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?


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.

  • 2
    \$\begingroup\$ "the primary winding will have a higher resistance" Is that always true? They don't necessarily have the same wire gauge. \$\endgroup\$ – endolith Dec 17 '10 at 21:25
  • \$\begingroup\$ The primary winding of an N:1 transformer will have a smaller cross section and be longer, both by approximately a factor of N. So the resistance grows very roughly by the square of N. \$\endgroup\$ – starblue Dec 17 '10 at 21:36
  • \$\begingroup\$ Would phase relationship have any impact if I want to connect two inner coils serially? \$\endgroup\$ – AndrejaKo Dec 17 '10 at 22:06
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    \$\begingroup\$ @endolith: Not necessarily, but if it's a step-down transformer, the primary will probably have a higher gauge (thinner wire) than the secondary if the wire is going to be different. In a step-down, the secondary has a higher current than the primary. \$\endgroup\$ – W5VO Dec 17 '10 at 22:18
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    \$\begingroup\$ @AndrejaKo: Yes, the phase will matter. If there really aren't any diagrams and you have two identical 12V secondaries, then you could hook them up "backwards" which would give you 0V in series. If you want to use them in series, connect the two windings in series and measure the AC voltage from end to end. If it's 24V, good! Otherwise, swap the wires of one of the secondaries. \$\endgroup\$ – W5VO Dec 17 '10 at 22:24

The primary's wire is thinner and as W5VO says has a higher resistance. For the secondaries I presume they're separated. To find start and end of a winding you measure resistance with your multimeter. Depending on the transformer's rating you'll measure a few hundred ohms between begin and end of a winding. If you measure mega-ohms your measuring on two isolated windings. Let's say you can find windings A-B, C-D and E-F this way. To find the polarity you'll have the connect the primary to the mains. Measure the AC voltages of A-B, C-D and E-F. Let's say these are 19V, 19V and 12V. Those are unloaded voltages, and especially for toroidal transformers those may be much higher than the rated voltage. In my example rated voltages might be 2 x 12V and 8V.
Now connect A to C and measure the voltage between B and D. If this is 0V (very low anyway) A-B and C-D are in phase, so if A is the "start" of A-B, then C is the "start" of C-D. If the voltages are equal you can place them in parallel to double the current.
If the voltage measured between B and D was 38V the windings are in anti-phase: if A is the start of A-B then C is the end of C-D. You can connect the windings this way to double the voltage or swap C and D to have the windings in phase again.

You can do something similar with the third winding. Connect A to E and measure between B and F. If the voltage is higher than 19V (still working with my example) then you have connected the start of one winding to the end of the other. If the measured voltage is the difference of the two windings' voltages then you have connected start with start. You can use either way of connecting to create higher or lower voltages.

  • \$\begingroup\$ How can I determine where to put the "dots" for dot notation. Are the dots the same as "start" above? \$\endgroup\$ – Eyal Apr 11 '16 at 8:21

In multiple winding transformer, each winding(including a centre tapping if any and multiple tappings) are denoted by a seperate cable color.that is, the winding with a center has a different color,and the one with no tapping has a different color.in addition,they have different cable size


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