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We have LED bulbs in our apartment. In the bathroom, the bulb flickers on and off when it's switched off. Same when it was a CFL bulb before I switched to LED cause I hate CFLs. Our mains voltage is 220v. The bulb we have looks like this

If I remember correctly, I should add a capacitor in series to the bulb to fix this. But what kind of capacitor? and what rating?

Update:

  • The switch is a combined switch and plug in one part. Looks like a normal one line switch.
  • This apartment and the whole building doesn't have grounding. We don't have this in requirement in our building codes. I'm in Lebanon.
  • Only this one bulb in the apartment does this. Other bulbs are also LED and two more CFL not changed yet.

Our bulb looks like this!

The switch in question

Any help appreciated, thanks.

Resolution: I fixed it with an incandescent bulb! I'm too lazy to go electrician detective for now.

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closed as off-topic by Daniel Grillo, Matt Young, Null, JIm Dearden, nidhin Jan 3 '16 at 6:29

This question appears to be off-topic. The users who voted to close gave this specific reason:

  • "Questions on the use of electronic devices are off-topic as this site is intended specifically for questions on electronics design." – Daniel Grillo, Matt Young, Null, JIm Dearden, nidhin
If this question can be reworded to fit the rules in the help center, please edit the question.

  • \$\begingroup\$ Is the switch a regular on-off switch with two terminals or is it something fancier? \$\endgroup\$ – Transistor Dec 23 '15 at 12:40
  • \$\begingroup\$ If the bulb is flickering ON and OFF when the supply is turned OFF indicates a (mains) switch/wiring/earthing problem. Get a qualified electrician to check it out. The bulb shown in the photo has an internal circuit (converting AC to asuitable DC level for LED) that allows it to run from 220V AC mains. Adding a capacitor will not sort out this problem and could be quite dangerous. \$\endgroup\$ – JIm Dearden Dec 23 '15 at 15:35
  • \$\begingroup\$ Updated my answer \$\endgroup\$ – Slabo Dec 23 '15 at 16:22
  • \$\begingroup\$ In the bathroom, is there any type of fan that comes on with the light? Such as an exhaust fan? \$\endgroup\$ – rdtsc Dec 23 '15 at 17:10
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    \$\begingroup\$ @PeterGreen My main concern is that it happens with the power turned off. It would be a totally different situation if the power was on, and yes, I have seen flickering LED bulbs. Flickering with the power off implies the LED must be drawing power from somewhere - perhaps a few volts difference between neutral/earth charging up an internal capacitor forming a relaxation type oscillator. I'm suggesting taking a safety first approach to check out the wiring first. Changing the type of bulb may well cure this (see Sperhro's answer). I agree with you that this wasn't an issue with incandescents. \$\endgroup\$ – JIm Dearden Dec 24 '15 at 11:13
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You might find that a different brand of bulb is much less prone to this kind of behavior. The internal circuit varies quite a bit from bulb to bulb. If you have a store with a generous return policy that applies to such products you could simply buy a few different types and try them. In particular, I would expect bulbs based on Microchip's CL8800 to not suffer from this problem. See this excellect article if you are interested in the gory details. Most bulbs have a small switching supply in them, but this type is different.

enter image description here

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    \$\begingroup\$ What a great article you cited. I've read similar for CFLs, this is the first LED teardown that I've seen. Nice nice nice. \$\endgroup\$ – Adam Lawrence Dec 23 '15 at 16:53
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The cause of this is the design of the electronics inside the bulb combined with stray capacitance between live and switched live. In the UK it is most commonly seen on 2-way switching installations due to the physical conductor layout in the cable used for those.

Basically the circuitry inside the bulb contains a rectifier followed by a capacitor, followed by some electronics that has a minimum voltage threshold. The capacitor charges (through the stray capacitance) until it reaches some threshold where the electronics in the bulb try to start up producing a flicker of light and discharging the capacitor. This process repeats.

One fix is to add a a capacitor in parallel with the bulb. This forms a voltage divider with the stray capacitance preventing the voltage build-up. (for those wondering why this is different from the capacitance inside the bulb it is because it is before the rectification while the capacitance in the bulb is after it)

In theory to calculate a minimum value for the capacitor you would need to know

  1. How low the voltage needs to be held to stop the LED bulb flickering.
  2. What the stray capacitance is between live and switched live.

Unfortunately neither of these values is readilly available so it becomes a case of rules of thumb rather than an exact science.

The capacitor needs to be of a type suitable for connecting directly across the mains. It's usually also good if it has some resistance in series to limit current surges at switch-on.

One forum thread I found suggested http://uk.rs-online.com/web/p/rc-network-capacitors/2067869/ which seems fairly reasonable to me.

The capacitor needs to be mounted inside a suitable electrical enclosure.

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  • \$\begingroup\$ I'm wondering, with the type of switch shown in the question, when it is turned off (seperating live from switched live), how much capacitance exists across/between the open contacts. I don't see it being more than (say) 20pF and this is not really going to make any noticeable effect if you consider that the wiring to the lamp will probably have 200pF as a switched-live to neutral "shunt". Also when we're talking pico farads, any effect is going to be over in a 1000x more than a blink of an eye. Maybe I'm not reading your answer correctly. \$\endgroup\$ – Andy aka Dec 26 '15 at 1:19
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A lot of different electronic circuits will produce odd-ball behaviour as the power is lost. Think about some music systems producing little pops when turned off (or on). Microprocessor boards will produce odd behaviour when being forcibly powered down and a PC has to be powered down in a proper way or you might get hard disk failures (extreme but not unheard of) or possibly data corruptions.

TVs used to produce a white dot in the centre of the screen when switched off (it gradually faded of course) and when you remove power from a motor it slowly comes to a halt. These are all after-effects following the removal of electricity. I had an old two stroke motor bike that got so hot even though I turned the ignition off it was revving like mad - now that really was annoying.

A typical LED circuit might use a chip like this: -

enter image description here

As the AC voltage is removed, the DC voltage across Cin remains high-enough for a few tens of milli-seconds to keep the LEDs powered but then it will inevitably drop below a certain theshold and the chip will turn off the LEDs but, this removes the load from Cin and its voltage then falls more gradually and, who amongst us can say that at some low voltage the chip doesn't have some perculiar behaviour that flashes the LEDs for a couple of instants.

Yes it's annoying but will sometimes happen with some designs. BTW I'm not saying the above chip will exhibit this behaviour, I'm just using it as an example of how strange things can appear to happen in some designs.

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All the neutral wires carries the current back to the transformer neutral point that is grounded at the supplying transformer secondary side. If it is not properly grounded, it can cause incidents such like yours.

Check if the neighbors also having the same issue. If it is, you better call the breakdown services.

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  • \$\begingroup\$ only the bulb in the bathroom does this. Strange that in the apartment only one bulb does this. \$\endgroup\$ – Slabo Dec 23 '15 at 17:09
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    \$\begingroup\$ @Slabo Does moving the "suspect" bulb to another room solve the issue? \$\endgroup\$ – Andy aka Dec 23 '15 at 17:32
  • \$\begingroup\$ Will try and report back \$\endgroup\$ – Slabo Dec 23 '15 at 19:10

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