Christmas tree lights 24v or 26 power supply  - Page 1
Forum section: electronics / general

author: MrMobodies
author_rank: Super Contributor
timestamp: November 13, 2022, 06:48:11 pm
content: I had been given this power supply to look at from a Christmas tree light that stopped working.I plugged it in a draws no load and I put the probes on the dc output and no voltage.JT-DC24V7.2W-E2Quote24/7.2=3.333 amps26/7.2=3.611 ampsMade mistake with calculation as Zero999 pointed out.7.2/24=0.3 (300 milliamps)7.2/26=0.27 (270 milliamps)I see many of these selling at all sorts of voltages but not with that exact part number or ones with those two different voltages. I have no means of measuring it to find out which as it doesn't power.I found a video of Bigclive tearing one of these down.https://wwwyoutube.com/watch?v=5EsnrO45ObAI opened it last night which is a bit different inside to the one Bigclive tore apart and I see no damage and no smell.Maybe I will post the pictures later.Google search:https://www.google.com/search?q=%22JT-DC24V7.2W-E2%22The closest I can find is this listing:https://www.ebay.co.uk/itm/255757493895CHANGZHOU JUTAI 24V DC FAIRY LIGHT POWER SUPPLY JT-DC24V7.2W-E1 (CH 1199)QuoteBoth 24V.I am not sure whether the leds need 24v or 26v, what Urated or Prated corresponds to in order to work out the correct power supply and if there are other components that maybe needed on the output as I don't want to blow up the LED'S.I messaged a few sellers selling them to find out if they sell that exact that model but not response from them.What do you think?

author: Zero999
author_rank: Super Contributor
timestamp: November 13, 2022, 07:26:33 pm
content: It's probably current limited. 26V might be the open circuit voltage and 24V the nominal, with the LEDs connected.You calculation is wrong. It should be I = P/V

author: MrMobodies
author_rank: Super Contributor
timestamp: November 13, 2022, 08:45:36 pm
content: So I got to find one that is 26v unloaded and 24v loaded or maybe easier, find one that is clamped at 24v with at least a 300ma load.I'll look at my box of power supplies. I have some adjustable regulators I brought 10 years ago that might do it.Do you think I better add a fuse if the power supply ampage is significantly greater?

author: MrMobodies
author_rank: Super Contributor
timestamp: November 13, 2022, 11:30:38 pm
content: I am not sure what to look for and I don' see anything that matches the open circuit and nominal voltage.I have found something and many like it but I am still confused with the voltages and what they mean:https://www.savemylight.co.uk/product/self-slt6-350ils-24v-6w-led-driver-constant-current/SELF SLT6-350ILS 6W LED DRIVER CONSTANT CURRENTQuoteQuoteI don't suppose this will be okay?

author: eti
author_rank: Super Contributor
timestamp: November 13, 2022, 11:38:10 pm
content: TBH mate you can buy a whole new set of lights *including* one of these, for about £6-15 - once the sealed unit is open, it will have lost its safety rating (not sure how these things are rated… “IPXX”?). I’d write it off and replace it if it takes more than an hour to fix, and ensure they’re not gonna use it outside!

author: MrMobodies
author_rank: Super Contributor
timestamp: November 13, 2022, 11:42:52 pm
content: I was told that the lights were run outside and that is interesting that you mentioned it.I am going to ask them if they are indeed rated for outside use.

author: eti
author_rank: Super Contributor
timestamp: November 14, 2022, 12:03:07 am
content: Quote from: MrMobodies on November 13, 2022, 11:42:52 pm🙌😃 Good old boy.

author: Zero999
author_rank: Super Contributor
timestamp: November 14, 2022, 10:25:34 am
content: Quote from: MrMobodies on November 13, 2022, 11:30:38 pmThat appears to be a constant voltage/current power supply. It will give 350mA, into a 7.5V to 21V load and will have a constant output voltage of 24V to 25V, as long as the load current doesn't exceed 300mA. It might overdrive the LEDs a little. Measure the current and if it's too high, add a series resistor. You might have to play around with different values. The current should ideally be measured with a clamp meter, but 300mA is a little low, so you can wrap multiple turns through the clamp, and divide the reading, by the number of turns.

author: madires
author_rank: Super Contributor
timestamp: November 14, 2022, 12:41:47 pm
content: Many outdoor LED lights come with those 24V wall warts (rated IP44). We got some too. Presumably a new wall wart costs as much as new lights.

author: tunk
author_rank: Super Contributor
timestamp: November 14, 2022, 01:07:25 pm
content: Maybe you could post pictures of your dead one?It could be a simple fix.If you have a bench PSU then you could try to startat e.g. 15V and ramp it up, and monitor the currentdraw.

author: Zero999
author_rank: Super Contributor
timestamp: November 14, 2022, 01:15:06 pm
content: Quote from: madires on November 14, 2022, 12:41:47 pmCounterintuitively, it still might be worth it. If the power supplies which come with new lights are crappy and tend to fail long before the lights, it might make sense to buy a decent quality one separately, even if it costs more than a new set.

author: madires
author_rank: Super Contributor
timestamp: November 14, 2022, 03:11:15 pm
content: Of course, the wall warts included are usually of the cheap variety. If you replace them with a good quality one you might need to change the connectors. The lights have a modified DIN speaker connector.

author: MrMobodies
author_rank: Super Contributor
timestamp: November 14, 2022, 03:59:59 pm
content: High resolution images attached.Flipped image to match the above:Used Peekview to merge the back and front:Flipped view:I tried to sharpen it a bit further with Irfanview.https://imgur.com/a/Vw4Ggs6

author: tunk
author_rank: Super Contributor
timestamp: November 14, 2022, 04:08:43 pm
content: A bit difficult to see, but some of the solder jointscould be suspect (cracked?). No expert, but maybeyou could test the green capacitor?

author: MrMobodies
author_rank: Super Contributor
timestamp: November 14, 2022, 04:29:34 pm
content: Took some more pictures on the sides:Higher resolution images attached.I'll test the capacitors soon.

author: AVGresponding
author_rank: Super Contributor
timestamp: November 14, 2022, 04:51:20 pm
content: Big Clive has loads of videos on fault-finding and repairing simple switchers like this. Might be worth watching a few. Usual suspects are failed electrolytic caps, sometimes they take out surrounding components unfortunately.Shorted windings on the trafo are less common but not impossible; with no power being drawn, the fusible resistor may well have blown.If you decide to source a replacement supply, I wouldn't worry too much about exactly matching the output voltage, these things aren't precision devices and +/-10% should cause no issues, the lower the better (within reason) as the LEDs are likely being overdriven anyway, so dropping the supply voltage to 20 or 22 will probably make them last ten times as long, for very little difference in brightness.Remember to check the 400V caps are discharged before you stick your fingies on 'em!

author: madires
author_rank: Super Contributor
timestamp: November 14, 2022, 05:27:00 pm
content: ... shorted diode on the secondary side or shorted bridge rectifier.

author: MrMobodies
author_rank: Super Contributor
timestamp: November 14, 2022, 05:29:15 pm
content: Thanks. The green 4.7uf cap measuring 4.59uf.Tracing from the live terminal to BD1, the resistor in the middle doesn't seem to measure anything. I think that resistor might have sometime to do with it.Just noticed what looks like a crack in the middle.Left: Red Black Black GoldenJust found:https://www.make-it.ca/resistor-color-code-calculator/So that will be:20 Ohms +/- 5% tollerance.So something else is likely to have failed to cause that resistor to break.Just noticed something.F1 next the resistor.So that is acting like a fuse?Took a look closer RF1 and I guessed it, a fusible resistor:Another crack on the side.Apart from the color coding it seems to looks just like the one in this link:https://www.elprocus.com/fusible-resistor/No signs of overheating so maybe it shorted out somewhere.Thanks for your help.I'll see if I can order another one and see what happens.

author: Zero999
author_rank: Super Contributor
timestamp: November 14, 2022, 06:05:13 pm
content: My bet is U1, which contains the switching transistor.

author: MrMobodies
author_rank: Super Contributor
timestamp: November 14, 2022, 06:39:34 pm
content: If the power supply works again then the problem was likely triggered outside from the cables or lights and this could be a waste of time but interesting.What I forgot to mention was that they wired it around a tree from a window.Now thinking about this I feel a bit stupid that I didn't take that into consideration.They wanted a replacement power supply but that could blow too in the same way.When I meet them next time I'll ask if they can take the down the lights and give them to me to test.