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I have this 2.5VA Split-Bobbin Power Transformer 115/230VAC. According to page 3 of the Data Sheet (PDF) it looks like there is a line on the "DSW" version on the pin 1-4 side. This seems to correspond to a small step down (maybe 1-2 mm or so) on the top of the physical transformer.

Is it safe to assume that is the identifier for the pin 1-4 side? If not, how do I identify which side is the input and which side is the output?

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The dimple on the transformer should indeed be the index mark for the 1-4 side.

If you're still nervous, take your DMM and measure winding resistances. The input windings will have much (at least 10x) greater resistance than the output windings.

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If you enlarge the photos in the datasheet, you will see that the pin numbers are printed on the coil former, just above the pins.

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  • \$\begingroup\$ Thanks, sorry, I should have included that. My 16 yo soon, with good eyes, a bright light & a magnifying glass is seeing no numbers on the plastic frame near the pins or anywhere else. \$\endgroup\$
    – FreeMan
    Commented Jul 3, 2015 at 1:27

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