I have a number of mains-powered devices that have begun humming (I assume at 50Hz, though I haven't measured it). These include clock radios, a speaker system, a lamp (20W halogen with a transformer before the switch), and a temperature-controlled soldering iron. With some of these devices, it can be really annoying (like the radio and lamp and the speaker that vibrate the floor, and my bed, and make it hard to get to sleep). I assume it's also wasting a small amount of energy. I have tried taking one of the clock radios apart, and cleaned some dust and crap off the PCB (I couldn't see any other obvious problems), and it did reduce the hum a little, but not much, and it started getting worse again.
Is there a common cause for these hums? And is there anything that can be done about it?
Best Answer
A transformer, as you well know, is made up of two or more coils around a core of ferrous material. That ferrous material is not a solid lump of metal, but a series of plates laminated together with adhesive. This is done because:
So you have lots of steel plates stuck together, but not only that:
Lots of metal plates, each with an induced magnetic field. That magnetic field acts between the adjacent plates stretching and squeezing the adhesive and insulation between them. Over time that adhesive starts to break apart and the laminated layers separate from each other slightly. This is the humming noise you can hear. It's always present, but once the adhesive starts to break it gets louder. These micro-fractures in the adhesive may not be visible to the naked eye, but in extreme situations they may be so bad the layers of lamination become loose and the transformer literally rattles as you shake it.
Also, the more current you draw through a transformer the larger the induced magnetic fields, and thus the louder the transformer hums (and the shorter its life span).