By Doug Leenhouts
Updated Mar 24, 2022
A transformer consists of two windings wrapped around an iron core – the primary and the secondary. When current flows through the primary, it generates a magnetic field that induces voltage in the secondary. By selecting the appropriate ratio of turns on each winding, a transformer can step voltage up (and reduce current) for long‑distance transmission, or step voltage down (and increase current) for local use.
Determine the number of turns on both the primary and secondary windings. A step‑down transformer has fewer turns on the secondary, while a step‑up transformer has more turns on the secondary relative to the primary.
In the U.S., residential outlets supply 110 V (or 120 V in many regions). If the input voltage is unknown, measure it with a multimeter: place the positive probe on the live wire feeding the transformer and the negative probe on the device’s ground.
The relationship between voltages and turns is expressed as:
Vs / Vp = Ns / Np
Rearrange to find the secondary voltage:
Vs = Vp × (Ns / Np)
For example, a 240 V source feeding a transformer with 500 primary turns and 100 secondary turns produces:
Vs = 240 V × (100 / 500) = 48 V
