I've been using a 'Universal Laptop Power Supply' for the last year or so to test laptops where either the owner hasn't brought the charger or there is potentially a problem with theirs. It seems to do the job, it's a 120W unit with selectable voltages from 14V to 22V, and a selection of tips that have suited every laptop I've used it with, except for one old Dell laptop.
Recently though it's been making a buzzing noise under load. Not necessarily a problem, but it's been getting louder and more annoying. So, I thought it was time to take it to bits and see what makes it tick (or rather buzz).
It seems to be fairly well made compared to some units I've seen. There is some unpopulated area at one end, which appears to be a 5V USB charging supply option not on this model.
Turns out the fan on there was making the noise, an 8V 30mm unit. I've removed it and tested it on a bench power supply, at 8V and it runs fine, as noisy as expected from a small fan, but not as bad as it had been inside the PSU. It seems it was being driven by a voltage which slowly rises with load, and at about 3-4V it is trying to start but fails to spin and just sits there and buzzes. Not a very good design really, it shouldn't be run below a safe guaranteed starting voltage, in this case, about 5V.
UPDATE: Comments disabled on this post as it seems to attract a lot of spam.