Help with repair to led light - Page 1
Forum section: electronics / repair

author: RobH
author_rank: Regular Contributor
timestamp: December 24, 2023, 05:27:38 pm
content: I was asked if I could repair this led light as none of the 3 leds came on. It is powered by x3 1.5v AAA batteries and they all read about 1.4v.I tidied up some of the wires and measured 4.33v between VDD on the pcb and -VCC on a battery point. I can't see what is missing , thinking there should be a +VCC wire connection to the pcb, but I don't know where.The red and yellow wires go to a small rocker switchI have just read on google that VDD is the Positive Supply Voltage and VSS is 0V or ground.. I can't help thinking that some part of the wiring is wrong, ie the black wire from the -VCC of a battery.I tried this but no leds lit up.Any ideas pleas

author: Haenk
author_rank: Super Contributor
timestamp: December 25, 2023, 09:00:26 am
content: Something has been messed up. I assume that switch just shorts the 3 batteries (it's hard to tell).Wheter or not that black wire (-) is correctly attached to the PCB, you are still missing (+) to the PCB.Very likely the yellow wire should connect to the PCB, somewhere, and certainly *not* to the (-) of the battery holder.The solder blob below the "D6" marking looks like a good candidate, but check with a magnification glas before, there should be some hints to a once attached cable.

author: Ian.M
author_rank: Super Contributor
timestamp: December 25, 2023, 09:17:19 am
content: Take a close look at the battery holder contacts. *IF* they are consistent with the hand-written polarity markings, then it would seem that the original fault was both wires from the battery + switch breaking off the PCB board, then the previous repairer soldering them in stupid places.I agree with Haenk: I *think* moving the PCB end of the black wire to the soldered pad next to the silkscreen 'D6' ('VSS') and moving the yellow wire from the battery holder to where the black wire was on the PCB ('VDD') may fix it.However if there's evidence the batteries may be in backwards, the black wire may actually be positive and in the right place, and the yellow wire switched negative, to be reconnected to the pad by 'D6', and the messed up colour coding is simply a consequence of using badly paid semi-skilled labour to assemble it with no quality inspection other than "Does it switch on?".N.B. If it has ever had both wires connected with reverse polarity the odds are the COB is blown and it will never work again.

author: RobH
author_rank: Regular Contributor
timestamp: December 25, 2023, 03:38:42 pm
content: Thanks for the replies, and the batteries are in the correct way, as I checked that first. Yes I think that someone else ha had a go at this and messed up somewhere.I moved the 2 wires as was suggested, but with just 3v now from the batteries, none of the led lights even show any signs of coming on.The pictures show what it is like now, after moving the wires.

author: Ian.M
author_rank: Super Contributor
timestamp: December 25, 2023, 04:49:00 pm
content: So something's pulling the voltage down. The COB is probably bad and if so, is shorting the batteries. There are a couple of things to check before giving up: Is the voltage drop at each battery(so all at 1V with it on) or is it all in one place due to a bad connection e.g. at the switch or one of the battery contacts? If so, fix the bad contact. If not, knowing its probably blown anyway, try swapping the yellow and black wires at the PCB (i.e. put the black back where it was), on the assumption the PCB VDD and VSS markings are wrong.However we are grasping at straws here and most likely you'll finally return it whence it came as not repairable and if you get any grief, tell them to be glad you aren't charging £30 to look at it!

author: RobH
author_rank: Regular Contributor
timestamp: December 25, 2023, 06:13:45 pm
content: Each battery reads approximately 1.1V to 1.2V now and I have 3.5V between the red wire on the battery and the black wire on the pcb connections. I must have had a dodgy connection before with my MM probes.Swapping the yellow and black wires over doesn't get it working either.Anyway, thanks for your help and time , and I'll give up on it now. I'll give the person who it belongs to , the bad news tomorrow

author: Ian.M
author_rank: Super Contributor
timestamp: December 25, 2023, 07:19:07 pm
content: 1.1-1.2V per cell is low enough it may not work anyway. If you can, put it on a bench supply @4.5V, current limit set to 500mA, connected positive to VDD, negative to VSS and see if it shows any signs of life. Otherwise try fresh batteries, but only switch on for a couple of seconds so as not to kill the batteries, as I reckon even dollar store batteries are worth more than it is, and the odds of it working are less than winning the lottery with a ticket found blowing down the sidewalk!

author: RobH
author_rank: Regular Contributor
timestamp: December 25, 2023, 08:29:38 pm
content: I tried what you suggested , bench PSU at 4.5v and 0.012A, positive on VDD and negative on VSS, nothing! I then reversed the connections with positive on VSS and negative on VDD, again nothing.So I guess that is the end of the line now.Thanks again