I recently ordered a custom transformer and under load with half of its rated current it heats up to 55 °C, and now it's started to radiate a "weez" noise.

Is this normal? How hot is too hot for a transformer?

How can I get rid of the noise? Pushing it down doesn't help!

Update: the iron core was loose (or wasn't tight enough), I returned the transformer to be fixed with a new core.

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    \$\begingroup\$ What does this transformer do? Is it for a switch mode power supply? What frequency does it operate at? \$\endgroup\$
    – SteveSh
    Jan 28, 2021 at 12:31
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    \$\begingroup\$ 55 deg is not that much, but it depends to the insulation class. Weezing noise is due to loose tight of lamination. Both phenomena together may also indicate a low quality product. \$\endgroup\$ Jan 28, 2021 at 12:33
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    \$\begingroup\$ @SteveSh It's a step down transformer working with 220V at 50Hz. for a linear regulator. the core is iron. \$\endgroup\$ Jan 28, 2021 at 12:50
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    \$\begingroup\$ The transformer's nameplate should state service current limits. If it doesn't have a nameplate it's not a real transformer, it's mystery meat. \$\endgroup\$ Jan 28, 2021 at 22:59
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    \$\begingroup\$ If the magic smoke gets out, it was too hot. \$\endgroup\$ Jan 28, 2021 at 23:47

2 Answers 2


The maximum allowed winding temperature VS. insulation class:

enter image description here

  • 6
    \$\begingroup\$ Except in a quick catastrophic failure, you can usually smell transformer insulation overheating before any permanent damage is done. It's hard to describe smells, but it's a bit like what you would get if you were cooking something which had "gone horribly wrong" for some reason. \$\endgroup\$
    – alephzero
    Jan 29, 2021 at 1:40
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    \$\begingroup\$ @alephzero I'm not that good at cooking, but even when it goes "horribly wrong" for me it doesn't smell as bad as fried insulation ;).. \$\endgroup\$
    – Vinzent
    Jan 31, 2021 at 20:27

The transformer should have a specification sheet that details such things are max. temperature. Since this one is custom, it seems that you should have that available.

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    \$\begingroup\$ I wonder why this got downvoted, it's a perfectly valid answer. There's no one-size-fits-all safe temperature, as what you measure on the outside is likely way below the temperature of the inner layers of the wiring. Only the designer of the transformer knows what temperature is the insulation rated for and how that translates to external/ambient temps. \$\endgroup\$
    – TooTea
    Jan 28, 2021 at 12:36
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    \$\begingroup\$ re: why could the answer be downvoted: it reads more like a comment than an answer. \$\endgroup\$ Jan 29, 2021 at 1:07
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    \$\begingroup\$ No, the above is an answer, this is more like a comment: "Use comments to ask for more information or suggest improvements. Avoid answering questions in comments" \$\endgroup\$ Jan 29, 2021 at 8:49

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