A step-up transformer takes 120 V from a wall outlet and boosts it to 1200 V for a neon sign. The transformer has 100 turns on the primary coil and 1000 turns on the secondary coil. The neon sign draws 0.05 A of current.
How much current does the transformer draw from the wall outlet?
The primary draws 0.5 A! A transformer does NOT create energy. Power in = Power out: V₁I₁ = V₂I₂. When voltage is stepped up by 10×, current is stepped DOWN by 10×. The power is the same on both sides — 60 watts in, 60 watts out.
Loading simulation…
Watch AC current flow through the primary coil, creating a changing magnetic flux in the iron core. The flux links both coils, inducing a voltage in the secondary. Adjust the turns ratio and watch voltage and current change — but power stays the same!
V₂/V₁ = N₂/N₁ = I₁/I₂
Voltage and turns go together (both up or both down). Current goes the opposite way. Power is always conserved: V₁I₁ = V₂I₂.
Electricity travels at 400,000 V on high-voltage lines, then step-down transformers reduce it to 120/240 V for homes. Without transformers, we’d lose most of our electricity as heat in the wires.
Your phone charger contains a tiny transformer (or switching converter) that steps 120 V down to 5 V. The current goes up, but the voltage is safe for your device.
Tube amplifiers use transformers to step up voltage for the vacuum tubes and match impedance to the speakers. The warm tone of a tube amp starts with a transformer.
Arc welders use step-down transformers to produce very low voltage but extremely high current — enough to melt steel at the contact point.
“A transformer is nature’s currency exchange: it trades voltage for current, but the total power — the energy per second — stays the same. No free energy, just a brilliant conversion.”
Transformer Lab
Build your own transformer by adjusting the number of turns on the primary and secondary coils. Connect a load and see how voltage, current, and power flow through the system. Compare step-up and step-down configurations.
Loading simulation…
Start with equal turns (1:1 ratio) and note that V₂ = V₁. Now double N₂ — the voltage doubles but the current halves. Try an extreme ratio like 10:1000 and watch the current on the primary spike to keep power balanced. Set the efficiency slider to 95% and see where the lost power goes as heat.
Transformers are the reason we can transmit power over hundreds of miles. High voltage means low current, and low current means tiny I²R losses in the wires. Without transformers, the electrical grid would be impossible.