Repairing el-cheapo LED downlight. Your thoughts? - Page 1
Forum section: electronics / repair

author: Peter Tryndoch
author_rank: Contributor
timestamp: February 06, 2019, 02:57:59 am
content: I recently visited a friend's shop. She had a blinking LED downlight (One Hung Lo at it's finest) so I took it away to see if I can fix it.I only took the light and not the PSU which was hard wired to 240V and I had no tools with me. Secondary = 20 - 42V, 350ma. ("42" yey!) My psu only goes up to 30V, so this is what I tested with.The leds are arranged in approx 6 banks of 10 leds in series each. (I do say "roughly" because I did not count them or pay too much attention to it either)4 leds, 1 in each bank were dead. Shorting them lit the others ok. I replaced these with some I had lying around which looked "roughly" similar. This was not an easy job as the leds were made of melty plastic which covered the pads completely and the solder tabs were underneath. I wonder how they managed to solder these on, either with a reflow oven or hot air blowy thingy. They melted at the mere sight of my soldering iron. after attaching 4, 2 of them failed, so I just grabbed 2 regular 3mm Red leds and soldered them in. After all, all I cared about is getting the volt drop across them and not how they light up. (dim Red). Tested, returned, plugged back in and working. Don't know for how long.OK if you are still reading this, here are some questions:Does the voltage drop across the Red leds differ to the original ones (White phosphur), and what sort of life shortening can I expect? I know this can be a bit vague, but give me an approximation.During testing, I noticed that some of the banks with faulty leds would light up briefly as I turned up the voltage from 0 to 30. But in the shop, all the leds were blinking. Is there some sort of weird protection cct in the psu? "Oh the current is too low - restart!" I originally swapped the lamp with a good one and the fault followed the lamp, so the psu was ok.

author: sleemanj
author_rank: Super Contributor
timestamp: February 06, 2019, 04:42:59 am
content: Blinking typically indicates that a bond-wire (microscopic gold wire inside that connects the silicon die to the led package's connections) has come unstuck, LED heats up, bond wire expands and lifts off die, connection breaks, LED cools, bond wire contacts and touches die again...A red LED die has a forward voltage around 1.7v, a blue LED die (which is what is used for white) is about 3v. A single LED package may contain more than 1 die, and those die can be connected in series or in parallel or both.If the supply is constant current, the differing forward voltage won't make a difference as long as it's still in the supply's output voltage range, the current will be the same, since, it's constant current, more important then is the ability for the replacement LED to handle the same current as the one you replaced would have (should have, but didn't).

author: Peter Tryndoch
author_rank: Contributor
timestamp: February 06, 2019, 09:48:04 am
content: I agree that something heating up, going open, cooling then close is the most common explanation for blinking in this sort of circuit.However when I was testing at 30V it should have in theory blinked faster (less heating - faster cooling) but it did not.As I ramped up the voltage, some of the segments would start to glow and then cut out, and only relight after I had dialed back to 0V (or low voltage anyway) they didn't blink. At the same time the good segments were permanently lit. I could have sworn that in the shop the whole light blinked on and off. But now I'm starting to have self doubt.Next time I will use some White 3mm leds.I warned my friend that that the light might work for only a day, week or longer. There about 5 years old so she got her moneys worth out of them.Oh and each led only had one die in it.Thank you for your response.