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for the past 3 months I have removed several transformers from PCBs to try and use it for a different purpose ,but as I connect it to a 230v main supply they just keep exploding . I don't think my problem is that I'm using the wrong terminals , frequency or voltage because the transformer came from a circuit using exactly 230 volts .Can someone please explain to me what am I doing wrong .

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    \$\begingroup\$ This question is unanswerable without the details of the transformers in question. However, it's rather rare for transformers suited to direct connection to mains to be found on circuit boards. Perhaps you're extracting high frequency transformers (or even chokes or other inductors) from switching power supplies and mistakenly trying to use them to make linear supplies. Generally speaking if it's not a distinct hunk of iron with a label giving specs, you shouldn't be trying to do this with it. \$\endgroup\$ – Chris Stratton Aug 14 '18 at 18:14
  • \$\begingroup\$ Are we supposed to guess all the details? I'm willing to be that all transformers are not created equal. I'm also willing to bet that most users here don't know the circuits that your transformers came from. \$\endgroup\$ – Bort Aug 14 '18 at 18:14
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    \$\begingroup\$ don't you think that you should have asked after the first one blew up? \$\endgroup\$ – jsotola Aug 14 '18 at 20:05
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    \$\begingroup\$ @patrick If you are a bit of a scavenger, then you should be collecting good iron transformers now (the ones with thin steel laminations that can go across 230V), as they will all be gone in a few years. Old audio gear is good for them. Toroidal transformers are especially useful, always snaffle them if you see one. For connecting things to the mains there is a Variac, and if you ever see one, acquire it. \$\endgroup\$ – Henry Crun Aug 14 '18 at 21:10
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    \$\begingroup\$ @patrick, you may have been thoughtful and careful in your experiments, however your description implies otherwise. ..... it seems to say that you were repeating the same thing over and over .... that is why i made the comment \$\endgroup\$ – jsotola Aug 15 '18 at 1:18
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Assuming you are talking about the ones from switch mode power supplies (tiny things), they don't run at 50Hz, the run at several tens of kHz.

In SMPS's, the mains is rectified to DC then switched through the transformer at a much higher frequency. This allows for physically smaller transformers.

If you connect one straight to the mains, you'll be running it at much lower frequency than it is rated for at that voltage, and hence orders of magnitude higher current than it's rated for (remember, inductor impedance is directly proportional to frequency). The net result is bang.

The moral really is don't try to wire up any component without reading it's datasheet. If you don't have a datasheet, don't use it.

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