Author Topic: Help me understand this AC/DC rechargeable LED bulb (Read 437 times)

ReallyBigCabbageBoi · « **on:** October 16, 2024, 06:44:09 pm »

I am an undergrad electronics student and I live in a not so well off part of the planet so power cuts are pretty common, so I bought this really cheap LED bulb (a little more than a $1) that stays on for about 2-3 hours after electricity gets cut. Out of curiosity, I opened it up and it has a Li-ion battery within it, which is connected to this circuit(Attached). The LEDS labeled 18V/1W glow with regular current and the ones labeled 3V/0.5W glow with the backup battery. They are radially placed and I can notice one transistor (Q1), 3 ICs [U1 being labeled SM2082EAJ - a google search taking me to some LED driver IC chip and - and U2 not being labeled at all and BD1 being M810F, which seems to be a diode]. There are also two Zener Diodes, a capacitor and several resistances. Can you tell me what's doing what here or was I scammed and the backup will last as long as the battery has juice, after which it will go back to being a regular LED bulb

?

Also, this thing turns on if I press the bottom against my palm/fingers while being disconnected from the socket.

Kim Christensen · « **Reply #1 on:** October 16, 2024, 07:33:31 pm »

Can't see much from the picture, but the 3V LEDs are all in parallel and are probably powered directly from the battery. Hopefully with a constant current IC, but probably just a resistor to limit the current.
I suspect they are just relying on the protection IC in the battery to stop the charging but it's hard to say exactly how they charge the battery.
The 18V LEDs are all in series and current limited by the SM2082EAJ when the bulb is running on AC. (Rectified and filtered 1st)

ReallyBigCabbageBoi · « **Reply #2 on:** October 16, 2024, 07:52:09 pm »

I looked around on the internet for similar circuits and most of them use diodes something like this.
Maybe the M810F is achieving a similar function here.

ReallyBigCabbageBoi · « **Reply #3 on:** October 16, 2024, 08:06:38 pm »

comes in handy as a torch

Kim Christensen · « **Reply #4 on:** October 16, 2024, 08:10:41 pm »

I think the bridge rectifier is used to power the AC LEDs. So I don't think there can be a capacitive dropper like in your diagram (C1 on the AC side of the bridge)
They are probably using some clever way of putting the battery in series with the 18V LEDs to charge it and then switching it out when it's charged. You need to trace out all the connections and draw a diagram. I can't really do that from a picture. Or send it to Big Clive to reverse engineer. He likes exploring oddball lights.

Here's a typical circuit using the SM2082. (No battery or low voltage LEDs)

PGPG · « **Reply #5 on:** October 16, 2024, 10:30:31 pm »

Quote from: ReallyBigCabbageBoi on October 16, 2024, 06:44:09 pm

I am an undergrad electronics student

When I was student (70s/80s) I used to repair (for money) any circuit that someone had a problem with it. In most cases I had to trace out relatively complicated schematics (learning by the way). The longest was 3 days spend on 15x30cm PCB from a car wheel balancer (what I have never even saw in working before as it was not times people change tires 2 times a year). I needed 3 days to have schematic (many transistors, 0 ICs, taking two A4 pages). Then having schematic of PCB only it took me next 3 days to got the idea of what external elements are connected to its connectors (AC supply, axle vibration detectors, rotation detector, analog meters to show results, switches to select working mode (taking measurement when wheel is rotating and showing at meters the angle and mass of the required weight) and then understood how it probably works (when showing results the AC 50Hz was used to get back from 2 samples the needed information and everything fully analog).
I managed to fix it even though I didn't have an oscilloscope or the whole machine to have real signals on the board.

The first step to understand any electronic is to discover its schematic. Your PCB is simply (I don't expect it being 4 layer with tracks hidden inside). One hour should be enough to draw its schematic (if you are not sure of connections help yourself with ohmmeter). You should faster have the schematic of what you have than you find in net a schematic of something that you are not sure if is even close to your device. Fully understanding can be complicated by ICs but general overview should be clear.