Cheap Chinese buck converters (and I actually soldered an SMD component) - Page 1
Forum section: electronics / general

author: rrinker
author_rank: Super Contributor
timestamp: April 12, 2017, 03:01:02 pm
content: (sort of - with those big tabs sticking out, a largish electrolytic cap isn't exactly a challenge)In the interests of not having to deal with voltage drop when starting at 5V, I was planning to run a higher voltage power bus around my model railroad, along the lines of 14-16V DC. So I picked up some cheap adjustable buck converters on eBay, a Chinese seller that ships from the US so I got them in days, not weeks. I've had them sitting around for a while - when they arrived I opened one and tested it out, saw that it worked, maintained output setting over a wide range of input, etc. Recently I took them out to experiment with and test more thoroughly. First one was the one I already tried, no problem. Second one I opened, two things fell out of the static bag - the board, and the output capacitor. Looking at the board, it's like it was never soldered on. Or rather, the tabs on the cap never got any heat - there was a blob of soldered with the perfect outline of the tabs molded in on the pads. I set that one aside, opened a third - looked ok. Voltage checked out, then I decided to hook them up to my scope and check the noise and ripple level. First one - mostly clean but a few peaks over 150mv when set to deliver 5V. sheesh. Then I tried the next one - peaks over 500mv! Insane. Then I noticed the input cap on the bad one seemed wobbly. Looked closer under magnification - only one tab soldered on, the other one was just the tab resting on top of the tab-shaped indentation in the solder. That one was super easy to get to - it was the tab on the edge of the board, no other components in the way so I fired up the soldering iron and easily attached it. That brought the noise down to the same 150mv as the first one.So I decided to see if I could solder the the completely detached cap back on the other board. One side again was super easy, it was at the board edge. But the other one was down next to the inductor. A little checking and the tip of my iron did clear the inductor, so it turned out to be no big deal, really. Upon hooking that one up, same results as the other 2.Not too impressed, but I guess what can you expect for about $1.50 each. Not sure where I want to head next - I really don't want to build my own, I will need a few dozen of these. Some of them need to handle 2A peaks, others that drive only logic can have lower current capacity, but I might as well just get all the same. There are others out there using the same design (LM2596S) - though actually I did just now see one that may solve the poor assembly problem. The ones I have (and all of them, I suppose) are panelized. The ones I have are very crudely cut apart, and the input and output caps are both aligned such that the leads are parallel to the long axis of the board - the cuts being VERY close to the one tab. The ones I just found have the caps turned 90 degrees so there are no solder pads right at the edge. I will have to give that seller a try, unless someone has something they can recommend. The poor quality ones I have come from Frentaly. I was planning to add additional power filtering to my design, so the noisy output may not be a problem - it does drive an Arduino on a breadboard with no issues. My design is based around the Nano - I'm actually using ALL of the pins so a Pro Mini is not usable.

author: whalphen
author_rank: Regular Contributor
timestamp: April 12, 2017, 06:24:46 pm
content: Over the past couple of years I've bought many thousands of parts directly from Asia via AliExpress. Originally, my expectations on quality were low. But my experiences have been very good. I rarely get a part that's bad -- about the same as when I buy from the big US parts suppliers. And, anytime I've had a problem, the vendors have been eager to resolve the issue. So I would consider your experience to be out of the norm. My biggest problem has been the lack of part specifications from the vendors. You usually don't quite know what you're getting until it arrives.Congratulations on soldering an SMD component! Once you know a few techniques, it's not difficult. For years I avoided doing surface mount. Now I prefer it over through-hole. Like most things, it just takes a few special tools and some practice.

author: rrinker
author_rank: Super Contributor
timestamp: April 13, 2017, 07:25:52 pm
content: That comes on the heels of totally blowing it trying to fix one of my Nanos. A small SMD cap on the bottom side came loose, so I attempted to resolder it. With very disastrous results. It still works but mainly because the cap is I believe part of the power filtering - so as long as I run it from a good stable power supply it is probably fine. These electrolytics on the buck converters were more like soldering a piece of wire to a pad or trace. I doubt I will be laying out my board for my project using SMD outlines. It's mostly a vision thing. I have to get used to working under my microscope because otherwise I can't see the pads clearly at any sort of normal distance, and my magnifier lamp doesn't magnify enough. Severe nearsightedness since childhood plus a partial loss of vision in my right eye from glaucoma aren't too conducive to SMD work.