Mains LED Light bulb still glows when turned off - Page 1
Forum section: electronics / beginners

author: Simon
author_rank: Unknown
timestamp: December 14, 2014, 04:58:14 pm
content: I bought 2 3 what LED lightbulbs on eBay. They work fine but when I turn them off they still glow slightly if I take them out of the socket the amount of light they make reduces to a slight glow due to the phosphor in the LEDs and eventually they go out. If I put the ball is back in the light socket they start to glow again not enough to make much light but they are definitely passing current. How is this possible?

author: SeanB
author_rank: Super Contributor
timestamp: December 14, 2014, 05:19:18 pm
content: Capacitance on the wiring to the mains switch, which is by you a parallel pair of cables in a conduit. This few hundred pF to 2nF capacitor is enough to charge the capacitors in the lamp till it lights dimly. Some lamps will flash at around one flash every 2-4 seconds as they build up enough charge to start the SMPS inside the lamp.Solutions are a bleed resistor of around 470k 1W across the lamp or an incandescent lamp in parallel, or separate the wiring into 2 separate conduits, or use 4 wire cable, with neutral and earth, to every switch to provide shielding. It is not common on old installations with steel conduit as this attenuates the phantom voltage a lot with the capacitance of the cables to the grounded conduit making an attenuator with a higher capacitance to the cable than between the cables.

author: Simon
author_rank: Unknown
timestamp: December 14, 2014, 05:23:46 pm
content: The strange thing is that another 3W LED light is not glowing in the same fitting. So there has to be a difference in design.I don't know how much power this "bleed" is using, it looks like house wiring circuits for lights might need to be look at in the future as the nature of the lights changes.

author: SeanB
author_rank: Super Contributor
timestamp: December 14, 2014, 05:36:08 pm
content: Cheap lights use a simple capacitive constant current source to drive the multiple dies in series ( typically 20-30 5mm or 3mm regular LED units) so any leakage current will provide enough power to drive the series string dimply. Other ones use a bridge rectifier, capacitor and SMPS controller, where the SMPS start up resistor provides enough drain to not allow the voltage to build up to a level where the controller will start, pulse the lamp and then turn off.

author: Simon
author_rank: Unknown
timestamp: December 14, 2014, 05:38:56 pm
content: sounds like it's time to have a DC light circuit mains supply

author: Zero999
author_rank: Super Contributor
timestamp: December 14, 2014, 06:06:47 pm
content: It's most likely the persistence of the phosphor on the LED. It's fairly normal. Running on DC won't solve the problems as I've seen this on some torches too.

author: Simon
author_rank: Unknown
timestamp: December 14, 2014, 06:09:34 pm
content: No it's not just the phosphors, they do hold some light due to phosphors but when you put the light into the fitting it starts to light up even more depite the switch is off

author: Seekonk
author_rank: Super Contributor
timestamp: December 14, 2014, 06:26:35 pm
content: That is wiring capacitance. Those that are dimable don't have that problem, the inverter takes too much power. A couple of nano amps is enough to light some cheap LED lamps. Most lamps have an internal resistor. These can draw a 1/4W and I have removed them from the lamps I use at camp that run on 140V DC. At home I have a dog lamp for the dog to get down the stairs at night. That is a standard 3.5W 120V LED wit a .01uF capacitor in series running at about 5mw. Had someone in Asia that wanted to monitor pumps in the field at night, something about not wanting to run into alligators on the 3 foot wide paths. I worked up a solution with a 120V LED spot running off the points. It takes next to nothing to run a LED if you remove the ghost resistor.If you put a .1uf X2 capacitor at the socket that will also eliminate the problem.

author: Simon
author_rank: Unknown
timestamp: December 14, 2014, 06:36:52 pm
content: So either way I'll still use power anyway, but maybe the capacitors in parallel won't ? as they will just keep giving the power back.

author: David_AVD
author_rank: Super Contributor
timestamp: December 14, 2014, 08:27:54 pm
content: It will be using no more power when off than if you had any other lamp (incandescent, etc) in there.

author: Simon
author_rank: Unknown
timestamp: December 14, 2014, 08:35:39 pm
content: true

author: Yansi
author_rank: Super Contributor
timestamp: December 14, 2014, 08:43:04 pm
content: cable capacitance might not be the real contributor. Most people forget that some light switches have a small neon lamp indication circuit, which is in series with the load, when switched off. The neon lamp indicator draws maybe two or three orders larger current, that the cable capacitance itself could make. And with a few miliamps running through a neon lamp, sure the LED lamp will glow.

author: Simon
author_rank: Unknown
timestamp: December 14, 2014, 08:49:03 pm
content: Well I suppose we are reaching a point where the bulbs use so little that leackage current is enough to make them work. These are 3W and they adequately replace 11 & 14W CFL's, I'm not saying that they make as much light but the overall solution with some loss of light is good enough for me to settle with 3W where i had 11 or 14W CFLThese are 60 LED lights so I'm guessing they are just all in series with a capacitor.Does the capacitor do anything for the power factor ?

author: Monkeh
author_rank: Super Contributor
timestamp: December 14, 2014, 09:49:19 pm
content: Quote from: Yansi on December 14, 2014, 08:43:04 pmThis is fairly rare, and usually people will know if they have a neon in their light switch..

author: Zero999
author_rank: Super Contributor
timestamp: December 14, 2014, 10:18:24 pm
content: Quote from: Simon on December 14, 2014, 06:09:34 pmWhat voltage does a meter indicate?Another thing you could try is to measure the voltage across the bulb when it's connected and the switch is off but you'll need to unscrew the ceiling rose and make sure you measure the wires going to the lamp, as it's quite normal for some of the connections to be permanently live in there.

author: Yansi
author_rank: Super Contributor
timestamp: December 14, 2014, 10:26:49 pm
content: Quote from: Monkeh on December 14, 2014, 09:49:19 pmRare maybe in some countries. Here it is relatively common to have such thing. It is useful also at night, when you need to find a light switch. But other countries seem not to discover that useful gizmo yet.When someone knows of the neon light in the switch, doesn't mean he knows how that thing is connected and what consequences it will have.

author: Monkeh
author_rank: Super Contributor
timestamp: December 14, 2014, 10:27:29 pm
content: Quote from: Yansi on December 14, 2014, 10:26:49 pmPlease define 'here'.

author: Yansi
author_rank: Super Contributor
timestamp: December 14, 2014, 10:33:26 pm
content: Sorry, ma here is defined by a black rectangle.But lets say, central Europe.

author: Seekonk
author_rank: Super Contributor
timestamp: December 14, 2014, 10:44:04 pm
content: I love threads like this. When there is a real technical problem, one or two answer. If it is a light bulb, everybody is an expert.This problem shows up mostly in three way light circuits (two switches one lamp) where there will be a long powered wire parallel with a wire that will power the lamp.

author: Simon
author_rank: Unknown
timestamp: December 15, 2014, 06:58:50 am
content: Yes capacitance of standard single switch wiring will mostly be in parallel with the bulb and help reduce the effect. I could go probing for the voltage on the bulb but that will have to wait.

author: SeanB
author_rank: Super Contributor
timestamp: December 15, 2014, 06:55:48 pm
content: Well, just grabbed a piece of solid core wire, pretty much similar to the stuff used in the UK for lighting, used here for lighting as well (1.5mm cable, but here it is twin and earth while in the UK it is twin only) and for a length of a little over 2m it has 300pF capacitance between the 2 conductors. From any core to the earth wire it is 470pF. this forms a pretty good capacitive divider so that you do not get much phantom voltage on the light fittings, helped as well by the earth being between the 2 power conductors.

author: nuno
author_rank: Frequent Contributor
timestamp: December 15, 2014, 08:22:26 pm
content: I've seen that at home too, in a living room wall lamp socket (never tried other sockets). My DMM measures 70 - 80VAC on the socket when the switch is off. It's a single circuit, from a double switch to the lamp; the double switch has one switch for each of 2 lamps in the room.