Broken LED Lamp - Page 1
Forum section: electronics / beginners

author: Balefati
author_rank: Contributor
timestamp: March 16, 2023, 03:59:57 pm
content: Hello everyone,a few weeks ago my lamp from my front door stopped working. I opened it up, changed the blown fuse and and it worked again. Now it stopped working again. I opened it up again and water leaked into lamp. One component was burnt. I assume it was a diode. But it is to burned to read anything.I desoldered the burned component and powered the circuit to see what happens (only for a few seconds at a time). The lamp works fine. I took pictures of the front and back of the PCBAnd also tried to reverse engineer the circuit and draw a diagram. The "?" is the burnt component.I measured the output of the bridge rectifier to be 98V. The LED PCB is populated with 30 LEDs in series i assume. The LEDs are labled with 0.2W max each.Maybe the burnt component was a Zener Diode to prevent the LEDs from dieing when a voltage peak occurs.Now my question is whether i can run the lamp as it is right now or do i have to replace the ?diode?Sorry if this is very obvious. I'm not the brightest mind when it comes to electronics. But i love to repair and tinker with stuff before throwing it away.Thanks in advance

author: nekelly
author_rank: Newbie
timestamp: March 16, 2023, 05:13:33 pm
content: I'm guessing the missing component is a capacitor.

author: Balefati
author_rank: Contributor
timestamp: March 16, 2023, 05:18:54 pm
content: Quote from: nekelly on March 16, 2023, 05:13:33 pmI'm relatively sure it's a diode of some sort. But just in case i'm gonna add a picture of it:

author: mariush
author_rank: Super Contributor
timestamp: March 16, 2023, 06:51:46 pm
content: Maybe a zener diode as a sort of protection for over voltage on the leds?

author: pcprogrammer
author_rank: Super Contributor
timestamp: March 16, 2023, 07:37:41 pm
content: The silkscreen on the PCB shows the symbol used for a diode (rectangle with a smaller rectangle at one end) and the text TVS. So it is some transient voltage suppressor, but the question is the voltage it was rated for.It did get hot, judging by the browning of the pcb.Edit: I made the markings red. See attached picture.

author: Balefati
author_rank: Contributor
timestamp: March 16, 2023, 07:54:46 pm
content: Quote from: pcprogrammer on March 16, 2023, 07:37:41 pmI see, thought it was TU and then something. Thanks for thatIs there a way to determine what voltage the zener diode has to have? The voltage coming from the bridge rectifier is 98V. I could also measure the current going through the LEDs if thats necessary. But i dont think its to much. I think its all LEDs in series so it shouldnt be more than 50mA, or maybe 100mA. I Know that regular voltages for zener diodes around this voltage are for example 91V and 100V. Would the zener diode voltage have to be higher or lower then the voltage for the string of LEDs?

author: pcprogrammer
author_rank: Super Contributor
timestamp: March 17, 2023, 06:16:39 am
content: Quote from: Balefati on March 16, 2023, 07:54:46 pmI could be wrong of course, because the picture is low resolution and when enlarged it deformed and can be something else. Though TVS makes sense for a protection diode.Quote from: Balefati on March 16, 2023, 07:54:46 pmIt depends on the circuit if it is used as a transient voltage suppressor or as a regulator based on a zener diode. Due to the browning of the pcb it is a bit hard to tell how the connections are. Could you draw up the schematic?The resistors on the board will provide a clue about the current, because if they are in series with the leds the same current runs through them. With the rated power of them and the voltage across them you can do calculations the see if the current might be to high.Also the leds will have fixed forward voltages that vary a bit with the current through them. Based on the number of leds you can determine this voltage. What is the wattage rating for the lamp? This will provide insight in the current too.The leds can be multiple series strings in parallel too.

author: Balefati
author_rank: Contributor
timestamp: March 17, 2023, 07:47:29 am
content: QuoteThere is a schematic in the very first post in this thread. The questionmark in the schematic is the diode in question. There are 30 LEDs each rated for a maximum power of 0.2W. On the label it says 230V 5W.

author: pcprogrammer
author_rank: Super Contributor
timestamp: March 17, 2023, 09:28:35 am
content: Quote from: Balefati on March 17, 2023, 07:47:29 amSorry my bad for not checking thatSince the TVS diode is parallel to the 510K resistor it is used to suppress transients. So its rating needs to be above the led plus resistor voltage. The 5W rating would mean only ~20mA drawn from the net. The 300 ohm resistor takes about ~0.14W. It looks beefy enough to take that. There will be ~6.5V across that resistor.You mentioned some 98V without the TVS diode and the lamp working properly, so I would say at least 100V, but it might be better to go a bit higher. The fact that it blew and burned the pcb shows it either protected your lamp for some heavy event or it was underrated for the job.The 98V minus the ~6.5 across the resistor gives ~3V per led, which sounds about right for white leds.

author: Balefati
author_rank: Contributor
timestamp: March 17, 2023, 09:49:03 am
content: QuoteThe heavy event was likely water getting into the lamp and shorting something out.I looked on the internet and found that zener diodes above 100V usually go up in steps by 10V so should i get a 110V diode? And what wattage should it be able to handle? I've seen diodes from 0.5W up to 5W of power handling. Or does it not matter to much since its just protecting from very short pulses of overvoltage?Thank you very much for your help so far

author: pcprogrammer
author_rank: Super Contributor
timestamp: March 17, 2023, 10:06:50 am
content: Yes some contaminated water can cause things to go boom. It then did serve its purpose right.Something likethiscould do the trick.These can surge quite a bit and will be better then a zener diode. They have a faster response time.

author: Zero999
author_rank: Super Contributor
timestamp: March 17, 2023, 10:25:23 am
content: As mentioned above, it's a transient voltage suppressor. It works by short circuiting high voltage spikes. The circuit will work with out it. It will just be more vulnerable to high voltage spikes. I wouldn't bother replacing it. Put it back together and if a high voltage spike fries the LEDs, just replace the whole unit.

author: Balefati
author_rank: Contributor
timestamp: March 17, 2023, 10:50:18 am
content: Yap, there was a little puddle in the bottom of the lamp. Didnt see any water where the electronics were, but some water most likely got in there as well.The Littelfuse diode looks very good. And its priced around what other Zener diodes would cost. But unfortunately they are not as readily available as Zener diodes. They are mostly availaible through big names like Mouser and DigiKey. Since i only need one, the shiping costs outweigh the price of the diode significantly (around 20€ at Mouser)I think a simple zener diode will have to do hereunless you know where to get them easily somewhere in Europe. Found one or two resellers in Germany but they only sell in large quantities above 100 pieces.Otherwise i would resort to getting something likethismaybeIts probably not going to live in the outside world anymore. I think i'm going to 3D print something for it and repurpose it as a lamp for soldering or when i have to do work on small things. Maybe print a much smaller housing for it and put it on a flexible arm of some sort.

author: Balefati
author_rank: Contributor
timestamp: March 17, 2023, 10:57:17 am
content: Quote from: Zero999 on March 17, 2023, 10:25:23 amThat might be even easier. Since its probably only going to be used for short durations every now and then anyways.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: March 17, 2023, 12:54:42 pm
content: Quote from: Zero999 on March 17, 2023, 10:25:23 amThat sounds like a bad Idea.There is no current limiting though the capacitive dropper, which means the output can be as high as the mains voltage, each time this thing is switched on, and it depends on the momentary mains voltage at the moment it's turned on. Relying on a TVS (or is it a "TUS on the silk screen?) to suppress this seems quite dubious.

author: pcprogrammer
author_rank: Super Contributor
timestamp: March 17, 2023, 01:09:33 pm
content: Aliexpress does sell them.https://nl.aliexpress.com/item/32978756942.html(You can search for the best deal there of course)I have bought several batches for lower voltages there and had no problems. But Ali is always a gamble.

author: pcprogrammer
author_rank: Super Contributor
timestamp: March 17, 2023, 01:21:59 pm
content: Quote from: Doctorandus_P on March 17, 2023, 12:54:42 pmWell maybe they provided the high impedance voltage divider to balance that. (The 2 510K ohm resistors) No expert on this, so can't tell if it makes any sense though.Maybe the TVS needs to have a rating above 230V then?

author: Balefati
author_rank: Contributor
timestamp: March 17, 2023, 01:34:35 pm
content: QuoteCan you explain what that means? I powered the lamp a few times now after i removed that diode. Was i just lucky it didnt blow the LEDs? Or is the duration simply not long enough and the LEDs can handle such a short spike? And what other measure would you suggest to protect the LEDs or what do you think was installed there in the first place? Again, i dont know too much about all this. Trying to learn as much as i can.

author: Zero999
author_rank: Super Contributor
timestamp: March 17, 2023, 01:59:59 pm
content: Quote from: Doctorandus_P on March 17, 2023, 12:54:42 pmI doubt it does anything. There's already a 300R resistor in series with the LEDs, which will limit the inrush current if it's turned on when the mains is at its peak. A better modification would be to put a capacitor in parallel with the LEDs, that will both soak up the inrush and smooth out the voltage, which will reduce flicker.

author: Balefati
author_rank: Contributor
timestamp: March 17, 2023, 02:58:50 pm
content: QuoteWhat type and capacity capacitor would you recommend in this case?

author: Victorman222
author_rank: Regular Contributor
timestamp: March 17, 2023, 04:06:59 pm
content: Electrolytic, rated for the mains voltage (not rms but peak rectified), i took a look at a few lamps i have and some common values are 4.7uF 400V, 6.8uF 400V, you could go larger, but make sure that the capacitor fits in the enclosure. Lamps like this without a capacitor will flicker with 100 Hz frequency, and while it is not easily visible, it can cause headache or even vision issues, you can check for flicker by using your camera and varying shutter speed, or moving your hand and checking if the motion blur is normal.

author: Zero999
author_rank: Super Contributor
timestamp: March 17, 2023, 06:03:27 pm
content: Quote from: Victorman222 on March 17, 2023, 04:06:59 pmIn this case you can get away with a lower voltage, since it's clamped by the LEDs, so 160V will do. The only downside to that is, if the LEDs fail open circuit the capacitor will explode, but modern electrolytic capacitors are vented, so it's not that dangerous. If you've got room, go for 400V.The capacitor forms a divider with the input capacitor, when the power is first applied so it's a good idea to choose a capacitor with a large enough value to ensure the output voltage is less than the LED's forward voltage, when the power is first applied. In this case 3.3µF would do, but I agree it's definitely a good idea to use a larger value to avoid flicker.

author: Balefati
author_rank: Contributor
timestamp: March 18, 2023, 01:30:43 pm
content: Thank you Zero999 and Victorman222I dont have any capacitors on hand but i had an old phone charger laying around that wastn working anymore. I figured it should have one to smooth the voltage. And voila, turns out it has two 400V 8.2µF. I'm gonna check them with my multimeter to see if there capacitance checks out to be what it claims and they havent gone bad which cause the charger to fail. If they are good i'm gonna solder one of them to the lamp. I'm gonna do a before and after, maybe i can see the flicker. But in anycase, the lmap works and i feel a bit better that it is somewhat protected from overvoltage and inrush currentThanks to all the people that helped

author: Balefati
author_rank: Contributor
timestamp: December 03, 2024, 05:47:45 pm
content: I think i used the capacitor i stated in the post before. 400V 8.2µF. I eventually abandoned the project but if i remeber correctly, the led panel worded.