A Twenty-to-One Transformer Turns Five Amps Into a Quarter of One

A transformer does nothing at all on direct current

The whole device works by a changing magnetic field inducing a voltage in the second winding, and a steady current produces a steady field, which induces nothing. Connect a mains transformer to a battery of the same voltage and the primary behaves as what it physically is — a long coil of thin wire with very little resistance — and draws whatever current the battery can supply until something gives. That is the practical reason a transformer belongs to alternating current and why direct-current systems need switching converters instead, which manufacture a changing field electronically before they can use the same trick.

The ideal balance is never quite reached

Sixty watts in and sixty watts out assumes a perfect transformer, and a real one loses a few per cent to resistance in the windings and to heating of the iron core. The losses in the core are there even with nothing connected, which is why an unloaded transformer left plugged in is warm and consuming power for no purpose. Efficiency also depends on load: a transformer running near its rating is usually at its best, while one delivering a tenth of what it was designed for wastes a much larger fraction of what it draws. Any figure calculated from the turns ratio alone is therefore a ceiling on what the secondary can deliver rather than a promise.

Frequently asked questions

Can the same transformer be used backwards?

Electrically the physics is symmetric — feed 12 volts into the 12-volt winding and 240 appears on the other — but the construction is not. The low-voltage winding is wound with thick wire and little insulation because it was never meant to hold 240 volts, and the high-voltage side is thin wire that cannot carry the current a reversed connection would demand. Devices built for this, such as isolation transformers with equal windings, say so; anything else run backwards is being used outside what its insulation was designed for, and the failure mode is not a gentle one.

Why is high voltage used for transmission?

Because the loss in a cable depends on the current squared, and the transformer lets you choose the current. Sending a given power at twenty times the voltage means a twentieth of the current, and a twentieth of the current wastes a four-hundredth of the heat in the same conductor. That single ratio is why long-distance transmission runs at hundreds of thousands of volts and steps down repeatedly on the way to a house, and it is the reason the transformer is arguably the most consequential piece of nineteenth-century electrical engineering.