Fixing a prelit LED Christmas tree - Page 1
Forum section: electronics / repair

author: kevinheb
author_rank: Newbie
timestamp: January 31, 2025, 11:16:00 am
content: Hi everyone, I would be grateful for any advice with this.Over Christmas, I noticed that our Christmas tree had lots of lights that weren't on. The tree was bought a few years ago with pre installed strings of LED lights. I have previously replaced a few LEDs that had stopped working, but currently it seems as though a third of the lights aren't working.The tree is powered by a 24V power supply that leads into a controller that sends out 3 strings of lights. The tree is in 3 sections with a 3 pin plug that fits into a socket on the lower section.I have taken apart the PSU to expose the cables, and have found that the voltages on each string are 12V, 12V, -12V. I have also tested continuity along the bottom section of the tree, and each string has continuity despite some of the LEDs not working.I confused whether or not the -12V is correct, or is this likely to be the problem, a faulty power supply?Any help would be gratefully received (and I'm looking to sort this issue out so that I can put the tree back up in the loft)

author: Zero999
author_rank: Super Contributor
timestamp: January 31, 2025, 11:36:12 am
content: What to you mean by -12V, 12V and 12V? Where's the 0V? Does it have a +/-12V power supply? Or are you talking about a single 12V supply, which would conventionally referred to as +12V and 0V?Have you tested the LEDs on a bench power supply? I suspect it's more likely the wiring, than the power supply.

author: bingo600
author_rank: Super Contributor
timestamp: January 31, 2025, 12:05:17 pm
content: Quote from: Zero999 on January 31, 2025, 11:36:12 amMy guess is swapped probe pins on one of the measurements

author: kevinheb
author_rank: Newbie
timestamp: January 31, 2025, 12:22:19 pm
content: Hi there, sorry I wasn't clearer, with the black probe of the multimeter connected to the neutral input of the power supply, I pushed the red probe into each of the 3 sockets of the 3 pin connector, and got 12V, 12V and -12V (I've attached a picture to hopefully explain clearer.Yes I did test one of the LEDs that wasn't lighting up on a bench power supply with 3V and it did light up.with thanks

author: kevinheb
author_rank: Newbie
timestamp: January 31, 2025, 06:20:37 pm
content: Definitely not swapped the probed pins for the -12V reading, left the black probe where it was, and just put the red probes into each of the sockets of the connector

author: ozcar
author_rank: Frequent Contributor
timestamp: January 31, 2025, 06:26:05 pm
content: Some random thoughts/comments.Any voltage supplied to the LEDs from the controller would be across the output wires of the controller. There might be continuity between one of the (24VAC) input wires and one of the output wires, but I would not assume that without checking.Normally controlling three sets of LEDs independently would require four wires (one "common" of either polarity, and then one wire of opposite polarity for each of the three sets). However, sometimes they rely on the fact that LEDs are, well, diodes, with not all of them connected "the same way around". For example you can have two sets of LEDs independently controlled with only two wires, with the voltage across the wires periodically reversing (so sort-of AC and probably too fast for you to see what is happening with just a multimeter.).How are the sets of LEDs connected to those three output wires? In series, parallel, or some combination, and are they all wired the same way around?There are also more fangled LEDs, where a large number of them can be controlled independently with only three wires, but I don't think that is what you have there.

author: kevinheb
author_rank: Newbie
timestamp: February 03, 2025, 06:45:55 pm
content: I have tried inspecting the lights to find out the answers to your questions but it is very difficult to trace the individual wires because they route all around the tree and are bundled together.How would I tell if the sets are in serial, parallel or a combination, and certainly no idea of knowing if all the LEDs are in the same direction because of the routing.

author: themadhippy
author_rank: Super Contributor
timestamp: February 03, 2025, 07:42:29 pm
content: id use a 9v battery and try all combinations of wires and polarity,use different coloured tape over the leds to identify what lights with what wire combination

author: ozcar
author_rank: Frequent Contributor
timestamp: February 04, 2025, 07:09:30 am
content: I suggested, or at least hinted, that you should measure the voltage between the three output wires, but you don't seem to have done that. If you call the wires 1, 2 and 3, measure from 1-2, then 1-3, and then 2-3 (ie black probe to 1 with red to 2, then black to 1 with red to 3, and finally black to 2 with red to 3). I'm guessing that the different sections/sets of LEDs light up at different times. However, it is unknown to me whether each section just turns on and off, or perhaps fades in and out. I also don't know if the controller gives you some way to influence the behaviour - if it does set it to produce a static pattern on the tree if possible or you will have a moving target. Even with a static pattern, the voltages may not be static, and may be varying too fast for you to see with a meter.You have mentioned replacing LEDs in the past (BTW, what type are they?), so presumably you can get to the wires going to the individual LEDs. You could try measuring the resistance (with the thing turned off) from each of wires that go to a particular LED, to each of the three pins that plug into the controller in turn. Do this for at least one LED that works, and one that does not, but preferably do it for several of the LEDs in each category. Without knowing how the LEDs are connected, the results may be difficult to interpret, but might give some clues.Can you open the controller box any more without destroying it? If so get some pictures of the inside showing what those three output wires connect to.

author: Fryguy
author_rank: Regular Contributor
timestamp: February 04, 2025, 08:04:19 am
content: Quote from: kevinheb on January 31, 2025, 12:22:19 pmThis sounds to me as if some of his string wires are broken .He did mention that this tree is already a couple of years old . . .Maybe it is time to just change the lights on this tree