Weird issue with my ceiling lamp. Faint glow on a LED that is turned off. - Page 1
Forum section: electronics / beginners

author: Decoman
author_rank: Regular Contributor
timestamp: December 27, 2017, 11:18:36 pm
content: I noticed only today, that after flipping the power switch on my ceiling lamp, that has three LED "bulbs" in it, one of the bulbs glows dimly. The other two are dark, as expected.How is this even possible, if there shouldn't be any current? I am simply assuming that I am looking at an electrically lit LED, though I suppose it could be some kind of chemical reaction.When I bump this one led "bulb" with a pencil, the led bulb seem to flicker weakly.Previously, all five led bulbs I had used, all failed over a few months, in that same ceiling lamp, I am now thinking that something might be wrong with the wiring or something, somewhere. Btw, I know that this lamp has no grounding wire. When I installed it, there were only two wires to use.

author: 0xdeadbeef
author_rank: Super Contributor
timestamp: December 27, 2017, 11:33:18 pm
content: Are you sure the LED doesn't go completely dark after a few minutes or so?All of my (ceiling) LED lamps fade out to some degree. One in my parents house needs like 10 minutes or so until it's completely off (but this can be only observed in complete darkness).I suspect there is some kind of capacitor discharge that keeps a small current flowing even after the input voltage is switched off.

author: Decoman
author_rank: Regular Contributor
timestamp: December 27, 2017, 11:37:58 pm
content: I only noticed this odd thing this evening just a few minutes ago, but I will take notice of the time passing now and see if the glow dissipates.I managed to take a photo of what this odd led looks like in the dark. This LED is the first in a row of four, though I only have three of the four sockets occupied by the very same type of LED's (came in the same pack). 5W or 6W bulbs iirc.Turning off the mains fuse, didn't make the glow go away.

author: Yansi
author_rank: Super Contributor
timestamp: December 27, 2017, 11:40:09 pm
content: Litle current is needded for LED excitation. The cable stray capacitance is enough for the LED lamps to glow.The effect is much more significant, if the switch on the wall has the typical "off light" with a neon lamp in it.Nothing new!We have even had a problem with one of the LED bulbs that it would flash continuously. The stray current from both the cable and through the neon "off light" was enought for the LED inverter to charge its capacitor up and start running for a brief moment.

author: Decoman
author_rank: Regular Contributor
timestamp: December 27, 2017, 11:54:43 pm
content: I've stood around for about 20 minutes, and the glow is still there, as bright as it was previously.I will have to go to bed now though.Update: 7 hours later and the glow was still there.

author: floobydust
author_rank: Super Contributor
timestamp: December 27, 2017, 11:55:52 pm
content: LED phosphor can be excited by UV, radiation, electric fields etc. so I'd play around and see if "something" is exciting the phosphor.

author: Yansi
author_rank: Super Contributor
timestamp: December 27, 2017, 11:56:47 pm
content: The glow is not supposed to go away, until you cut it dry out ofanycurrent.floobydust: Yeah... it is excited by the current flowing through the LEDs, that are emitting the blue light!

author: cdev
author_rank: Super Contributor
timestamp: December 28, 2017, 12:18:36 am
content: Back in the pre-LED lighting days I used to live near a very large TV tower and would often walk my dog in the wooded area and neighborhoods around its base. In the summer, the area around it would often get very densely foggy and when that happened electronic equipment and fluorescent lights around there would go absolutely berserk.You could sometimes see an eerie flickering outside of houses as you walked by.

author: sleemanj
author_rank: Super Contributor
timestamp: December 28, 2017, 12:23:01 am
content: Quite common, a modern LED just needs such incredibly low amounts of energy to emit a visible glow that capacitive coupling can provide the microamps or even nanoamps it might require.Here's bigclive observing the effectHere's another video from somebody else.And of course there are situations where the switch isn't actually off and pass a small amount of current to parasitically power themselves (or just leaky), for a standard incandescent lamp that's not an issue because the filament wont' light up, but for an LED that's going to be plenty to show a glow, or flash.

author: bw2341
author_rank: Regular Contributor
timestamp: December 28, 2017, 05:29:11 am
content: Is the lamp part of another device such as a ceiling fan?Some ceiling fans may incorporate their own switching components internally. If it's an electronic switch, there might be some leakage current by design.Is the wall switch a standard on-off switch?Many fancier wiring devices need some leakage current to power their functions. Some of these are:Electronic programmable timersMotion sensor switchesTouch control dimmersIlluminated wall switchesSmart-home remote controllable switchesand more...If these are installed with a two-wire connection, they need to leak some current through the light bulb to work.You can check for leakage current with an old incandescent light bulb. When you screw in the incandescent bulb into another socket on the same lamp, the problem LED lamp should go out. Since the incandescent bulb has a lower resistance, most of the leakage current should pass through it.

author: Decoman
author_rank: Regular Contributor
timestamp: December 28, 2017, 06:17:01 am
content: Quote from: bw2341 on December 28, 2017, 05:29:11 amNo.Quote from: bw2341 on December 28, 2017, 05:29:11 amYes, it looks like it. A big flat flip switch with no LED indicator or anything to it.

author: Seekonk
author_rank: Super Contributor
timestamp: December 28, 2017, 10:19:48 am
content: Is it a three way switch, controlled by two locations? That would have enough wire capacitance. I have some 3.5W spots I repurposed since they were a base I didn't use. Internally they had a resistor to bleed off any wire capacitance leakage. That consumed a 1/4W on its own. Seemed like a lot of wasted power for a 3W lamp. Your only recourse is to put a load in parallel with the lamp. I like using RC network for this .1uf 220 ohm in series. Hide it in switch or lamp box. Or buy some different lamps with internal resistors.

author: SparkyFX
author_rank: Frequent Contributor
timestamp: December 28, 2017, 10:32:56 am
content: Is there a second, powered pair of wires running parallel to the lighting?Another possibility would be that indeed the neutral wire is switched instead of the phase (trap for young players). Leakage current to protective earth might then lead to this effect.A fault in the installation might also lead to loads connected in series instead of parallel, which has effects like dimly glowing lights as well.

author: Decoman
author_rank: Regular Contributor
timestamp: December 28, 2017, 10:40:34 am
content: Quote from: Seekonk on December 28, 2017, 10:19:48 amI am not able to discern the wiring of the wall flip switch and this wall switch isn't something I ever installed myself. Must have been there for some 30 years or more I think.Quote from: SparkyFX on December 28, 2017, 10:32:56 amThere should be no other wires running parallel I think, I only saw two wires up inside the socket.When I mounted the four-bulb ceiling lamp, I noticed there were only two wires. I forgot how I wired this lamp, but it could be that I wired the L and N randomly, as it sort of shouldn't matter anyway with my apartment's 230V AC power from what I had read up to that point.

author: SparkyFX
author_rank: Frequent Contributor
timestamp: December 28, 2017, 11:16:19 am
content: Thats right, on the lamp polarity should not matter (both must be supposed to be hot, regulation in your country may differ). I mean that the switch is wired in a way that phase is connected permanently, while neutral is switched. If the neutral wire between lamp socket and switch has got a small leakage to ground (like broken isolation or water in a distribution box), that such a leakage current would not make a regular lamp glow at all, but might be sufficient for those LEDs.You could ask an electrician to check the wiring with an isolation tester or multimeter while loads are removed and the circuit is isolated and not powered (e.g. at the switch) to sort that out.

author: Decoman
author_rank: Regular Contributor
timestamp: December 28, 2017, 11:27:49 am
content: Thank you. I might do that and solicit an electrician for a quick check. Shouldn't cost too much I think.

author: ogden
author_rank: Super Contributor
timestamp: December 28, 2017, 12:20:33 pm
content: #1 reason of such problem usually is switch with neon lamp indicator inside. Those neon lamps fail after some time and conduct more current than initially - which in result is enough to glow or pulse low power electronic-ballast lamp.

author: bw2341
author_rank: Regular Contributor
timestamp: December 28, 2017, 02:19:21 pm
content: Many styles of illuminated switches look identical to their non-illumitnated counterparts. The indicator lamp glows through the plastic itself.Even if the switch is a normal one and working correctly, it’s worth confirming the leakage current with an incandescent bulb, if you have one.

author: Zero999
author_rank: Super Contributor
timestamp: December 28, 2017, 03:53:10 pm
content: Quote from: Decoman on December 27, 2017, 11:37:58 pmThe yellow colour of the glow seems to imply it's the phosphor, rather than a low current. Try removing the LED from its fitting.

author: floobydust
author_rank: Super Contributor
timestamp: December 28, 2017, 04:46:08 pm
content: Yes I'm saying it's phosphor alone glowing, not some phantom power to the LED.I'm guessing a small radioactive element is there

author: Zero999
author_rank: Super Contributor
timestamp: December 28, 2017, 07:08:58 pm
content: Quote from: floobydust on December 28, 2017, 04:46:08 pmI doubt there's any radioactive element in there.LED phosphors can glow for a long time, after the light source has been removed. The initial light seems to disappear very quickly, but the afterglow can last for awhile. Try this with a bare LED, with just a constant current source or resistor driving it.

author: Yansi
author_rank: Super Contributor
timestamp: December 28, 2017, 07:27:29 pm
content: Yeah, try it. Yall see there is no light decay in the led. Absolutely none (<1us).

author: KE5FX
author_rank: Super Contributor
timestamp: December 28, 2017, 07:40:23 pm
content: The phosphor used by at least some of these bulbs use is ridiculously persistent. When I installed LED bulbs in my bedroom's ceiling fixtures, I discovered that they remained bright enough to keep me awake for at least a couple of hours. Had to go back to CFLs.

author: Zero999
author_rank: Super Contributor
timestamp: December 28, 2017, 07:49:52 pm
content: Quote from: Yansi on December 28, 2017, 07:27:29 pmI have and yes there was some afterglow. I've even got two LEDs of the same type, blind folded myself, switched an unknown LED on and off again and was able to tell which one was switched on by the afterglow. I don't even need to turn the LED on for this to occur. The effect is clearly visible with a blacklight. I haven't tried a blind test on myself with that though.You're right that there's hardly any delay in the LED, but the phosphor has significant persistence.

author: Yansi
author_rank: Super Contributor
timestamp: December 28, 2017, 08:12:51 pm
content: I do not know what LEDs do you have guys, but have just tried (to be sure) using bog standard 5mm white LED and a chinese 10W multichip. There is really none afterglow.My friend did extensive testing on different white LEDs and measured up to 1.4us maximum light decay and that was by the way an IR LED and the decay mos probably caused by something very else, as obviously IR LEDs do not have phosphors.I have also a few craptacular 230V chinese LED bulbs here with dead electronic ballasts. I will try those too. Gimme a minute to crack one open...//EDIT: Found one. Unfortunately the chips are all potted under the phosphor on a glass disc. Failed open. My 200V supply is not doing anything to it.

author: Zero999
author_rank: Super Contributor
timestamp: December 28, 2017, 09:30:27 pm
content: Quote from: Yansi on December 28, 2017, 08:12:51 pmI have tested white LEDs in a strobe application and can confirm that most of the light output stops very quickly, after the power is disconnected from the LED, even though there is some very dim afterglow.When you did your test with the 10W LED (it's not so noticeable with small LEDs) were your eyes dark adapted?Another test you could do is exiting the phosphor with a blue LED.I am interested in properly investing this, yet lack the tools to properly do so.

author: Yansi
author_rank: Super Contributor
timestamp: December 28, 2017, 09:39:04 pm
content: Not much eye adaption has been done, but I could certainly distinguish the "glow of stray current" we are talking about here from the beginning. That is actually relatively bright and am almost sure it is not the phosphor glowing by itself. (I think there even is a reason behind why the phosphor has deliberately very low decay - look up "LED phosphor quenching")If your LED lamp still dimly glows after being dismantled from the light fixture: I have interesting news for you!There usually is a filter capacitor inside to suppress the strobe effect. Even the slightest of uA are very visible on todays high efficiency LEDs. Disconnect the LED completely from the circuit and voila, no glow!

author: Zero999
author_rank: Super Contributor
timestamp: December 28, 2017, 10:03:35 pm
content: Quote from: Yansi on December 28, 2017, 09:39:04 pmI know about the storage capacitor. I have tested the bare LEDs, unsoldered from the board and still found there's perceptible afterglow.

author: Decoman
author_rank: Regular Contributor
timestamp: December 28, 2017, 10:17:23 pm
content: I unscrewed this one LED "bulb" this evening, and the thing apparently has a glow to it still. One mystery solved then.Update: And the glow seems to have dissipated now almost entirely, not that long after I unscrewed it.

author: BrianHG
author_rank: Super Contributor
timestamp: December 28, 2017, 10:46:24 pm
content: Quote from: Decoman on December 28, 2017, 10:17:23 pmCongratulations, you stumbled onto the perpetual light engine! Place a super sensitive solar cell in fron and you will have free energy forever...Since you took it out of the socket... Look closely at the led's phosphor if you can. If there is a hot spot in the center of each LED, then, your LEDs are being powered most likely by an AM radio broadcast and it is a fluke that the internal circuity's leads inductance and orientation just manages to tune into the signal. If the phosphor glows evenly across each LED, without central hot spots, then, somehow there is a UV or IR light source within your room which just happens to excite the LED's phosphor to a visible light.

author: floobydust
author_rank: Super Contributor
timestamp: December 28, 2017, 10:53:41 pm
content: Quote from: Decoman on December 28, 2017, 10:17:23 pmWhat are you drinking lately, I'd like to try someYou'd have to keep the LED bulb in the dark for a long time, to see if this is from the phosphor remaining excited by light, or if this isradioluminescence.

author: Decoman
author_rank: Regular Contributor
timestamp: December 28, 2017, 11:11:41 pm
content: This is getting complicated. I guess I can tape the bulb to the socket, but without feeding it electricity, if there happens to be something affecting the bulb from the exterior.

author: SparkyFX
author_rank: Frequent Contributor
timestamp: December 29, 2017, 03:19:44 pm
content: The LEDs i use in ceiling lights afterglow visibly (2 meters away) for a couple of seconds after a few hours of being switched on.

author: Yansi
author_rank: Super Contributor
timestamp: December 29, 2017, 03:25:31 pm
content: ... until the internal filter cap discharges.

author: ConKbot
author_rank: Super Contributor
timestamp: December 29, 2017, 03:45:00 pm
content: Don't know if it's the same in the EU, but in the US it's common for the run to a breaker to run to a light fixture, then another lead from the light fixture to the switch, where the black lead is the power, and the white lead (normally neutral) gets marked with black tape and carries the switched power back to the light fixture itself. This could have enough capacitance depending on cable construction. (Usually in the US it's flat romex with the ground between power and neutral, which should reduce capacitive leakage between the two)Throw an incandescent bulb in an adjacent socket. If the LED goes out, it's capacitive leakage. If it doesn't, remove the bulb, and if it stays on, it's either residual glow or external excitation. No need for an electrician.