Mini Trees - how are they wired?

[RichieNorthcott]

Hi - I'm wondering if an electronics guru has any thoughts on the following? I have a reasonable understanding of electronics but without dissecting this product it's proving hard for me to work out...

I have a set of 6 'mini trees' each of which has 15 LEDs (so 90 in total). Each tree is connected together, the entire set runs off a small 24Vdc transformer. All sorts of retailers have these, I'm sure they're from the same factory!
Screen Shot 2019-12-07 at 17.29.47 copy.png

I wanted to split these into two, so I cut at the half way point as shown below. This drawing is NOT intended to show how they are actually wired, that's what i'm trying to work out. Each tree does link to the next however.
trees.jpg

My findings were as follows:

- Trees 1-3 did not work without reconnecting the cut 'link wire' back to itself. This suggests the six trees are in series, not in parallel as I had first thought.
- Running off 12v, trees 1-3 are less bright than they were before. These needed the circuit closed as per previous point (seems odd...)
- Trees 4-6 are brighter than they were when connected to 12v at the cut point

So this suggests there are some differences between the six trees, maybe the first/last has some extra resistance say?

Does anyone have any thoughts on how these might actually be wired? My ideal would be to have two sets of three trees which can be run from 12v, also dimmed as if they were LED strip. I was looking to use two channels on some existing 12V LED dimming hardware which I have.

The documentation with the trees mentions the LEDs are each 3v 0.015W (so 5mA per LED), the transformer is rated 3.6VA.

My other finding was some weird spikes in my DMX signal when connecting through my control systems - my thinking on this is there might be some capacitors or inductors present. So these were affecting the signal to other lights in the same system. I have disconnected them for now in case they damage anything.

[dirknerkle]

I'm going to guess that "essentially," the 90 lights are akin to a single string of lights just spliced with connectors. Are they are numbered, too so that the connections always go 1->2, 2->3, 3->4, etc.? If that's the case, I'll bet that hidden somewhere on a couple of the trees are some rectifier/current control pods spliced into the wire.

I'd measure the voltage at the outputs of each of the trees to see whether it's constant throughout or whether it decreases with each tree. If it decreases it's more indicative of a series setup while if it stays constant, it's likely more of a parallel setup.

How many wires are in the cable that goes from tree to tree and between the bulbs? If it's 3 wires, one of them is likely a "through" wire and connects elsewhere to a bulb to re-energize a new set of bulbs.

But.... I'm just guessin' here....

[RichieNorthcott]

Hi thanks for the reply - they are permanently wired so no connectors. It's a 2 core wire between each tree. So each tree has a wire 'in' and 'out' except for the last one. I agree with your thoughts re the rectifier/current control pods, that would explain what I'm seeing to some extent.

[Barnabybear]

Hi, just a guess / thought based on stuff and a couple of beers (it’s late UK side). What can we pick out of this?

A white LED needs at least 3V to light even at the manufacturer’s spec of 5mA confirms this they say the LED’s are 3V.

3V * 15 LEDs would need 45V to light each tree if the LEDs where in series and transformer is only spec’d at 24V.

If we split each trees LEDs into two sets as fifteen doesn’t divide by two this means that one set would have eight LEDs and the other seven LEDs. Given the price of LEDs and the fact that the manufacturer has designed this fifteen must have a meaning? More on this later.

There are only two wires between each tree.

You connected them to 12V and each half lit. Given the 3V minimum required for each LED this means that at most there can only be a maximum of four LEDs in series in any point in the circuit.

Again this doesn’t divide by fifteen but three does.

For 12V to light fifteen LEDs my take would be that there are five sets of three series LEDs in parallel in each tree. Are there third wires in each tree between sets of three LEDs?

At this point I’m inclined to think that there are three series circuits of three LEDs in each tree making a total of eighteen series circuits all in parallel to be fed with something just over 9.6V.

A good question is why the ‘transformer’ is rated at 24V?

It could be that the ‘transformer’ which I’ll call a power supply from now on is just a capacitor dropper and feeds about +12V and -12V to two opposed sets of LEDs. Whilst you could class this as 24V DC it stretches the imagination somewhat.

- Trees 1-3 did not work without reconnecting the cut 'link wire' back to itself. This suggests the six trees are in series, not in parallel as I had first thought.

From above this sounds about right.

Running off 12v, trees 1-3 are less bright than they were before.
I’m guessing as you suspected there are some current limiting resistors in the first part of the string.
My other finding was some weird spikes in my DMX signal when connecting through my control systems - my thinking on this is there might be some capacitors or inductors present.

I’m not sure if you have the opportunity to check the original wiring connection between the two haves but it wouldn’t surprise me if the second half of the LEDs where in the opposite polarity to the first three and the power supply was just a capacitor dropper – no rectifier and a current limiting resistor.

Anyway that gives you something to think about.

[RichieNorthcott]

Thanks so much!

Transformer is I believe a normal 'wall wart', I measure 24v on the output terminals with a voltmeter with no load.

I think I need to have a poke around one of the trees a little better, it's not as easy as it sounds, everything is very small and hidden!

Also note there were some differences between when I connected 1-3 on their own (needing to close the output wire) and 4-6 on their own - this suggests all trees are not the same, just need to work out what the circuit of 90 LEDs looks like altogether as a whole. 1-3 were quite dim, 4-6 were over bright. The original brightness was probably somewhere in the middle of the two for the set of 6. I haven't been running as I believe I am applying over-voltage in the second case.

[oh and thanks for your help/info on the Arduino implementation - my Wi-Fi controller is actually working quite well, but I got the ethernet-Arduino version working anyway - I'll post a video at some point]

[Barnabybear]

Hi, my mind got to thinking about this again today. I played with some 32V AC strings earlier this year (or last I can't remember) the schematic is below:

LED Reindeer_2.jpg

These could offer an explanation as to the wiring on your trees. Make the supply 24V DC, make all the diodes face the same way, make it only six groups instead of the ten shown and add a couple of current limiting resistors.

[RichieNorthcott]

Definitely a DC supply, confirmed both in the instructions and measured with a DC voltmeter. I'm going to have a poke around the trees a little more and see if I can work out what's going on....

[RichieNorthcott]

Okay so on closer inspection the below is I THINK how they are wired. The instructions also mention a series-parallel arrangement, without going into detail.

So the LEDs are in parallel on each tree, there is some sort of box of tricks on the input to each one. I can't tell you what it is because it's sealed up in plastic. So like this:
trees.jpg
I'm trying to work out when the 'thing' is!

To convert for 12V use, I know it's not recommended (I think) to wire LEDs in parallel with a 'shared' resistor, but for each tree (assuming the LEDs are in parallel) then total current at 3V is 75mA (15 x 5mA), and the voltage drop resistor I'd need is 120 ohms therefore? I'm keen to run them from my 12V striplight DMX decoders... I could then have each tree on its own channel for Christmas light show epic-ness. Is it as simple as that or is there more to it? The minimum power rating of the resistor would be 0.675W. i have a bench PSU so can do some testing, it does mean dissecting a tree though!

**edit**
Okay I did this using the bench PSU and a 1W 120 ohm resistor. The resistor gets fairly warm, but it's not going to be a show stopper. Are there any better options for dropping the voltage/limiting the current? I'm looking at each tree in isolation now (so a parallel chain of 15 LEDs), they it's being controlled by 12V PWM so I don't want to be introducing any capacitors etc. It needs to stay responsive to dimming.

One thought I did have is perhaps there is NO additional components, and the system just relies on the voltage drop across a 15m not very thick cable? Without the drop each tree would receive 4V, maybe it's as simple as relying on 4 dropping to 3?

[Barnabybear]

Hi, yep everything in that post looks good. If you use 2 x 56 or 68 ohms (feed and return) then 0.5W give a good overhead even accounting for the heat shrink you’ll probably put over them unless you live somewhere hot (you need to finish your profile).

[RichieNorthcott]

Hi, yep everything in that post looks good. If you use 2 x 56 or 68 ohms (feed and return) then 0.5W give a good overhead even accounting for the heat shrink you’ll probably put over them unless you live somewhere hot (you need to finish your profile).

North London is quite chilly point taken...