Electrolytic in capacitive dropper light bulb - Page 1
Forum section: electronics / projects

author: Fungus
author_rank: Super Contributor
timestamp: June 22, 2024, 04:56:03 am
content: Update: Ignore this thread...I just did a Big Clive and pulled apart a failed LED bulb (Phillips brand).The only things inside apart from the LEDs were a 22 Ohm resistor and an electrolytic capacitor.I've traced out the circuit as best I can and it seems to be the basic capacitive dropper circuit as shown.OTOH I was always told that electrolytics in capacitive droppers were aHUGEno-no, so what gives?nb. Ifound the cap on the weband itdoessay "special for LED lighting" so I guess they know what they're doing. I'm just trying to get my head around it.Is is just a specially tough capacitor for this application?

author: floobydust
author_rank: Super Contributor
timestamp: June 22, 2024, 05:19:31 am
content: I've found electrolytics in a few brands of LED night lights, but they use cap dropper+rectifier diodes and a single LED.I came home and it stinks like blown capacitor, did a search and found the night light dead. Took it apart and what happened is the LED failed went open-circuit, so then the cap overvoltaged and popped. Your cap 400V rated so that would not be it, did it just take the piss?Flicker is more a problem with high voltage LED strings I think.

author: Kim Christensen
author_rank: Super Contributor
timestamp: June 22, 2024, 05:25:14 am
content: Well, they do make non-polarized electrolytic capacitors. But....Quote from: Fungus on June 22, 2024, 04:56:03 amThat is a polarized capacitor. I suspect you've traced the circuit incorrectly

author: Fungus
author_rank: Super Contributor
timestamp: June 22, 2024, 07:18:34 am
content: OK!I managed to smash through the crud at the base of the bulb, it's horrible stuff, seems like epoxy mixed with fiberglass PCB shavings.It turns out there was more components in there.(Can't find any data on them though)

author: pcprogrammer
author_rank: Super Contributor
timestamp: June 22, 2024, 07:44:40 am
content: Yep special capacitor to go boom. It even is branded as such "AXBOOM"

author: Messtechniker
author_rank: Frequent Contributor
timestamp: June 22, 2024, 07:50:35 am
content: Thereshouldmust be a rectifier diode somewhere.

author: Fungus
author_rank: Super Contributor
timestamp: June 22, 2024, 07:59:35 am
content: I was mostly after the LED filaments but it seems they're really high voltage.My 32V bench supply won't light one up at all so maybe 96V filaments - no use for what I had in mind for them.FWIW: The bulb lit up but flickered on/off when plugged in so I assume it's the chip that failed, not the LEDs.

author: Fungus
author_rank: Super Contributor
timestamp: June 22, 2024, 08:05:08 am
content: Quote from: Messtechniker on June 22, 2024, 07:50:35 amThere's a .7V drop between some of the exposed legs on that IC but I can't get at all of them.That brown gunk covering everything is as hard as glass.

author: tunk
author_rank: Super Contributor
timestamp: June 22, 2024, 10:20:15 am
content: Here's a Chinese datasheet for the BP5133HC - could be a linear driver:https://www.bpsemi.com/upload/cn/file/2023-07/col337/1688454214770.pdf

author: Kim Christensen
author_rank: Super Contributor
timestamp: June 22, 2024, 03:14:28 pm
content: Tunk,That's the chip I found too...First time I've seen a LED driver IC with the bridge rectifier built into it. Single chip solution.

author: Fungus
author_rank: Super Contributor
timestamp: June 23, 2024, 01:15:52 pm
content: FWIW I've dug a bit more and this is what I've found with continuity tester. Not exactly what's shown in the datasheet. The pin spacing looks different on my chip, too.nb. The 22 Ohm resistor is an axial resistor (1/4W size) that goes from mains to the PCB, which is why I initially thought it was a simple capacitive dropper.

author: SeanB
author_rank: Super Contributor
timestamp: June 23, 2024, 07:06:58 pm
content: Well, a single component solution saves cost, after all that extra 4mm of PC board trace and the 0.5g of copper alloy for the extra 4 pins, is a big cost. So single chip, and thus easier assembly

author: amyk
author_rank: Super Contributor
timestamp: June 23, 2024, 10:11:29 pm
content: Quote from: Fungus on June 22, 2024, 07:59:35 amFlickering is a common symptom of bond wire failures, or bad solder connections.