I purchased a dual-voltage step-down transformer. It had 3 input leads for either 115V or 230V and could output either 12V or 24V on the center-tapped secondary. I taped off one of the input hot wires and connected the other two to a wall plug. I then connected the three outputs to a screw terminal so I could safely test them with my meter, but they were otherwise unconnected.

I went to plug the transformer in (to 120V) and it buzzed loudly for a second and then violently exploded with wisps of some pleasant-smelling magic smoke. On the primary side I could see that it had spewed some copper from the primary winding onto the insulation of the wires (I could scrape it off to see insulation below, so the hot/neutral wires weren't shorted because of cracked insulation). The transformer was hot to the touch even after sitting unplugged for a few minutes. Testing the resistance of the primary now I get an open circuit between one hot and neutral and about 1 Ohm between the other hot and neutral.

Why did my transformer blow up? The first thing I thought of is that it was defective and the primary winding was shorted somewhere. Is there something else that I'm missing? Did I need to connect the primaries in parallel? The wiring diagram on the datasheet is very simplistic but doesn't seem to indicate such - it quite literally has 3 lines coming from the primary side that say 0V, 115V, and 230V.

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    \$\begingroup\$ are you certain that you used the correct terminals? a photo of the diagram could help \$\endgroup\$ Commented Oct 16, 2020 at 3:25
  • \$\begingroup\$ @Jasen Yep - the NEMA plug I had was idiot-proof with "white" and "black" labels on the terminals. Since you asked I just double-checked and confirmed that I wired it correctly. I was using lever nuts to connect it to the input of the transformer so it would have been pretty tough to not notice if the polarity was wrong there as well. \$\endgroup\$
    – flashbang
    Commented Oct 16, 2020 at 3:29
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    \$\begingroup\$ Lesson learned - always put a suitably rated fuse on the primary side! \$\endgroup\$ Commented Oct 16, 2020 at 3:29
  • \$\begingroup\$ @BruceAbbott - I was thinking that, and then I went "nah, I'm just doing a quick test with no load to make sure the thing works. Surely nothing could go wrong..." Famous last words before Murphy's law hits, I suppose. \$\endgroup\$
    – flashbang
    Commented Oct 16, 2020 at 3:31
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    \$\begingroup\$ That data sheet doesn't seem to have enough information to identify the primary and secondary windings. don't assume anything about wire colours that is not written on the transformer, or written in the data sheet. \$\endgroup\$ Commented Oct 16, 2020 at 3:50

1 Answer 1


You connected it backwards. Primary color code Black/Yellow/Red.

enter image description here

In the future, you should read the datasheet and catalog and not make assumptions and also you can double-check by measuring the resistance. The primary for 115/230VAC will have much higher resistance than the secondary.

Also a small fuse in series with the primary would have saved it.

Note: On the primary, leave the unused connection open. If you connect the black and red together to one side of the line and yellow to the other and apply 120VAC you'll burn out the transformer (and the fuse might not even save it!).

  • \$\begingroup\$ Wow, yes, you are correct - it does say this on the product page. I would like to say that the datasheet doesn't have any information about the colors of the wires. This is my first time working with a transformer and I guess I (stupidly) believed white = neutral and black = hot since I didn't see anything in the datasheet. At least this was a cheap lesson, and now I can say I built a kilovolt transformer. \$\endgroup\$
    – flashbang
    Commented Oct 16, 2020 at 4:30
  • \$\begingroup\$ It's a very dubious datasheet. I think the good makers of 50/60Hz transformers have long moved onto other things and only garage level operations are left. When I bought linear transformers from Taiwan and China there was about 20 pages to the specs. \$\endgroup\$ Commented Oct 16, 2020 at 4:35
  • \$\begingroup\$ Man, that would have been nice. Since I bought it from Amazon instead of Jameco's site directly I never saw the description and just went straight for the datasheet, and since I'm used to working with low-voltage DC electronics I automatically think of red, yellow, and black wires as 5V, 3.3V/"lower than red," and ground. I guess I shouldn't make such assumptions when working with higher voltage AC now. Boy, do I feel dumb. \$\endgroup\$
    – flashbang
    Commented Oct 16, 2020 at 4:45

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