Two coils, one iron core, no moving parts — the turns ratio, why only AC works, and the transformer types you will actually meet.
A transformer changes voltage with no moving parts and ~98% efficiency: alternating current in the primary coil creates a magnetic field, the iron core carries that field to the secondary coil, and the secondary produces a different voltage. The turns ratio is everything — twice the turns, twice the voltage, half the current. Every building you have ever been in has at least one.
Working or dead — we buy both. Value guide →
100 turns on the primary, 50 on the secondary: 480V in becomes 240V out, and the current doubles (minus small losses). Step-down transformers (the common kind) reduce voltage for use; step-up transformers raise it for transmission. Power in ≈ power out — voltage and current trade against each other.
The laminated silicon-steel core concentrates the magnetic field so nearly all of it links both coils — that is the 98% efficiency. The laminations (thin insulated sheets) block eddy currents that would waste energy as heat. A humming transformer is magnetostriction: the core physically vibrating at line frequency.
Transformers need a changing magnetic field — DC is steady, so nothing transfers. This is the deep reason the grid is AC: voltage can be transformed up for efficient long-distance transmission and back down for safe use. Edison’s DC lost to Tesla’s AC on this fact alone.
Air-cooled, in commercial buildings everywhere: the 480V-to-208Y/120V workhorse in electrical rooms. VPI (vacuum pressure impregnated) or cast-resin. Simple to handle, simple to sell used.
Padmount and substation units for utilities and heavy industry. Oil cools and insulates; DGA oil testing reveals internal health. Bigger tickets, heavier logistics — and the pre-1979 PCB question (see our value guide).
Isolation transformers (1:1 ratio — no voltage change, just electrical separation for sensitive equipment). Autotransformers (single shared winding — cheaper, smaller, no isolation). Buck-boost (small corrections to line voltage). Each has its own used market.
Follow a kilowatt from power plant to phone charger and it passes through four or five transformers: stepped up to hundreds of kilovolts for transmission (high voltage = low current = minimal line losses), stepped down at substations, stepped down again at the pole or padmount, and finally inside your devices. Each transformation loses only ~1–2%. Without transformers, every generator would have to sit next to its load — no grid, no cities as we know them. The transformer is arguably the most important electrical device ever invented, and it has no moving parts, makes no noise beyond a hum, and lasts 30–40 years. When you sell a used transformer, you are selling a piece of that chain — which is why working units hold value so well (see our transformer value guide).
AC in the primary coil creates a changing magnetic field; the iron core carries it to the secondary coil, which produces a different voltage. The turns ratio sets the voltage change — power in roughly equals power out.
Magnetostriction — the iron core physically vibrates at line frequency (60 Hz). Louder than usual humming can indicate loose laminations or overload.
Step-down reduces voltage (the common building kind: 480V to 208V); step-up raises it (generation and transmission). Same device, different turns ratio.
No — large transformers run ~98–99% efficient. The small losses become heat, which is why big units need oil or forced-air cooling.
Because transformers only work on AC, and transforming voltage up for transmission (then down for use) is what makes long-distance power practical.
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