LED Bulb power supply board modification? - Page 1
Forum section: electronics / projects

author: stormyb
author_rank: Newbie
timestamp: June 19, 2023, 09:03:56 am
content: TL;DR I've got a smartbulb with a busted socket connection, so I wanna figure out where the high-voltage mains power supply part of the PCB ends, chop it off, and wire up a USB PD adapter instead to turn this nifty little RGBW smartbulb into a USB-powered one! But, I don't want to go poking around mains voltage with my multimeter, so I'm asking the EEVBlog forums for help!- - -I've got one of those nifty $20 4-pack smartbulbs from Amazon (brand is Teckin, model is SB50). It's got everyone's favorite Espressif inside, an ESP8266-something-or-the-other. Fun little gadget, naturally, but one of them got aggressively unscrewed by a roommate recently and the socket base popped off. Normally I'd just put the thing back together but it's been having a hell of a time with flickering ever since that incident, and to be quite frank I'm not much interested in high-voltage troubleshooting and repairOn that note, I have recently acquired some niftyUSB PD sink adapters- you plug in a USB C cable to one end, and it spits out a configurable voltage on the other end. I've been using 'em for building cosplays, laptop charger replacements, even powering a small NeoPixel lamp I helped someone design and build.Now, I had a thought - since the LED board seems to still be fine, and the shoddy connection to wall/mains is the big issue here, why not just combine the two products? If I've learned anything from the EEVBlog channel, ElectroBoom, bigclive, etc, then the light bulb's PCB is just going to be a capacitor/resister bridge rectifier with some step-down components (probably not too far off from what would be in a USB wall wart or something) and then on the other end, the actual electronics for the LED part of things! assuming I can correctly identify current/voltage paths, I figured I'd be able use a dremel or other small tool to cut off the parts of the PCB that are purely for converting mains/wall power to what the LEDs need, and then replace that with one of the aforementioned USBC PD adapters.Normally I'd just take my several-years-of-electronics-college experience, and a multimeter, and start poking around. This works fine when I'm trying to troubleshoot which pin on an arduino's shorted out, or find a dead trace on some AAA-powered kid's blinking toy. But this is mains voltage here! Frankly I feel confident in my abilities when working with low-voltage components but am mildly terrified at the idea of hooking this thing up to mains power just so I can poke around with my multimeter. (Even if the multimeter is rated and tested on wall voltages and currents, I don't exactly trust myself to not slip up and prod the wrong thing!) I wouldn't be too upset about accidentally having the multimeter in DC mode and probing an AC pin and blowing a fuse, but I'd be pretty upset about injuring myself, or worse.So, I spent some time poking around on the internet! Funny enough, these little espressif devices are famous for being able to flash with open-source firmware for a project called Tasmota, which is neat! Even more neat, is the fact that people need to open up their devices most of the time to access pins to flash firmware! I was able to follow a few github posts and that sort of thing to find out how to get my bulb open the rest of the way, but because all of those folks are only concerned with the software side of things, not the hardware, I'm actually finding an incredible lack of documentation for the mainboard functionality!Now, lucky for me, the 'brain' of it seems to have its pins already labeled for Vcc, ground, R/G/B/W/Y pwm channels, all that good jazz. A this point you might think it's easy to just pop the board off with a quick desolder job and then feed it the necessary voltage! But unluckily for me, the brain daughterboard isn't connected to the LEDs - the connection comes up through the power section, into the brain board, and then back down into the main board before going out to the LEDs! So even if I tap the Vcc/gnd with my multimeter, it's just the dedicated power source for the microcontroller, not the total power necessary for everything including the LEDs! (My best guess is there's some mosfets on the main board that handle this!)So, I'm posting here for advice! Normally in a situation like this I'd do something sane, like finding someone else'swell-photographed teardown procedureto look at the layout of the board and its components, and then maybe I'd cross-reference that with a handy dandyFCC bill of materials for the board's componentsand then just maybe even start de-soldering things working from the AC end up toward the LED end until I had enough of the board cleared to properly reverse engineer it. But unfortunately for me, I only have one of these bulbs to tinker with! So as much as I'd love to happily whip out some desoldering tools and a dremel to rip this thing to shreds and figure it out in the process, it seems I oughtta enlist some experts who know a thing or two!I've got a flatbed scanner, a camera with decent zoom, and (ironically) some decent lighting, so I'll have to dig it all out and see if I can get something set up for posting a pic of my exact board. It's nearly identical to the one in the linked gallery, but I'm on the V1.8 revision of the LM99E power board, so things might be just a tiny bit different! Poking around with a multimeter (without connected power!!) has been pretty handy for tracing things back from the LED section toward the power supply section, but any time a trace disappears, the readings get funky if I accidentally read through a capacitor or something instead of a direct trace, so it's a been a bit of a headache trying to figure out this compact little puzzle!

author: tunk
author_rank: Super Contributor
timestamp: June 19, 2023, 10:32:20 am
content: Post some photos.Big Clive recently tore down a wifi bulb where the white LEDs were runfrom rectified 230Vac, the coloured LEDs from 14V and the wifi from 3.3V:https://youtu.be/B_HSXRpDWyw

author: stormyb
author_rank: Newbie
timestamp: June 19, 2023, 11:06:55 pm
content: Alright, so here's a macro shot of the espressif chip, as well as a shot of the back/front of the whole thing. I tried to get the flatbed scanner set up but it's some HP piece of junk that only scans things on the exact surface of the flatbed, so the PCB scan was beyond useless - a waste of a few hours rummaging around for cables and drivers. Ah well. I've taken the current photos with my phone in an attempt to get the board as close to the phone as possible without starting to lose focus or deal with distortion; I've taken a whole grid of macro shots and am currently attempting to make a tiled image in photoshop so for now, here's the 'main' shots.Additionally, here are the photos from the other teardown I'd already linked, as it is much of the same thing:One,two,three,four.Additionally,the LED board,which is the same in mine.

author: stormyb
author_rank: Newbie
timestamp: June 19, 2023, 11:53:21 pm
content: Aaand here's the stitched image from almost two dozen macro shots. Image is slightly warped, but it's more than good enough for following traces.

author: stormyb
author_rank: Newbie
timestamp: June 19, 2023, 11:56:19 pm
content: As I've never used a board designer or circuit simulator before, this may be beyond the scope of what's feasible, but since I have a PCB layout (backside) which is fully labeled and traceable, and I have a PCB layout (frontside, populated) which is also fully labeled and, with careful inspection, mostly traceable - and, of course, I have the BOM that lists every single labeled component - would it be possible to recreate this in some software and 'test' the voltage at each point? I've seen some simple things demoing this, and aside from a few components (primarily, the 'U1' with its lasered-off face) everything seems fully labeled and identifiable, particularly thanks to the BOM page.

author: DavidAlfa
author_rank: Super Contributor
timestamp: June 20, 2023, 12:20:30 am
content: This circuit is not mains-isolated.Connect it briefly to the mains and measure voltage in RS10 (3301), seems the main power output, I'm pretty sure it'll work when feeding the same voltage there.The esp board has its own buck regulator, so it might take the same voltage as in RS10.

author: stormyb
author_rank: Newbie
timestamp: June 20, 2023, 12:24:24 am
content: Quote from: DavidAlfa on June 20, 2023, 12:20:30 amI was poking around (with it unplugged) and came to the same conclusion, that RS10 would be the effective 'power injection' spot, but didn't want to be too self-certain. I'll see if I can get it back in the screw base safely, and give this a poke! Which part is the buck converter? Is that, presumably, the unlabled/lasered chip?

author: DavidAlfa
author_rank: Super Contributor
timestamp: June 20, 2023, 12:40:18 am
content: Yep, that's the buck regulator for the mains power.Don't bother trying to identify it, you're not using it after all...Where do those small shunt resistors (RTx) go?

author: stormyb
author_rank: Newbie
timestamp: June 20, 2023, 01:19:06 am
content: It's quite hard to tell where the shunt resistors go, even with the board in my hands turning it under a light to try and follow the vias.I made a side-by-side version of the board by putting the top and bottom images beside each other and then mirroring one of them, so it's a little easier to get an idea of where things are; I've highlighted a pair of massive inductors (not sure what their purpose is, frankly) with green, and highlighted a very large cap (35V, 200uF) which is where you marked the Ground and Main connections. It all seems a bit odd - if I'm reading the PCB correctly, there's a resistor that bridges the cap (RS10). Does this all make sense? Hopefully the image helps, do remember I mirrored the 'bottom' side of the board so the images would align side-by-side.(also, congrats on your 4000th post!)