LED spotlight glowing dimly with power off

[zerobug]

I wondered if I could ask for some advice - I have a ceiling lamp fitting comprising 4 gu10 type 35w halogen spotlights. Some time ago I swopped the bulbs for LED versions which worked fine, but I decided to put the originals back for the darker mornings. When I took the led bulbs out, I noticed that when left with only one LED bulb in situ ( the other 3 holders empty) the one remaining bulb glowed, very very dimly, although switched off. On installing one of the original halogen bulbs into any of the other holders, ( still switched off) it went out. Could anyone suggest a cause as to why this might be happening? Cheers.

 

[MarkieSparkie]

This phenomenon is caused by the final circuit cable capacitance. It is more common in conventionally wired two way and intermediate switching circuits due to the wide spread use of 3+E cable and the extra parallel capacitance that creates.
The GU10 mains LED lamp is essentially a number of LEDs driven by a constant current driver powered by a mains rectifier and smoothing capacitor.
Even though the live feed to the LED lamp is switched OFF the cable capacitance allows leakage current to flow to the lamp slowly charging the smoothing capacitor in the lamp until the potential is sufficient for the constant current driver to "fire" the LEDs. This discharges the smoothing capacitor, the constant current driver stops, the LEDs extinguish and the process of charging starts again ... if conditions are suitable this continues indefinitely.
In your case the charge/discharge cycle is short causing the LEDs to appear to be 'ON' continuously, however the energy is limited so the LEDs are only barely lit. Once more more lamps are added the loading on the cable capacitance is such that the individual smoothing capacitors do not reach "firing" potential.
A tungsten halogen filament lamp doesn't suffer this problem because the filament is permanently loading the cable capacitance and any acquired charge is unable to overcome the thermal inertia.

 

[telectrix]

excellent , concise explanation, markie. or you could say it's the fairies.

 

[Kamikaze]

I would have blamed it on the gremlins...lol

 

[telectrix]

or coulds it be those pesky faster than the speed of light neutrinos? ( used to get 'em in bags from the sweetshop )

 

[Robinson]

this phenomenon is caused by the final circuit cable capacitance. It is more common in conventionally wired two way and intermediate switching circuits due to the wide spread use of 3+e cable and the extra parallel capacitance that creates.
The gu10 mains led lamp is essentially a number of leds driven by a constant current driver powered by a mains rectifier and smoothing capacitor.
Even though the live feed to the led lamp is switched off the cable capacitance allows leakage current to flow to the lamp slowly charging the smoothing capacitor in the lamp until the potential is sufficient for the constant current driver to "fire" the leds. This discharges the smoothing capacitor, the constant current driver stops, the leds extinguish and the process of charging starts again ... If conditions are suitable this continues indefinitely.
In your case the charge/discharge cycle is short causing the leds to appear to be 'on' continuously, however the energy is limited so the leds are only barely lit. Once more more lamps are added the loading on the cable capacitance is such that the individual smoothing capacitors do not reach "firing" potential.
A tungsten halogen filament lamp doesn't suffer this problem because the filament is permanently loading the cable capacitance and any acquired charge is unable to overcome the thermal inertia.

wow ! ! !

 

[zerobug]

Markie, thanks very much for the very detailed explanation, appreciated. (Also pleased it sounds as though the wiring is all ok.) Thanks.

 

[poppypiesdad]

Ahhh thats why the upstairs compact fires off at night time , next question , how to stop it , apart from the filament lamp

Jamie

 

[MarkieSparkie]

Ahhh thats why the upstairs compact fires off at night time , next question , how to stop it , apart from the filament lamp

Jamie

Hi Jamie, I didn't post a solution to the problem as it was irrelevant in Zerobugs situation. However, I do have a solution for you:
A simple solution is to permanently discharge the cable capacitance by wiring two 160K ohm, 1/4W, resistors rated at >=250V, in series across the lamp holder terminals. Do not try to use a single 330K ohm resistor because this may fail due to voltage breakdown.
If this solution is implemented a notice should be placed at the DB stating which final circuits are effected and which lamp holders have been modified, why and how.

 

[Robinson]

Hi Jamie, I didn't post a solution to the problem as it was irrelevant in Zerobugs situation. However, I do have a solution for you:
A simple solution is to permanently discharge the cable capacitance by wiring two 160K ohm, 1/4W, resistors rated at >=250V, in series across the lamp holder terminals. Do not try to use a single 330K ohm resistor because this may fail due to voltage breakdown.
If this solution is implemented a notice should be placed at the DB stating which final circuits are effected and which lamp holders have been modified, why and how.

Excuse my ignorance but would this be across just one within the circuit or each point. And in what way do you mean across terminals ? sorry i just intrigued and like to get clear in my head.

 

[telectrix]

anywhere across L and N on the circuit. the resistors just act as a drain of the capacitance.

 

[clicon357]

I have something similar with a couple of low energy bulbs and some two way switches. Not bothered doing anything about it but I've heard people use these "snubbers" from Maplin:

Contact Suppressor : Suppression : Maplin Electronics

 

[Robinson]

anywhere across L and N on the circuit. the resistors just act as a drain of the capacitance.

Thanks Tele. makes sense.

 

[MarkieSparkie]

Excuse my ignorance but would this be across just one within the circuit or each point. And in what way do you mean across terminals ? sorry i just intrigued and like to get clear in my head.

Hi, Robinson, I've expanded the solution explanation to give you more information:
A simple solution is to permanently discharge the cable capacitance by wiring two 160K ohm, 1/4W, resistors rated at >=250V, in series across L and N on the final circuit, it is good electronics practice to do this as close to the effected lamp as possible, if this is done across the switched L and N at the lamp holder terminals it allows simple switching out of circuit (or disconnection) for testing. Do not try to use a single 330K ohm resistor because this may fail due to voltage breakdown.
If the final circuit has a large number of switched lamps it may be pragmatic to connect the series resistors between permanent L and N at a convenient point e.g. at the DB to reduce the number of resistors required, however the number of points that will require treatment will depend on how the cable capacitance is distributed on the final circuit. Additional points particularly where 3+E is used for 2 way and intermediate switching may well require additional pairs of resistors.

 

[JM7]

Hi, MarkieSparkie, seems pretty simple. I have this issue with a light fitting that I have just fitted with LED bulbs. I have have understood correctly you are suggesting 2 resistors at 160K (0.6W and min 250V) wired in series and then connected in parallel with the light fitting. Is that right? Only seems to affect this fitting so this seems to be the simplest solution.