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For starters, I am not electrically capable, so bear with me. But, I did read similar questions that did not address, as far as I could tell, what I see happening (they may have, but I was too illiterate to understand).

I hired an electrician to replace an existing chandelier (19 years old, including dimmer switch is 19 years old). I bought 6 expensive ($10 each) Energetic LED soft white 3000K bulbs. These are marked 'Dimmable'. The chandelier is on a dimmer switch where off has meant off, in the past.

I installed 6 bulbs into the chandelier, turned them on bright. All is well. Turned them off, they remained 'on' (rather dim, but still not all that dim).

I also bought $9 bulbs (not dimmable) for another fixture. I also have a lamp nearby with the old fashioned light bulb. I unscrewed 1 of the 6 on the chandelier. It, of course, went out. The other 5 stayed lit. I then screwed in the old fashioned light bulb. All 5 LED lights went out!!! I turned on the light switch, and all 6 turned on. I turned it off and then unscrewed the old fashioned bulb - all 5 LED came back on!!!! I then replaced it with a $9 LED bulb, non dimmable. $9 bulb did not light. But the 5 LEDs that were still glowing continued to glow.

So, I have a problem. I want the lights to go off when turned off. Might the problem be the bulbs? The wiring of the new chandelier? The age/wiring of the dimmer switch? Something else?

Hopefully, you can tell from the content of this, that I know very very little. If you start talking to me about MOSFET, capacitors, snubbers and the like, it will likely go over my head. I will ask my electrician to come back out, but thought I would see what enlightenment I could obtain here.

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    \$\begingroup\$ This question belongs on diy.stackexchange where it's frequently asked. TLDR: your dimmer is obsolete. \$\endgroup\$ – Harper May 30 '16 at 0:44
  • \$\begingroup\$ Old X-10 switches that don't need separate common in the jbox are designed to be powered by a trickle of power through the filliment even when off. This dimmer might use a similar principle. Does it have two wires (plus ground) only, and is not purley mechanical? \$\endgroup\$ – JDługosz May 30 '16 at 5:09
  • \$\begingroup\$ I think Bigclive did a video sort of like this on youtube. youtube.com/watch?v=tzWz_guJHvY \$\endgroup\$ – Alex May 30 '16 at 19:19
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Buy a dimmer designed for LEDs. Your typical light dimmer puts out pulses of power; the brighter the setting the wider the pulses. This is OK for incandescent lamps as they draw lot of current and need a brief time to turn on and off.

LED lamps draw about 1/10th as much current and turn on and off in millionths of a second, so the same narrow pulses that make an incandescent light dim will make an LED light at least medium bright, maybe even flicker a bit.

You could "cheat" a bit by plugging in a incandescent bulb off in a corner to act as a minimum load which helps the LEDs behave better, but in the long run it is best to install the correct dimmer for the LEDs. Almost any hardware store should have them.

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I very much suspect that your dimmer is intended for use with incandescent bulbs, and it's not turning off completely. This is not a problem with regular bulbs, as you've found out.

However, your LED bulbs have a power supply circuit which is able to operate on the small amount of power being passed by the dimmer, and this creates your problem.

You have two solutions, I think. The first is to try replacing the dimmer - it may be defective.

The other solution you've already discovered. Leave one incandescent bulb in the chandelier.

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It sounds like the old dimmer switch is the problem. In the OFF position it is still leaking a small amount of current. The old fashioned bulbs are so inefficient that that small amount is not enough to light them. The new bulbs are very efficient so the small amount of leakage is lighting them up. By placing a single old fashioned bulb shunts the current away from the new bulbs causing them to shut off.

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An easy fix, just fit a 240 V neon bulb (with integral resistor) across the lamp input, this mops up any residual current. Inside the ceiling rose or the light fitting is a handy place were it cannot be seen. It worked every time.

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You have several options. If your chandelier has six sockets, can you run one incandescent bulb and five LED? That may keep the dimmer happy since the TRIAC inside it needs a minimum holding current and the LEDs have too low current consumption to satisfy it.

You can change the dimmer to one made for LEDs, made with MOSFETs inside and hopefully designed with very low "holding" current in mind.

Also, when you say on, how bright is that? I have some LEDs which shine even when the switch is off and no dimmer what so ever. If the plates inside the switch are too close (lamp lead switches mostly and not wall mounted), you will capacitivly couple a few uA of current which will bleed though the LED and make it glow. At 230 V, this effect is twice what you would see in a 115 V country. Far from bright, but still not off.

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Here is how a traditional switch is wired. Note the switch is in series with the load. Black is supply hot; red is switched hot; white is return (neutral).

enter image description here

A dimmer is a type of powered switch. That would include dimmers, PIRs, lighted switches, smart switches, any case where the switch needs power for its own onboard electronics. Here's how an ideal powered switch would be wired. Note that it has access to supply hot and neutral, and can power its own internal electronics indefinitely from these.

enter image description here

However, in many physical installations, the switch is off on a spur, fed by a 2-wire cable (supply and switched hot - no neutral present). That's outlawed as of 2011, but many installed switches are wired this way. As a result of this, many powered switches wire like this:

enter image description here

How do they do this? The powered switch places itself in series with the load. When on, it adds a tiny amount of voltage drop. When off, it leaks a small amount of current through the light bulb. This relied on a feature of incandescents: When cold, they resemble a dead short, and some current will flow before an incandescent starts to light.

So in fact, Off did not mean off in the past with the incandescents. Current leaked through them too. They were just so inefficient they didn't glow. LEDs are more efficient than that, and do indeed glow.

Once you screw in one old incandescent bulb in parallel with the LEDs, its "near dead short" provides a low impedance path for the dimmer's leakage current. It also greatly reduces voltage across the LEDs, so they too extinguish.

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