the real turns, and the picture is slowed down 100 times at 50 Hz (120 times at 60 Hz) so that you can see each swing. Voltages are rms; the graph shows the peaks, which are 1.41 times larger. The cable view sends 24 W along a cable of 1 Ω, a small power line, and the transformer's own loss is left out of that comparison; if the source voltage is very low the power is reduced so the cable never loses more than a quarter of it. The load resistance is not used in the cable view.