How hot should a 120V to 12V transformer be getting at low current ?

[lordvader88]

I have 2 transformers,  rated 120V:12V @1 or 2 amps, both weigh about 0.5lb or more. 1 from a little radio/ghetto BLASTER, and 1 is a certified wall wart. And just running well under 50mA most of the time, they still get VERY hot after a few hours.

Is that normal ?

[BrianHG]

Check the power supply input voltage ratings.  Old ones may be designed for 110v, and you power coming into you house may be above 125v.  This shouldn't do much, maybe make the transformers warm to the touch, but not 70 degrees centigrade.  Some older transformers are just designed to run near the core's saturation point, so, the get warm.

If you have a properly safe setup to do so, can you tell us how many ma AC the transformers are drawing on the 125v side when nothing is connected on the output.

[floobydust]

I find cheap wall wart transformers run around 35-45C;  Copper and steel are expensive so they push the flux-density and the core & primary gets quite warm, even at no load.
After a few years the plastic is all discoloured.
Older ones were not like this, I have a Sony 9V 1A adapter that is stone cold with 122VAC in, but it's huge.

Hammond power transformers (166/167) went through a phase where they made them in China instead of Canada. For the same specs/part number the chinese parts ran quite hot. It was a debacle, as they should have changed the part number. Manufacturers were used to a level of quality. Hammond had to change the design spec from being so cheap.

If it's too hot to touch like 80C, I had one transformer that I think has a shorted turn that caused that.

[amyk]

Check if there is any DC bias on the mains, which could be saturating the core partially on each cycle. Another thing you can do is look at the output waveform --- it won't be a perfect sine, but it should not be too distorted or unbalanced.

[lordvader88]

I'll check the AC input current later. But assuming it just runs that hot, I should just add a few holes to the plastic case and a mini fan....darn, they got thrown out years ago by accident.

[floobydust]

If you measure the transformer's primary current, you may get misleading (high) numbers.
You see magnetization current which has high peaks (not a sine wave, need true RMS), and mostly out-of-phase with mains voltage so the power (factor) is low.

All you can do is add cooling and ensure there's a primary fuse if the transformer does cook.