hot miniature transformer

[Grunff]

No, shouldn't be anywhere near that hot. Something's not right.

[mike]

I have a miniature 9-0-9 V (250mA) transformer from Maplin I was going to use for a simple power supply (fw bridge + smoothing cap). But the thing runs *very* hot. You can just about touch it. Even under no load with nothing connected to the output side it's just the same. I think it's faulty - but maybe these cheapo transformers normally run hot like that? I'm not going to use it as I think it's a fire risk anyway - but just like to know if that sort of heat is normal? ta!

[G&M]

Sounds like a shorted turn - but are you providing the ventilation or heat removal required. For a 5W transformer driving a diode bridge (the worse load for a transofrmer even though we all do it) you should look at removing two watts or so.

[Ian Stirling]

It may actually get cooler as a load is drawn. The core may be saturating due to them cutting costs and using a really small one. If you connect a 40 ohm load, does it cool down?

[Steven Briggs]

In message , mike writes

I'd disagree with the others, it's probably OK.

Just about touchable (i.e. keep yer finger on it for about a second) works out about 60C. Touchable continuously is about 55C.

For very small transformers, losses in the core & losses due to magnetizing current in the primary dominate, so even at zero load they run pretty hot.

As a power supply designer, I deal with temperature issues on windings & components all the time. You only worry when you can't meet class H temperatures (155C operating), as beyond that the materials start to get very exotic & expensive ;-)

Only thing I wouldn't probably do with your TX is stick in some cheap horrible small ABS box (unless it's UL 94V-0).

[Bob Minchin]

Some of these small transformers do run warm but should not get as hot as you seem to suggest. I'd return it to maplin for exchange. Even buying a new one is cheap compared to the problem that overheating might cause.

Bob

[Grunff]

Bearing in mind that you're a PSU designer and I'm not, I'd just like to say that in many years of building various bits of kit with small transformers in them I haven't come across one that runs too hot to touch - and if I had I'd have replaced it without question.

[Dave Plowman (News)]

They will run warm under load, and I'd not personally use one for something that was on continuously at near the full rated output. Their 'standard' range is fine for this, however, and I've never had problems with them. The 6VA one is only slightly bigger and slightly more expensive.

[raden]

In message , Steven Briggs writes

A simple 9-0-9 v transformer under no load conditions?

I think not

I could be more sarcastic but I can't be bothered

[N. Thornton]

I have designed PSUs too, but dont do it now. I would have to disagree with you on this one. If the case is too hot to hold we dont know what temp the TF is at, but clearly it will be much higher. A wall wart running that hot is just a recipe for trouble. It lacks the safety margin necessary in case something lands on it and reduces its ventilation further.

It is one thing thing to run transformers at the temp you discuss, but not the case, not in a consumer environment where no knowledge or skill can be assumed, and children can be expected.

I also dont know how it would affect your fire insurance when you know it to be running abnormally hot, yet continue to use it.

Regards, NT

[Ian Stirling]

I don't think it's a wall wart. Though the poster does not say so, it's sort of implied by the comment about 9-0-9. Few wall-warts bring out more than two wires, and I've never seen one that supplies AC as two equal voltages of opposite phases.

[Dave Plowman (News)]

No. Miniature transformer 250mA from Maplin gives the clue. That's what they describe one of their chassis types as - exactly.

[Ian Stirling]

However, without knowledge of the maplin catalog, it's not utterly impossible that they might have a wall wart that did that. Very unlikely.

[Steven Briggs]

In message , N. Thornton writes

Sorry to I should have made my original post clearer, I'm not saying the OP's TX should will be class H, indeed nothing like. Almost certainly bog standard class A, in which case 90C is the operating limit (varies a bit depending on which standard applies).

I read the OP as saying he had an open, uncased transformer, which to quote his words "can just about touch it", which is likely to be around

60C, which is IMO is fine.

The outside of a plastic wall wart at under similar conditions (i.e. just about touchable) would indeed be cause for concern. My finger is not calibrated for plastic touch temperatures, but with lower thermal conductivity / thermal mass, "just about touchable" I guess will be somewhat higher than 60, and as we agree, not good.

[Steven Briggs]

Read the OP again. "Can just about touch it". IME that's about 60C for something metal. Not a problem for a cheapo open transformer.

[Dave Plowman (News)]

As it happens I've got a few of those - not Maplin, but exactly the same dimensions so may be from the same maker. And one's been on for a few hours now with no load. Not even warm.

[Ian Stirling]

Depending on the design, it may actually get cooler if loaded.

[raden]

In message , Steven Briggs writes

Under no load conditions?

[Ian Stirling]

No, too small core leading to saturation on the peaks of the mains waveform. If you apply a load, it reduces the peaks, and may well reduce overall losses.

[raden]

Peltier transformers ?