Toughest challenge for an electronics engineer - Page 1
Forum section: electronics / general

author: Connoiseur
author_rank: Regular Contributor
timestamp: December 31, 2015, 08:19:24 am
content: Hi,I recently bought some Christmas lights off ebay which came with this incredibly shitty power supply. The label says that it can output 500mA @ 5V.....but guess what ,it provides 27V OC and 2.2V @ 100mA(voltage regulation anyone? huh, who cares).So I thought I would pop it open and see what's inside. AAAAnd horror of horrors, it was the worst thing I've ever seen.No input protection fuse, no common mode choke, 300V rated cap for 230V mains! blah blah blah. Someone seriously wants to kill a large number of people.I would suggest you consider generic Chinese mains adaptor for use so much as you would consider a rotten corpse for food.Now here's the challenge. Looking at the PCB try to say something good about the adaptor.

author: bktemp
author_rank: Super Contributor
timestamp: December 31, 2015, 08:51:15 am
content: Quote from: Connoiseur on December 31, 2015, 08:19:24 amAt least they have added a fuse in the layout: The thin track near the mains input.

author: Cubdriver
author_rank: Unknown
timestamp: December 31, 2015, 08:57:56 am
content: The mirrored silkscreen on the solder side is a nice touch...-PatETA - and I notice that it's rev 3 of the board. You'd think they might have corrected that on one of the two respins.

author: SeanB
author_rank: Super Contributor
timestamp: December 31, 2015, 09:03:06 am
content: Good thing is primary secondary isolation gap is correct, along with the reflow soldering being good and sufficient for the most part. Trace fuse is there, and it is thin enough that it will fail before any major fireworks occur that might damage the case integrity. Case is a relatively decent mould, though the join area should have had either solvent welding fully around it or some screws to hold it together.Are you sure it is not a constant current supply, sort of done by accident? Seems like that would be the right thing to drive those cheap light sets which rely on the source impedance to provide current limiting. Universal input as well, you have the right set of pads to handle a US socket spacing as well, and the silkscreen with the reversed numbers and mirrored s as a 2 are nice touches, along with a double sided silkscreen. They actually have 2 earlier revisions of this board? rev 4 will remove 3 diodes and lengthen the wiggly fuse to cover the gap, along with making the traces thinner. Copper is expensive, recover as much as possible from the etchant.

author: tszaboo
author_rank: Super Contributor
timestamp: December 31, 2015, 10:01:49 am
content: Ever seen power supply with capacitive dropper? This one tries to have isolation at least.

author: Connoiseur
author_rank: Regular Contributor
timestamp: December 31, 2015, 10:36:53 am
content: Quote from: bktemp on December 31, 2015, 08:51:15 amWow what an ingenuity! I wonder is there any way one can be sure about the current rating. I mean heat will be dissipated by the paper phenolic thingy increasing the current required to melt the trace.Quote from: SeanB on December 31, 2015, 09:03:06 amNo! It doesn't seem to be constant current as>It blew one of the lights (yes I was stupid enough to plug it in!)>The label says so>I measured DC current in series with a range of resistors. However didn't dare to short it.Quote from: NANDBlog on December 31, 2015, 10:01:49 amYes I have. They come with those mosquito swats and cheap rechargeable torches. However there is no way one can come in contact with mains when it is in use.

author: Gyro
author_rank: Super Contributor
timestamp: December 31, 2015, 10:45:16 am
content: Well I think the 0.1" LN header footprint for alternative mains input is a nice touch!P.S. Its implementation is thankfully flawed!

author: VK5RC
author_rank: Unknown
timestamp: December 31, 2015, 10:56:36 am
content: That's really nice strain relief on the cable (or so it appears)

author: VK3DRB
author_rank: Super Contributor
timestamp: December 31, 2015, 11:24:14 am
content: Mirror image silkscreen on some componentsBut there is something good about this PCB: It comes fully compliant with all the safety regulations set by Chairman Mao.

author: Stonent
author_rank: Super Contributor
timestamp: December 31, 2015, 04:48:07 pm
content: Watch some of bigclivedotcom. He takes apart lots of cheap led lights and many of them just use a dropper cap. Also many are 120/240 but have a 300 volt cap which is more than suitable for 120 volt operation but not 240.

author: ConKbot
author_rank: Super Contributor
timestamp: December 31, 2015, 05:15:26 pm
content: +1 to watching some bigclivedotcom videos, that adapter aint shitDoes it run the Christmas lights? I'm presuming they are LED, so the poor voltage regulation isnt much of a problem, if its meant to be a constant-ish current output. The 300v cap on 240v is obviously a problem, but I'd ignore the numbers on the casing. They obviously took an existing board from the cheapest usb charger they had(with no apparent form of feedback, unless they are using extra wingdings on the flyback transformer to sense, and its just not used on this particular adapter), took off a few components stuffed it in existing cases (why pay to change even the printing?) and the only thing custom for this run was the cable strain relief which fits in a USB sized slot in the case.Cheap, safe, meets specifications on the casepick 2

author: SeanB
author_rank: Super Contributor
timestamp: January 01, 2016, 12:13:21 pm
content: Quote from: ConKbot on December 31, 2015, 05:15:26 pmI would say 0/3 for that one. Not the cheapest, you can buy cheaper if you do not really care if they work on your test line, as you test using another power supply on the line and simply put the prepacked bag in the box.

author: dannyf
author_rank: Super Contributor
timestamp: January 01, 2016, 12:40:14 pm
content: QuoteYou bought cheap and you got cheap. What else should you expect? Quality is expensive.Shitty engineering and shitty products exist to satisfy shitty consumers' demand for shitty products. If you don't want shitty power supplies, buy from a reputable source. I'm sure guys like Hermes or Bloomingdale would be happy to sell you one that's more than properly engineered. And if that's still no good for you, you can always commission someone to engineer it until your are fully satisfied. You just need to be ready to sacrifice a leg or an arm of yours.So quality product or your arm? You cannot have your cake and eat it too.