What a Transformer Actually Does

6 min · difficulty 2/10

A transformer has two coils of wire wound around a shared magnetic core. The side connected to the source is the primary. The side connected to the load is the secondary. There is no direct wire between them. Power crosses the gap through the magnetic field in the core.

The ratio of turns sets everything. If the primary has twenty times as many turns as the secondary, the secondary voltage is about one twentieth of the primary voltage. Voltage scales with the turns ratio.

Current goes the other way. Because the transformer moves power rather than making or destroying it, stepping the voltage down raises the current the secondary can supply. That is the whole trick, and it is why the same device can sit at the top of the system and at the curb in front of a house.

Turns ratio: voltage down, current up, power across A transformer diagram with a primary coil of 1000 turns on the left and a secondary coil of 50 turns on the right, both wound on a shared rectangular magnetic core in the center. No wire directly connects the two coils. On the primary side, V primary is high and I primary is low. On the secondary side, V secondary equals V primary divided by 20 while I secondary is approximately I primary times 20. The turns ratio is 20 to 1. A banner reads Power in approximately equals Power out. A callout reads No direct wire connection, power crosses through the magnetic field. Turns ratio 20:1 Shared magnetic core magnetic field 1000 turns 50 turns Primary (source side) V_primary Voltage is HIGH steps down on secondary I_primary Current is LOW steps up on secondary Secondary (load side) V_secondary = V_primary ÷ 20 Voltage is LOW stepped down 20× I_secondary ≈ I_primary × 20 Current is HIGH stepped up 20× Voltage and current move in opposite directions No direct wire connection Power crosses through the magnetic field Power in ≈ Power out
The turns ratio sets everything. With 20 times more turns on the primary, the secondary voltage is about one twentieth as large, and the available current is about twenty times larger. Power crosses the core roughly unchanged, so voltage and current always move in opposite directions.

Nameplates and sizing decisions are the easy part; supplier capacity and lead time are what actually delay a transformer order. DistroForge report editions track transformer lead times, supplier capacity, and named-supplier scorecards for exactly this kind of procurement decision.

Question 1 of 2

A transformer steps voltage down from primary to secondary. What happens to the available current on the secondary side?

Educational material only. This is not engineering, safety, or procurement advice. Confirm any value against manufacturer documentation and a licensed professional before specifying equipment.