I was cleaning up and found an old Six-Letter usb charger that I remembered having removed from service but I couldn’t remember why. I opened it up and got a clue right away.
The unpopulated spots on the bottom are for the very important AC line noise filter to keep the high frequency switching pulses from the switch mode power supply from escaping into the house wiring and causing a myriad of interference to tv/radio reception and audio systems (among other things). In many cheap and nasty power supplies that have gone through quality fade, you will find this same issue. What should have been there is a common mode choke that looks like a small transformer, and a safety capacitor. The safety capacitor in this case is one rated to fail non-catastrophically if it suffers a short across its dielectric or a massive over current event from a surge.
Let’s have a look under the board. On a lot of the supplies I’ve seen cheapened like this, two wire jumpers bypass out the common mode choke. The way this works is it’s basically two inductors wound back to back. Any noise common to both sides is attenuated by being magnetically cancelled out. Neat trick, right? Well, it is if you don’t —-
Oopsie poopsie they made a fucky wucky! A real fucko boingo…. No wait, they didn’t, they just didn’t even want to have to change the build of the supply when they ripped out the inductor. The traces just cross over where it would have been inserted into the circuit. Cute. I’m guessing if they ever had to submit a sample of one of these to a potential reseller who was going to run it through EMC testing, they would have added the missing choke and capacitor and cut the shorting traces… then followed up with a production run missing all the parts when the actual order came in.
Of course, the fact this was directly sold via Amazon using a random, rotating six letter brand name, likely appended to a stolen ASIN (don’t even get me started) suggests the risk of anyone ever bothering to test one is approximately zero. I’m pretty sure if it didn’t get banished due to massive electrical noise issues, that unhappy looking melted diode at D1 probably got it hucked unceremoniously in what was supposed to be an e-waste box that I forgot to take for disposal.