By Chris Deziel Updated Mar 24, 2022
Transformers are compact yet powerful devices that rely on electromagnetic induction. A conductor in a changing magnetic field generates an electric current, while a changing current produces a magnetic field. Because this interaction requires alternating current, transformers excel in AC power systems.
Each transformer has two windings: a primary coil and a secondary coil. The ratio of turns in these coils determines the voltage transformation according to Faraday’s law:
\(\frac{N_s}{N_p}=\frac{V_s}{V_p}\)
In a step‑down transformer the secondary voltage is lower than the primary voltage, so the secondary coil must have fewer turns than the primary coil. By selecting the desired primary voltage and target secondary voltage, you can calculate the required turns for each coil.
For maximum efficiency, use a ferromagnetic core that magnetizes easily and transfers energy efficiently. A common, inexpensive core is a 2‑ to 3‑inch steel washer from a hardware store or salvage yard.
Wrap 28‑gauge magnetic wire around the core for the primary coil. Aim for at least 500 turns, keeping the wire snug and layered if necessary. Count and record each turn. After winding, leave both ends free for connection and secure the coil with masking tape.
The secondary coil should have fewer turns based on the voltage ratio you calculated. Wind it over the primary coil, keeping the ends free for measurement. Once both coils are wound, wrap the entire assembly in masking tape and finish with electrical tape to insulate the transformer.
To step down 120 V from a household outlet to 12 V, the voltage ratio is 12/120 = 1/10. If the primary has 500 turns, the secondary should have 50 turns.
Using mains voltage in a DIY transformer is for illustration only. The resulting current can overheat the wire and pose safety risks. Use this setup with low‑voltage, low‑current sources instead, and never leave it energized for extended periods.
Never connect a homemade transformer directly to mains outlets or circuits. Follow all electrical safety guidelines when testing or using your transformer.
