LED Ring Light Failure - Page 1
Forum section: electronics / repair

author: Lansteys
author_rank: Newbie
timestamp: August 18, 2025, 07:12:45 am
content: Hi All,- Not an Electrical Engineer -My partner has a LED ring light that failed recently.It has an external 240VAC to 18VDC, 4.5A adaptor. Label on the input of the device states it accepts 13-18VDC. Internally there is a main PCB and a smaller PBC that just extends the 7 segment displays that display brightness and colour temp, which are controlled by 2 POTs.Initial failure was reported as the LED’s suddenly turning OFF, as though unplugged. On inspection, I found that it would turn back ON when the brightness was reduced to below ~10%. Above this the adaptor seemed to be restarting repeatedly, as though the output was shorted.Guessing it was a power issue, I tried using a 12VDC, 5A adaptor (closest thing I had) which worked for initial testing, at all brightnesses, though the LED’s again turned OFF after ~30mins. This time they would not turn ON again when brightness was reduced and the power supply entered the same restart loop as if shorted.Next I tried replacing the voltage regulator on the PCB, though nothing changed.There is a buzzing coming from one of the components on the PCB, though I was unable to isolate this to a single component.Nothing is getting obviously hot and nothing looks burnt or to have released magic smoke, etc.This is about the extent of my ability to diagnose the issue, though I’m hoping someone can point me in the direction of further diagnosis/repair steps? I’m assuming the main part of the board is a typical LED driver circuit?I have a multimeter, no scopes or adjustable power supplies. Happy to attempt any measurements, etc.Thanks in advance for any clues anyone can provide!

author: Uunoctium
author_rank: Frequent Contributor
timestamp: August 18, 2025, 09:23:11 am
content: The unit appears to contain two independent, adjustable LED strings, each driven by its own PWM step-up (boost) converter. This is necessary because the combined forward voltage of the current-controlled LED chains exceeds the input voltage supplied by the external PSU.Both LED drivers are based on typical application circuits of the Richtek RT8482 LED controller (datasheet: RT8482https://media.digikey.com/pdf/Data%20Sheets/Richtek%20PDF/RT8482-00.pdf). The controller supports both digital (PWM) and analog dimming. In this design, overall brightness control—including decoding of the digital potentiometer and management of the display —appears to be handled by an STC15 series MCU (datasheet: STC15F2K60S2https://www.stcmicro.com/datasheet/STC15F2K60S2-en.pdf).The RT8482 provides built-in protection features such as over-temperature and over-current shutdown. A likely failure mode could be related to the electrolytic capacitors in the boost circuit: over time they may dry out, resulting in excessive ripple. In that case, the current-sense resistor would no longer represent the average LED current but instead the peak ripple current, which may far exceed the programmed limit.A practical next step would be to desolder and test the electrolytic capacitors (e.g., C18 and the capacitor above D5, label unclear).

author: tautech
author_rank: Super Contributor
timestamp: August 18, 2025, 09:38:20 am
content: Quote from: Lansteys on August 18, 2025, 07:12:45 amYup strong clue.Most likely failed short LED chips and the controller is sensing excessive current draw to protect itself and the other remaining good LED's.If your DMM has a LED test mode easy to find the culprit or get yourself a pair of SMD smart tweezers with a LED test mode to more quickly check the LED array.Replacing them can be fun depending on the substrate they are soldered to because if it is one big heatsink it's easy to melt the LED's soldering the buggers in.....don't ask how I know.You might get lucky and the traces aren't one big heatsink but as the LED's are soldered using reflow with carefully controlled temps you'll have to carefully solder in replacements being careful to get their polarity correct.Removing dud ones can be done with a scalpel and later remove their remaining pads with an iron leaving a clean pad which you might have to remove a little solder mask to increase pad size to help with fitting the replacements.(currently repairing LED ceiling fittings with multiple LED chip failures....)

author: cozza
author_rank: Regular Contributor
timestamp: August 19, 2025, 09:29:21 pm
content: The original power supply is 81WThe supply used for testing is 60W, perhaps it is being overloaded slightly by the current drawn by the ring light and going into over current or thermal protect mode after 30 minutes?Can you test the 18V power supply with an external load? Or measure the current drawn at 12V?

author: Cyclotron
author_rank: Unknown
timestamp: August 19, 2025, 09:35:07 pm
content: Quote from: tautech on August 18, 2025, 09:38:20 amIt’s helpful to use low temp solder. Seems to work a treat for leds for me.

author: tautech
author_rank: Super Contributor
timestamp: August 20, 2025, 10:51:50 am
content: Quote from: Cyclotron on August 19, 2025, 09:35:07 pmCertainly when you can.However on another smaller bulkhead mains fixture repair that uses 18x 18V 0.5W 30mA LED's on analuminum PCBnormal soldering procedures don't apply.A certain amount of diagnosis was required to just determine the type and rating of the LED chips used.......finding them online was easy.But just getting the old LED chip pads off the PCB is troublesome....these just aren't made to be reworked.Probably I can use preheat and hot air with solder paste but that's another project for a rainy day.....

author: Lansteys
author_rank: Newbie
timestamp: August 30, 2025, 03:32:37 am
content: Thanks for all the tips!I’m working through these slowly.Can confirm that all LED’s light up when tested with DMM in diode test mode. Readout shows ~2.95V.LED’s light up in groups of four. Not all appear the same brightness, but I wouldn’t say any where particularly dim or anything.I will order some electrolytic capacitors as I don’t have any the same value mentioned “(e.g., C18 and the capacitor above D5, label unclear)”.Following this I’ll attempt to test the original power supply under load.Thanks again!!