How to Determine Which Wire Is Which on a Transformer


Let's say I have a transformer with 3 coils. When primary coil is connected to 230V, two secondaries output 12V. Wires are color coded, but I don't know what colors stand for and there is no information about that on manufacturer's website.

Is there any relatively easy way to determine which wire is primary, which is secondary and which is start to the secondary and which is end of the secondary coil?

Best Answer

In a step-down transformer, the primary winding will have a higher resistance. The resistance will be low, but there should be a noticable difference unless you've got a really big transformer.

If you're trying to identify what a winding is, then here are some notes:

  • If 3 wires have continuity, then you probably have a center-tap winding (+, 0, -)
  • If only 2 wires have continuity, then that is a simple winding.
  • If 4 or more wires have continuity, then you have a multi-tap transformer winding.

Transformer windings have a phase relationship, but it's typically not important for power supplies. Unless one of the wires has continuity to the transformer core, the polarity doesn't matter.

A relatively safe way to test transformers is to hook up a small AC voltage (1-5VAC) and measure the voltages across the other windings.